[PATCH v4] Use open file description locks for lockfiles

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[PATCH v4] Use open file description locks for lockfiles
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* [PATCH v4] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 35b0b72f0a7..25ebcd3cc47 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 0e624fe36b9..677137207e7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index d88a7a70308..153fbb477bb 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..d4c2f80eb46 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+}			PGLockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1;
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	PGLockFile *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(PGLockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index ffb413ab612..ad34142e7d6 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1956,6 +1956,7 @@ PGIOAlignedBlock
 PGLZ_HistEntry
 PGLZ_Strategy
 PGLoadBalanceType
+PGLockFile
 PGMessageField
 PGModuleMagicFunction
 PGNoticeHooks

base-commit: 73dfe79fd6034b1e7e41e83d9c82c166dba8eb67
-- 
2.52.0


--tks2nr37zts5e6h7--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v4] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 35b0b72f0a7..25ebcd3cc47 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 0e624fe36b9..677137207e7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index d88a7a70308..153fbb477bb 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..d4c2f80eb46 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+}			PGLockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1;
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	PGLockFile *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(PGLockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index ffb413ab612..ad34142e7d6 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1956,6 +1956,7 @@ PGIOAlignedBlock
 PGLZ_HistEntry
 PGLZ_Strategy
 PGLoadBalanceType
+PGLockFile
 PGMessageField
 PGModuleMagicFunction
 PGNoticeHooks

base-commit: 73dfe79fd6034b1e7e41e83d9c82c166dba8eb67
-- 
2.52.0


--tks2nr37zts5e6h7--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v4] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 35b0b72f0a7..25ebcd3cc47 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 0e624fe36b9..677137207e7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index d88a7a70308..153fbb477bb 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..d4c2f80eb46 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+}			PGLockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1;
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	PGLockFile *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(PGLockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index ffb413ab612..ad34142e7d6 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1956,6 +1956,7 @@ PGIOAlignedBlock
 PGLZ_HistEntry
 PGLZ_Strategy
 PGLoadBalanceType
+PGLockFile
 PGMessageField
 PGModuleMagicFunction
 PGNoticeHooks

base-commit: 73dfe79fd6034b1e7e41e83d9c82c166dba8eb67
-- 
2.52.0


--tks2nr37zts5e6h7--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v3] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 5f77f3cac29..15bda6c6413 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 61cee42daa7..e24cb7a6f01 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index 568e0e150bf..3d641dc0403 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..26c3324542c 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+} LockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	LockFile   *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(LockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 1969d467c1d..185d69b5520 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1653,6 +1653,7 @@ LocationLen
 LockAcquireResult
 LockClauseStrength
 LockData
+LockFile
 LockInfoData
 LockInstanceData
 LockMethod

base-commit: 031904048aa22e7c70dc8e9c170e2743f9b0f090
-- 
2.52.0


--zdel3ow7bygx53fm--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v4] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 35b0b72f0a7..25ebcd3cc47 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 0e624fe36b9..677137207e7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index d88a7a70308..153fbb477bb 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..d4c2f80eb46 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+}			PGLockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1;
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	PGLockFile *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(PGLockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index ffb413ab612..ad34142e7d6 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1956,6 +1956,7 @@ PGIOAlignedBlock
 PGLZ_HistEntry
 PGLZ_Strategy
 PGLoadBalanceType
+PGLockFile
 PGMessageField
 PGModuleMagicFunction
 PGNoticeHooks

base-commit: 73dfe79fd6034b1e7e41e83d9c82c166dba8eb67
-- 
2.52.0


--tks2nr37zts5e6h7--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v3] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 5f77f3cac29..15bda6c6413 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 61cee42daa7..e24cb7a6f01 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index 568e0e150bf..3d641dc0403 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..26c3324542c 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+} LockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	LockFile   *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(LockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 1969d467c1d..185d69b5520 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1653,6 +1653,7 @@ LocationLen
 LockAcquireResult
 LockClauseStrength
 LockData
+LockFile
 LockInfoData
 LockInstanceData
 LockMethod

base-commit: 031904048aa22e7c70dc8e9c170e2743f9b0f090
-- 
2.52.0


--zdel3ow7bygx53fm--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v3] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 5f77f3cac29..15bda6c6413 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 61cee42daa7..e24cb7a6f01 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index 568e0e150bf..3d641dc0403 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..26c3324542c 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+} LockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	LockFile   *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(LockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 1969d467c1d..185d69b5520 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1653,6 +1653,7 @@ LocationLen
 LockAcquireResult
 LockClauseStrength
 LockData
+LockFile
 LockInfoData
 LockInstanceData
 LockMethod

base-commit: 031904048aa22e7c70dc8e9c170e2743f9b0f090
-- 
2.52.0


--zdel3ow7bygx53fm--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v3] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 5f77f3cac29..15bda6c6413 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 61cee42daa7..e24cb7a6f01 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index 568e0e150bf..3d641dc0403 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..26c3324542c 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+} LockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	LockFile   *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(LockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 1969d467c1d..185d69b5520 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1653,6 +1653,7 @@ LocationLen
 LockAcquireResult
 LockClauseStrength
 LockData
+LockFile
 LockInfoData
 LockInstanceData
 LockMethod

base-commit: 031904048aa22e7c70dc8e9c170e2743f9b0f090
-- 
2.52.0


--zdel3ow7bygx53fm--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v3] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 5f77f3cac29..15bda6c6413 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 61cee42daa7..e24cb7a6f01 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index 568e0e150bf..3d641dc0403 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..26c3324542c 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+} LockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	LockFile   *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(LockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 1969d467c1d..185d69b5520 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1653,6 +1653,7 @@ LocationLen
 LockAcquireResult
 LockClauseStrength
 LockData
+LockFile
 LockInfoData
 LockInstanceData
 LockMethod

base-commit: 031904048aa22e7c70dc8e9c170e2743f9b0f090
-- 
2.52.0


--zdel3ow7bygx53fm--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v3] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 5f77f3cac29..15bda6c6413 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 61cee42daa7..e24cb7a6f01 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index 568e0e150bf..3d641dc0403 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..26c3324542c 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+} LockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	LockFile   *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(LockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 1969d467c1d..185d69b5520 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1653,6 +1653,7 @@ LocationLen
 LockAcquireResult
 LockClauseStrength
 LockData
+LockFile
 LockInfoData
 LockInstanceData
 LockMethod

base-commit: 031904048aa22e7c70dc8e9c170e2743f9b0f090
-- 
2.52.0


--zdel3ow7bygx53fm--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v4] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 35b0b72f0a7..25ebcd3cc47 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 0e624fe36b9..677137207e7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index d88a7a70308..153fbb477bb 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..d4c2f80eb46 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+}			PGLockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1;
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	PGLockFile *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(PGLockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index ffb413ab612..ad34142e7d6 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1956,6 +1956,7 @@ PGIOAlignedBlock
 PGLZ_HistEntry
 PGLZ_Strategy
 PGLoadBalanceType
+PGLockFile
 PGMessageField
 PGModuleMagicFunction
 PGNoticeHooks

base-commit: 73dfe79fd6034b1e7e41e83d9c82c166dba8eb67
-- 
2.52.0


--tks2nr37zts5e6h7--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v4] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 35b0b72f0a7..25ebcd3cc47 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 0e624fe36b9..677137207e7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index d88a7a70308..153fbb477bb 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..d4c2f80eb46 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+}			PGLockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1;
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	PGLockFile *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(PGLockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index ffb413ab612..ad34142e7d6 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1956,6 +1956,7 @@ PGIOAlignedBlock
 PGLZ_HistEntry
 PGLZ_Strategy
 PGLoadBalanceType
+PGLockFile
 PGMessageField
 PGModuleMagicFunction
 PGNoticeHooks

base-commit: 73dfe79fd6034b1e7e41e83d9c82c166dba8eb67
-- 
2.52.0


--tks2nr37zts5e6h7--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v4] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 35b0b72f0a7..25ebcd3cc47 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 0e624fe36b9..677137207e7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index d88a7a70308..153fbb477bb 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..d4c2f80eb46 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+}			PGLockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1;
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	PGLockFile *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(PGLockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index ffb413ab612..ad34142e7d6 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1956,6 +1956,7 @@ PGIOAlignedBlock
 PGLZ_HistEntry
 PGLZ_Strategy
 PGLoadBalanceType
+PGLockFile
 PGMessageField
 PGModuleMagicFunction
 PGNoticeHooks

base-commit: 73dfe79fd6034b1e7e41e83d9c82c166dba8eb67
-- 
2.52.0


--tks2nr37zts5e6h7--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v3] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 5f77f3cac29..15bda6c6413 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 61cee42daa7..e24cb7a6f01 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index 568e0e150bf..3d641dc0403 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..26c3324542c 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+} LockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	LockFile   *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(LockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 1969d467c1d..185d69b5520 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1653,6 +1653,7 @@ LocationLen
 LockAcquireResult
 LockClauseStrength
 LockData
+LockFile
 LockInfoData
 LockInstanceData
 LockMethod

base-commit: 031904048aa22e7c70dc8e9c170e2743f9b0f090
-- 
2.52.0


--zdel3ow7bygx53fm--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v4] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 35b0b72f0a7..25ebcd3cc47 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 0e624fe36b9..677137207e7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index d88a7a70308..153fbb477bb 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..d4c2f80eb46 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+}			PGLockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1;
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	PGLockFile *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(PGLockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		PGLockFile *lock_file = (PGLockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index ffb413ab612..ad34142e7d6 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1956,6 +1956,7 @@ PGIOAlignedBlock
 PGLZ_HistEntry
 PGLZ_Strategy
 PGLoadBalanceType
+PGLockFile
 PGMessageField
 PGModuleMagicFunction
 PGNoticeHooks

base-commit: 73dfe79fd6034b1e7e41e83d9c82c166dba8eb67
-- 
2.52.0


--tks2nr37zts5e6h7--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v3] Use open file description locks for lockfiles
@ 2025-12-18 17:21 Dmitrii Dolgov <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Dmitrii Dolgov @ 2025-12-18 17:21 UTC (permalink / raw)

When starting up, postmaster checks for an existing data directory lockfile. If
this file contains current process PID, it's assumed to be stale. Turns out
there is another possibility: we might be running in a PID namespace, and there
is another postgres running inside another PID namespace using the same data
directory. The result is that we don't see another process due to namespace
isolation and start concurrently with the other.

To prevent such situations, at startup use fcntl to get an exclusive open file
description lock for data directory lockfile. Since such locks are associated
with open file descriptors, meaning they're not affected by PID namespace
isolation. It's a "best effort" locking, intended to work with already existing
mechanism, not replace it.

This approach was discussed multiple times in the past, and usually was
rejected as the main work horse for the data directory lockfile due to:

* Portability issues. Open file description lock was a non-POSIX extension in
  Linux and similar flock is from BSD standard. But looks like everybody agrees
  that such locks make more sense than a typical advisory locks, and
  F_OFD_SETLK made its way into POSIX.1 2024 [1].

* Issues with NFS. The current state of things here looks like this:

  - NFSv3 doesn't implement open file description locks, they're converted to
    advisory locks instead. Advisory locks are subject to namespace isolation,
    meaning that processes in different PID namespaces will not see each other
    advisory lock, and it's still possible to run multiple postgres
    instances on the same data directory.

  - NFSv4 uses a lease system for locking, I haven't found any mention of
    conversion to advisory locks neither in the man page nor in RFC [2].

To summarize, the approach is now considered POSIX and should fix the described
problem everywhere, except NFSv3.

Use open file description lock for both data directory and socker
lockfiles, since both are affected in the same way.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/fcntl.html
[2]: https://www.rfc-editor.org/rfc/rfc7530

Reviewed-by: Ilmar Yunusov <[email protected]>
---
 configure                         |  14 +++
 configure.ac                      |   3 +
 meson.build                       |   1 +
 src/backend/utils/init/miscinit.c | 136 ++++++++++++++++++++++++------
 src/include/pg_config.h.in        |   4 +
 src/tools/pgindent/typedefs.list  |   1 +
 6 files changed, 134 insertions(+), 25 deletions(-)

diff --git a/configure b/configure
index 5f77f3cac29..15bda6c6413 100755
--- a/configure
+++ b/configure
@@ -16444,6 +16444,20 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# Linux open file descriptor locks
+ac_fn_c_check_decl "$LINENO" "F_OFD_SETLK" "ac_cv_have_decl_F_OFD_SETLK" "#include <fcntl.h>
+"
+if test "x$ac_cv_have_decl_F_OFD_SETLK" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_F_OFD_SETLK $ac_have_decl
+_ACEOF
+
+
 ac_fn_c_check_func "$LINENO" "explicit_bzero" "ac_cv_func_explicit_bzero"
 if test "x$ac_cv_func_explicit_bzero" = xyes; then :
   $as_echo "#define HAVE_EXPLICIT_BZERO 1" >>confdefs.h
diff --git a/configure.ac b/configure.ac
index 61cee42daa7..e24cb7a6f01 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1913,6 +1913,9 @@ AC_CHECK_DECLS([memset_s], [], [], [#define __STDC_WANT_LIB_EXT1__ 1
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
+# Linux open file descriptor locks
+AC_CHECK_DECLS([F_OFD_SETLK], [], [], [#include <fcntl.h>])
+
 AC_REPLACE_FUNCS(m4_normalize([
 	explicit_bzero
 	getopt
diff --git a/meson.build b/meson.build
index 568e0e150bf..3d641dc0403 100644
--- a/meson.build
+++ b/meson.build
@@ -2901,6 +2901,7 @@ decl_checks = [
   ['strlcpy', 'string.h'],
   ['strsep',  'string.h'],
   ['timingsafe_bcmp',  'string.h'],
+  ['F_OFD_SETLK', 'fcntl.h'],
 ]
 
 # Need to check for function declarations for these functions, because
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 7ffc808073a..26c3324542c 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -69,6 +69,15 @@ static List *lock_files = NIL;
 
 static Latch LocalLatchData;
 
+typedef struct
+{
+	/* LockFile name. */
+	const char *filename;
+
+	/* File descriptor for open file description lock. */
+	int			fd;
+} LockFile;
+
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
  *
@@ -1119,6 +1128,48 @@ RestoreClientConnectionInfo(char *conninfo)
  *-------------------------------------------------------------------------
  */
 
+/*
+ * OFD lock the specified lockfile.
+ *
+ * Lock the lockfile with an open file description lock. If the lock is already
+ * taken, it's a hard stop. It's only a best effort test, and any other errors
+ * are ignored. On succes the file descriptor is duplicated, to make sure there
+ * will be at least one open copy of it to keep the lock.
+ *
+ * filename argument is used only for reporting purposes.
+ */
+static int
+OFDLockFile(int fd, const char *filename)
+{
+#if HAVE_DECL_F_OFD_SETLK
+	struct flock lock;
+
+	lock.l_type = F_WRLCK;
+	lock.l_whence = SEEK_SET;
+	lock.l_start = 0;
+	lock.l_len = 0;
+	lock.l_pid = 0;
+
+	if (fcntl(fd, F_OFD_SETLK, &lock) == -1)
+	{
+		if (errno == EAGAIN)
+			ereport(FATAL,
+					(errcode(ERRCODE_LOCK_FILE_EXISTS),
+					 errmsg("cannot lock the lock file \"%s\"", filename),
+					 errhint("Another server is starting.")));
+		else
+		{
+			elog(WARNING, "Failed locking file \"%s\", %m", filename);
+			return -1;
+		}
+	}
+	else
+		return dup(fd);
+#else
+	return -1
+#endif
+}
+
 /*
  * proc_exit callback to remove lockfiles.
  */
@@ -1129,9 +1180,16 @@ UnlinkLockFiles(int status, Datum arg)
 
 	foreach(l, lock_files)
 	{
-		char	   *curfile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
-		unlink(curfile);
+		/*
+		 * Close the file descriptor, which keeps the open file description
+		 * lock.
+		 */
+		if (lock_file->fd > 0)
+			close(lock_file->fd);
+
+		unlink(lock_file->filename);
 		/* Should we complain if the unlink fails? */
 	}
 	/* Since we're about to exit, no need to reclaim storage */
@@ -1161,7 +1219,9 @@ CreateLockFile(const char *filename, bool amPostmaster,
 			   const char *socketDir,
 			   bool isDDLock, const char *refName)
 {
-	int			fd;
+	int			fd,
+				flock_fd = -1;
+	LockFile   *lock_file;
 	char		buffer[MAXPGPATH * 2 + 256];
 	int			ntries;
 	int			len;
@@ -1173,22 +1233,32 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	const char *envvar;
 
 	/*
-	 * If the PID in the lockfile is our own PID or our parent's or
-	 * grandparent's PID, then the file must be stale (probably left over from
-	 * a previous system boot cycle).  We need to check this because of the
-	 * likelihood that a reboot will assign exactly the same PID as we had in
-	 * the previous reboot, or one that's only one or two counts larger and
-	 * hence the lockfile's PID now refers to an ancestor shell process.  We
-	 * allow pg_ctl to pass down its parent shell PID (our grandparent PID)
-	 * via the environment variable PG_GRANDPARENT_PID; this is so that
-	 * launching the postmaster via pg_ctl can be just as reliable as
-	 * launching it directly.  There is no provision for detecting
-	 * further-removed ancestor processes, but if the init script is written
-	 * carefully then all but the immediate parent shell will be root-owned
-	 * processes and so the kill test will fail with EPERM.  Note that we
-	 * cannot get a false negative this way, because an existing postmaster
-	 * would surely never launch a competing postmaster or pg_ctl process
-	 * directly.
+	 * If we find an already existing lockfile containing our own PID, there
+	 * are few options:
+	 *
+	 * - There is another process, that we don't see due to PID namespace
+	 * isolation, which is already running in this data directory.
+	 *
+	 * To prevent two concurrent processes working with the same data
+	 * directory, we first try to lock the lockfile exclusively.
+	 *
+	 * - The file must be stale, probably left over from a previous system
+	 * boot cycle. The same if the lockfile contains our parent's or
+	 * grandparent's PID.
+	 *
+	 * We need to check this because of the likelihood that a reboot will
+	 * assign exactly the same PID as we had in the previous reboot, or one
+	 * that's only one or two counts larger and hence the lockfile's PID now
+	 * refers to an ancestor shell process.  We allow pg_ctl to pass down its
+	 * parent shell PID (our grandparent PID) via the environment variable
+	 * PG_GRANDPARENT_PID; this is so that launching the postmaster via pg_ctl
+	 * can be just as reliable as launching it directly.  There is no
+	 * provision for detecting further-removed ancestor processes, but if the
+	 * init script is written carefully then all but the immediate parent
+	 * shell will be root-owned processes and so the kill test will fail with
+	 * EPERM.  Note that we cannot get a false negative this way, because an
+	 * existing postmaster would surely never launch a competing postmaster or
+	 * pg_ctl process directly.
 	 */
 	my_pid = getpid();
 
@@ -1224,7 +1294,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		 */
 		fd = open(filename, O_RDWR | O_CREAT | O_EXCL, pg_file_create_mode);
 		if (fd >= 0)
-			break;				/* Success; exit the retry loop */
+		{
+			/* Success; lock and exit the retry loop */
+			flock_fd = OFDLockFile(fd, filename);
+			break;
+		}
 
 		/*
 		 * Couldn't create the pid file. Probably it already exists.
@@ -1238,8 +1312,12 @@ CreateLockFile(const char *filename, bool amPostmaster,
 		/*
 		 * Read the file to get the old owner's PID.  Note race condition
 		 * here: file might have been deleted since we tried to create it.
+		 *
+		 * We're going to use the same fd for flock, and want to create a
+		 * write lock for the latter one. Since both fd and the lock have to
+		 * be of the same type, open the file for read and write.
 		 */
-		fd = open(filename, O_RDONLY, pg_file_create_mode);
+		fd = open(filename, O_RDWR, pg_file_create_mode);
 		if (fd < 0)
 		{
 			if (errno == ENOENT)
@@ -1249,6 +1327,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 					 errmsg("could not open lock file \"%s\": %m",
 							filename)));
 		}
+
+		/* Try to lock the file. We stop here, if it's already locked. */
+		flock_fd = OFDLockFile(fd, filename);
+
 		pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_CREATE_READ);
 		if ((len = read(fd, buffer, sizeof(buffer) - 1)) < 0)
 			ereport(FATAL,
@@ -1448,7 +1530,11 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * Use lcons so that the lock files are unlinked in reverse order of
 	 * creation; this is critical!
 	 */
-	lock_files = lcons(pstrdup(filename), lock_files);
+	lock_file = palloc0_object(LockFile);
+	lock_file->filename = pstrdup(filename);
+	lock_file->fd = flock_fd;
+
+	lock_files = lcons(lock_file, lock_files);
 }
 
 /*
@@ -1495,14 +1581,14 @@ TouchSocketLockFiles(void)
 
 	foreach(l, lock_files)
 	{
-		char	   *socketLockFile = (char *) lfirst(l);
+		LockFile   *lock_file = (LockFile *) lfirst(l);
 
 		/* No need to touch the data directory lock file, we trust */
-		if (strcmp(socketLockFile, DIRECTORY_LOCK_FILE) == 0)
+		if (strcmp(lock_file->filename, DIRECTORY_LOCK_FILE) == 0)
 			continue;
 
 		/* we just ignore any error here */
-		(void) utime(socketLockFile, NULL);
+		(void) utime(lock_file->filename, NULL);
 	}
 }
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 4f8113c144b..cc38c06dc13 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -85,6 +85,10 @@
    don't. */
 #undef HAVE_DECL_F_FULLFSYNC
 
+/* Define to 1 if you have the declaration of `F_OFD_SETLK', and to 0 if you
+   don't. */
+#undef HAVE_DECL_F_OFD_SETLK
+
 /* Define to 1 if you have the declaration of `memset_s', and to 0 if you
    don't. */
 #undef HAVE_DECL_MEMSET_S
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 1969d467c1d..185d69b5520 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1653,6 +1653,7 @@ LocationLen
 LockAcquireResult
 LockClauseStrength
 LockData
+LockFile
 LockInfoData
 LockInstanceData
 LockMethod

base-commit: 031904048aa22e7c70dc8e9c170e2743f9b0f090
-- 
2.52.0


--zdel3ow7bygx53fm--





^ permalink  raw  reply  [nested|flat] 17+ messages in thread

* [PATCH v1 3/3] Rename "fast" and "slow" popcount functions.
@ 2026-01-14 19:14 Nathan Bossart <[email protected]>
  0 siblings, 0 replies; 17+ messages in thread

From: Nathan Bossart @ 2026-01-14 19:14 UTC (permalink / raw)

Since we now how several implementations of the popcount functions,
let's give them more descriptive names.  This commit replaces
"slow" with "generic" and "fast" with "sse42".
---
 src/include/port/pg_bitutils.h |  8 +++---
 src/port/pg_bitutils.c         | 38 ++++++++++++-------------
 src/port/pg_popcount_x86_64.c  | 52 +++++++++++++++++-----------------
 3 files changed, 49 insertions(+), 49 deletions(-)

diff --git a/src/include/port/pg_bitutils.h b/src/include/port/pg_bitutils.h
index c9c508d4ba3..f2f94894d3a 100644
--- a/src/include/port/pg_bitutils.h
+++ b/src/include/port/pg_bitutils.h
@@ -307,10 +307,10 @@ pg_ceil_log2_64(uint64 num)
 #define POPCNT_AARCH64 1
 #endif
 
-extern int	pg_popcount32_slow(uint32 word);
-extern int	pg_popcount64_slow(uint64 word);
-extern uint64 pg_popcount_slow(const char *buf, int bytes);
-extern uint64 pg_popcount_masked_slow(const char *buf, int bytes, bits8 mask);
+extern int	pg_popcount32_generic(uint32 word);
+extern int	pg_popcount64_generic(uint64 word);
+extern uint64 pg_popcount_generic(const char *buf, int bytes);
+extern uint64 pg_popcount_masked_generic(const char *buf, int bytes, bits8 mask);
 
 #ifdef TRY_POPCNT_X86_64
 /*
diff --git a/src/port/pg_bitutils.c b/src/port/pg_bitutils.c
index 9f9f90ddd4d..2d8a7e0cf6a 100644
--- a/src/port/pg_bitutils.c
+++ b/src/port/pg_bitutils.c
@@ -97,11 +97,11 @@ const uint8 pg_number_of_ones[256] = {
 };
 
 /*
- * pg_popcount32_slow
+ * pg_popcount32_generic
  *		Return the number of 1 bits set in word
  */
 int
-pg_popcount32_slow(uint32 word)
+pg_popcount32_generic(uint32 word)
 {
 #ifdef HAVE__BUILTIN_POPCOUNT
 	return __builtin_popcount(word);
@@ -119,11 +119,11 @@ pg_popcount32_slow(uint32 word)
 }
 
 /*
- * pg_popcount64_slow
+ * pg_popcount64_generic
  *		Return the number of 1 bits set in word
  */
 int
-pg_popcount64_slow(uint64 word)
+pg_popcount64_generic(uint64 word)
 {
 #ifdef HAVE__BUILTIN_POPCOUNT
 #if SIZEOF_LONG == 8
@@ -147,11 +147,11 @@ pg_popcount64_slow(uint64 word)
 }
 
 /*
- * pg_popcount_slow
+ * pg_popcount_generic
  *		Returns the number of 1-bits in buf
  */
 uint64
-pg_popcount_slow(const char *buf, int bytes)
+pg_popcount_generic(const char *buf, int bytes)
 {
 	uint64		popcnt = 0;
 
@@ -163,7 +163,7 @@ pg_popcount_slow(const char *buf, int bytes)
 
 		while (bytes >= 8)
 		{
-			popcnt += pg_popcount64_slow(*words++);
+			popcnt += pg_popcount64_generic(*words++);
 			bytes -= 8;
 		}
 
@@ -177,7 +177,7 @@ pg_popcount_slow(const char *buf, int bytes)
 
 		while (bytes >= 4)
 		{
-			popcnt += pg_popcount32_slow(*words++);
+			popcnt += pg_popcount32_generic(*words++);
 			bytes -= 4;
 		}
 
@@ -193,11 +193,11 @@ pg_popcount_slow(const char *buf, int bytes)
 }
 
 /*
- * pg_popcount_masked_slow
+ * pg_popcount_masked_generic
  *		Returns the number of 1-bits in buf after applying the mask to each byte
  */
 uint64
-pg_popcount_masked_slow(const char *buf, int bytes, bits8 mask)
+pg_popcount_masked_generic(const char *buf, int bytes, bits8 mask)
 {
 	uint64		popcnt = 0;
 
@@ -211,7 +211,7 @@ pg_popcount_masked_slow(const char *buf, int bytes, bits8 mask)
 
 		while (bytes >= 8)
 		{
-			popcnt += pg_popcount64_slow(*words++ & maskv);
+			popcnt += pg_popcount64_generic(*words++ & maskv);
 			bytes -= 8;
 		}
 
@@ -227,7 +227,7 @@ pg_popcount_masked_slow(const char *buf, int bytes, bits8 mask)
 
 		while (bytes >= 4)
 		{
-			popcnt += pg_popcount32_slow(*words++ & maskv);
+			popcnt += pg_popcount32_generic(*words++ & maskv);
 			bytes -= 4;
 		}
 
@@ -246,20 +246,20 @@ pg_popcount_masked_slow(const char *buf, int bytes, bits8 mask)
 
 /*
  * When special CPU instructions are not available, there's no point in using
- * function pointers to vary the implementation between the fast and slow
- * method.  We instead just make these actual external functions.  The compiler
- * should be able to inline the slow versions here.
+ * function pointers to vary the implementation.  We instead just make these
+ * actual external functions.  The compiler should be able to inline the
+ * generic versions here.
  */
 int
 pg_popcount32(uint32 word)
 {
-	return pg_popcount32_slow(word);
+	return pg_popcount32_generic(word);
 }
 
 int
 pg_popcount64(uint64 word)
 {
-	return pg_popcount64_slow(word);
+	return pg_popcount64_generic(word);
 }
 
 /*
@@ -269,7 +269,7 @@ pg_popcount64(uint64 word)
 uint64
 pg_popcount_optimized(const char *buf, int bytes)
 {
-	return pg_popcount_slow(buf, bytes);
+	return pg_popcount_generic(buf, bytes);
 }
 
 /*
@@ -279,7 +279,7 @@ pg_popcount_optimized(const char *buf, int bytes)
 uint64
 pg_popcount_masked_optimized(const char *buf, int bytes, bits8 mask)
 {
-	return pg_popcount_masked_slow(buf, bytes, mask);
+	return pg_popcount_masked_generic(buf, bytes, mask);
 }
 
 #endif							/* ! TRY_POPCNT_X86_64 && ! POPCNT_AARCH64 */
diff --git a/src/port/pg_popcount_x86_64.c b/src/port/pg_popcount_x86_64.c
index f8643642613..13c922f7ab1 100644
--- a/src/port/pg_popcount_x86_64.c
+++ b/src/port/pg_popcount_x86_64.c
@@ -32,10 +32,10 @@
  * The SSE4.2 versions are built regardless of whether we are building the
  * AVX-512 versions.
  */
-static inline int pg_popcount32_fast(uint32 word);
-static inline int pg_popcount64_fast(uint64 word);
-static uint64 pg_popcount_fast(const char *buf, int bytes);
-static uint64 pg_popcount_masked_fast(const char *buf, int bytes, bits8 mask);
+static inline int pg_popcount32_sse42(uint32 word);
+static inline int pg_popcount64_sse42(uint64 word);
+static uint64 pg_popcount_sse42(const char *buf, int bytes);
+static uint64 pg_popcount_masked_sse42(const char *buf, int bytes, bits8 mask);
 
 /*
  * These are the AVX-512 implementations of the popcount functions.
@@ -64,7 +64,7 @@ uint64		(*pg_popcount_masked_optimized) (const char *buf, int bytes, bits8 mask)
  * Return true if CPUID indicates that the POPCNT instruction is available.
  */
 static bool
-pg_popcount_available(void)
+pg_popcount_sse42_available(void)
 {
 	unsigned int exx[4] = {0, 0, 0, 0};
 
@@ -161,19 +161,19 @@ pg_popcount_avx512_available(void)
 static inline void
 choose_popcount_functions(void)
 {
-	if (pg_popcount_available())
+	if (pg_popcount_sse42_available())
 	{
-		pg_popcount32 = pg_popcount32_fast;
-		pg_popcount64 = pg_popcount64_fast;
-		pg_popcount_optimized = pg_popcount_fast;
-		pg_popcount_masked_optimized = pg_popcount_masked_fast;
+		pg_popcount32 = pg_popcount32_sse42;
+		pg_popcount64 = pg_popcount64_sse42;
+		pg_popcount_optimized = pg_popcount_sse42;
+		pg_popcount_masked_optimized = pg_popcount_masked_sse42;
 	}
 	else
 	{
-		pg_popcount32 = pg_popcount32_slow;
-		pg_popcount64 = pg_popcount64_slow;
-		pg_popcount_optimized = pg_popcount_slow;
-		pg_popcount_masked_optimized = pg_popcount_masked_slow;
+		pg_popcount32 = pg_popcount32_generic;
+		pg_popcount64 = pg_popcount64_generic;
+		pg_popcount_optimized = pg_popcount_generic;
+		pg_popcount_masked_optimized = pg_popcount_masked_generic;
 	}
 
 #ifdef USE_AVX512_POPCNT_WITH_RUNTIME_CHECK
@@ -335,11 +335,11 @@ pg_popcount_masked_avx512(const char *buf, int bytes, bits8 mask)
 #endif							/* USE_AVX512_POPCNT_WITH_RUNTIME_CHECK */
 
 /*
- * pg_popcount32_fast
+ * pg_popcount32_sse42
  *		Return the number of 1 bits set in word
  */
 static inline int
-pg_popcount32_fast(uint32 word)
+pg_popcount32_sse42(uint32 word)
 {
 #ifdef _MSC_VER
 	return __popcnt(word);
@@ -352,11 +352,11 @@ __asm__ __volatile__(" popcntl %1,%0\n":"=q"(res):"rm"(word):"cc");
 }
 
 /*
- * pg_popcount64_fast
+ * pg_popcount64_sse42
  *		Return the number of 1 bits set in word
  */
 static inline int
-pg_popcount64_fast(uint64 word)
+pg_popcount64_sse42(uint64 word)
 {
 #ifdef _MSC_VER
 	return __popcnt64(word);
@@ -369,11 +369,11 @@ __asm__ __volatile__(" popcntq %1,%0\n":"=q"(res):"rm"(word):"cc");
 }
 
 /*
- * pg_popcount_fast
+ * pg_popcount_sse42
  *		Returns the number of 1-bits in buf
  */
 static uint64
-pg_popcount_fast(const char *buf, int bytes)
+pg_popcount_sse42(const char *buf, int bytes)
 {
 	uint64		popcnt = 0;
 
@@ -385,7 +385,7 @@ pg_popcount_fast(const char *buf, int bytes)
 
 		while (bytes >= 8)
 		{
-			popcnt += pg_popcount64_fast(*words++);
+			popcnt += pg_popcount64_sse42(*words++);
 			bytes -= 8;
 		}
 
@@ -399,7 +399,7 @@ pg_popcount_fast(const char *buf, int bytes)
 
 		while (bytes >= 4)
 		{
-			popcnt += pg_popcount32_fast(*words++);
+			popcnt += pg_popcount32_sse42(*words++);
 			bytes -= 4;
 		}
 
@@ -415,11 +415,11 @@ pg_popcount_fast(const char *buf, int bytes)
 }
 
 /*
- * pg_popcount_masked_fast
+ * pg_popcount_masked_sse42
  *		Returns the number of 1-bits in buf after applying the mask to each byte
  */
 static uint64
-pg_popcount_masked_fast(const char *buf, int bytes, bits8 mask)
+pg_popcount_masked_sse42(const char *buf, int bytes, bits8 mask)
 {
 	uint64		popcnt = 0;
 
@@ -433,7 +433,7 @@ pg_popcount_masked_fast(const char *buf, int bytes, bits8 mask)
 
 		while (bytes >= 8)
 		{
-			popcnt += pg_popcount64_fast(*words++ & maskv);
+			popcnt += pg_popcount64_sse42(*words++ & maskv);
 			bytes -= 8;
 		}
 
@@ -449,7 +449,7 @@ pg_popcount_masked_fast(const char *buf, int bytes, bits8 mask)
 
 		while (bytes >= 4)
 		{
-			popcnt += pg_popcount32_fast(*words++ & maskv);
+			popcnt += pg_popcount32_sse42(*words++ & maskv);
 			bytes -= 4;
 		}
 
-- 
2.50.1 (Apple Git-155)


--RzZ1MyDExwO9gtVK--






^ permalink  raw  reply  [nested|flat] 17+ messages in thread

end of thread, other threads:[~2026-01-14 19:14 UTC | newest]

Thread overview: 17+ messages (download: mbox mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
2025-12-18 17:21 [PATCH v4] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v4] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v4] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v3] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v4] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v3] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v3] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v3] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v3] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v3] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v4] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v4] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v4] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v3] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v4] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2025-12-18 17:21 [PATCH v3] Use open file description locks for lockfiles Dmitrii Dolgov <[email protected]>
2026-01-14 19:14 [PATCH v1 3/3] Rename "fast" and "slow" popcount functions. Nathan Bossart <[email protected]>

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