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* Re: improve performance of pg_dump --binary-upgrade
@ 2024-04-18 07:24 Daniel Gustafsson <[email protected]>
0 siblings, 0 replies; 17+ messages in thread
From: Daniel Gustafsson @ 2024-04-18 07:24 UTC (permalink / raw)
To: Nathan Bossart <[email protected]>; +Cc: pgsql-hackers
> On 18 Apr 2024, at 06:17, Nathan Bossart <[email protected]> wrote:
> The attached work-in-progress patch speeds up 'pg_dump --binary-upgrade'
> for this case. Instead of executing the query in every call to the
> function, we can execute it once during the first call and store all the
> required information in a sorted array that we can bsearch() in future
> calls.
That does indeed seem like a saner approach. Since we look up the relkind we
can also remove the is_index parameter to binary_upgrade_set_pg_class_oids
since we already know that without the caller telling us?
> One downside of this approach is the memory usage.
I'm not too worried about the worst-case performance of this.
> This was more-or-less
> the first approach that crossed my mind, so I wouldn't be surprised if
> there's a better way. I tried to keep the pg_dump output the same, but if
> that isn't important, maybe we could dump all the pg_class OIDs at once
> instead of calling binary_upgrade_set_pg_class_oids() for each one.
Without changing the backend handling of the Oid's we can't really do that
AFAICT, the backend stores the Oid for the next call so it needs to be per
relation like now?
For Greenplum we moved this to the backend by first dumping all Oids which were
read into backend cache, and during relation creation the Oid to use was looked
up in the backend. (This wasn't a performance change, it was to allow multiple
shared-nothing clusters to have a unified view of Oids, so I never benchmarked
it all that well.) The upside of that is that the magic Oid variables in the
backend can be removed, but it obviously adds slight overhead in others.
--
Daniel Gustafsson
^ 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 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 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 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 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
end of thread, other threads:[~2025-12-18 17:21 UTC | newest]
Thread overview: 17+ messages (download: mbox mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
2024-04-18 07:24 Re: improve performance of pg_dump --binary-upgrade Daniel Gustafsson <[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 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 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 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]>
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