Subject: [PATCH v14 3/3] Fix replay of create database records on standby

From: Alvaro Herrera <[email-masked]>
Date: Thu, 9 Jan 2020 17:54:40 -0300

Crash recovery on standby may encounter missing directories when
replaying create database WAL records.  Prior to this patch, the
standby would fail to recover in such a case.  However, the
directories could be legitimately missing.  Consider a sequence of WAL
records as follows:

    CREATE DATABASE
    DROP DATABASE
    DROP TABLESPACE

If, after replaying the last WAL record and removing the tablespace
directory, the standby crashes and has to replay the create database
record again, the crash recovery must be able to move on.

This patch adds mechanism similar to invalid page hash table, to track
missing directories during crash recovery.  If all the missing
directory references are matched with corresponding drop records at
the end of crash recovery, the standby can safely enter archive
recovery.

Bug identified by Paul Guo.

Authored by Paul Guo, Kyotaro Horiguchi and Asim R P.
---
 src/backend/access/transam/xlog.c      |   6 +
 src/backend/access/transam/xlogutils.c | 145 +++++++++++++++++++++++++
 src/backend/commands/dbcommands.c      |  55 ++++++++++
 src/backend/commands/tablespace.c      |   5 +
 src/include/access/xlogutils.h         |   4 +
 5 files changed, 215 insertions(+)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index e073121a7e..badda1deb2 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -8309,6 +8309,12 @@ CheckRecoveryConsistency(void)
 		 */
 		XLogCheckInvalidPages();
 
+		/*
+		 * Check if the XLOG sequence contained any unresolved references to
+		 * missing directories.
+		 */
+		XLogCheckMissingDirs();
+
 		reachedConsistency = true;
 		ereport(LOG,
 				(errmsg("consistent recovery state reached at %X/%X",
diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index b33e0531ed..99abf8b2f4 100644
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -79,6 +79,151 @@ typedef struct xl_invalid_page
 
 static HTAB *invalid_page_tab = NULL;
 
+/*
+ * If a create database WAL record is being replayed more than once during
+ * crash recovery on a standby, it is possible that either the tablespace
+ * directory or the template database directory is missing.  This happens when
+ * the directories are removed by replay of subsequent drop records.  Note
+ * that this problem happens only on standby and not on master.  On master, a
+ * checkpoint is created at the end of create database operation. On standby,
+ * however, such a strategy (creating restart points during replay) is not
+ * viable because it will slow down WAL replay.
+ *
+ * The alternative is to track references to each missing directory
+ * encountered when performing crash recovery in the following hash table.
+ * Similar to invalid page table above, the expectation is that each missing
+ * directory entry should be matched with a drop database or drop tablespace
+ * WAL record by the end of crash recovery.
+ */
+typedef struct xl_missing_dir_key
+{
+	Oid spcNode;
+	Oid dbNode;
+} xl_missing_dir_key;
+
+typedef struct xl_missing_dir
+{
+	xl_missing_dir_key key;
+	char path[MAXPGPATH];
+} xl_missing_dir;
+
+static HTAB *missing_dir_tab = NULL;
+
+void
+XLogReportMissingDir(Oid spcNode, Oid dbNode, char *path)
+{
+	xl_missing_dir_key key;
+	bool found;
+	xl_missing_dir *entry;
+
+	/*
+	 * Database OID may be invalid but tablespace OID must be valid.  If
+	 * dbNode is InvalidOid, we are logging a missing tablespace directory,
+	 * otherwise we are logging a missing database directory.
+	 */
+	Assert(OidIsValid(spcNode));
+
+	if (missing_dir_tab == NULL)
+	{
+		/* create hash table when first needed */
+		HASHCTL		ctl;
+
+		memset(&ctl, 0, sizeof(ctl));
+		ctl.keysize = sizeof(xl_missing_dir_key);
+		ctl.entrysize = sizeof(xl_missing_dir);
+
+		missing_dir_tab = hash_create("XLOG missing directory table",
+									   100,
+									   &ctl,
+									   HASH_ELEM | HASH_BLOBS);
+	}
+
+	key.spcNode = spcNode;
+	key.dbNode = dbNode;
+
+	entry = hash_search(missing_dir_tab, &key, HASH_ENTER, &found);
+
+	if (found)
+	{
+		if (dbNode == InvalidOid)
+			elog(DEBUG2, "missing directory %s (tablespace %d) already exists: %s",
+				 path, spcNode, entry->path);
+		else
+			elog(DEBUG2, "missing directory %s (tablespace %d database %d) already exists: %s",
+				 path, spcNode, dbNode, entry->path);
+	}
+	else
+	{
+		strlcpy(entry->path, path, sizeof(entry->path));
+		if (dbNode == InvalidOid)
+			elog(DEBUG2, "logged missing dir %s (tablespace %d)",
+				 path, spcNode);
+		else
+			elog(DEBUG2, "logged missing dir %s (tablespace %d database %d)",
+				 path, spcNode, dbNode);
+	}
+}
+
+void
+XLogForgetMissingDir(Oid spcNode, Oid dbNode)
+{
+	xl_missing_dir_key key;
+
+	key.spcNode = spcNode;
+	key.dbNode = dbNode;
+
+	/* Database OID may be invalid but tablespace OID must be valid. */
+	Assert(OidIsValid(spcNode));
+
+	if (missing_dir_tab == NULL)
+		return;
+
+	if (hash_search(missing_dir_tab, &key, HASH_REMOVE, NULL) != NULL)
+	{
+		if (dbNode == InvalidOid)
+		{
+			elog(DEBUG2, "forgot missing dir (tablespace %d)", spcNode);
+		}
+		else
+		{
+			char *path = GetDatabasePath(dbNode, spcNode);
+
+			elog(DEBUG2, "forgot missing dir %s (tablespace %d database %d)",
+				 path, spcNode, dbNode);
+			pfree(path);
+		}
+	}
+}
+
+/*
+ * This is called at the end of crash recovery, before entering archive
+ * recovery on a standby.  PANIC if the hash table is not empty.
+ */
+void
+XLogCheckMissingDirs(void)
+{
+	HASH_SEQ_STATUS status;
+	xl_missing_dir *hentry;
+	bool		foundone = false;
+
+	if (missing_dir_tab == NULL)
+		return;					/* nothing to do */
+
+	hash_seq_init(&status, missing_dir_tab);
+
+	while ((hentry = (xl_missing_dir *) hash_seq_search(&status)) != NULL)
+	{
+		elog(WARNING, "missing directory \"%s\" tablespace %d database %d",
+			 hentry->path, hentry->key.spcNode, hentry->key.dbNode);
+		foundone = true;
+	}
+
+	if (foundone)
+		elog(PANIC, "WAL contains references to missing directories");
+
+	hash_destroy(missing_dir_tab);
+	missing_dir_tab = NULL;
+}
 
 /* Report a reference to an invalid page */
 static void
diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index 029fab48df..0f483edb71 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -2143,7 +2143,9 @@ dbase_redo(XLogReaderState *record)
 		xl_dbase_create_rec *xlrec = (xl_dbase_create_rec *) XLogRecGetData(record);
 		char	   *src_path;
 		char	   *dst_path;
+		char	   *parent_path;
 		struct stat st;
+		bool	    skip = false;
 
 		src_path = GetDatabasePath(xlrec->src_db_id, xlrec->src_tablespace_id);
 		dst_path = GetDatabasePath(xlrec->db_id, xlrec->tablespace_id);
@@ -2161,6 +2163,55 @@ dbase_redo(XLogReaderState *record)
 						(errmsg("some useless files may be left behind in old database directory \"%s\"",
 								dst_path)));
 		}
+		else if (!reachedConsistency)
+		{
+			/*
+			 * It is possible that drop tablespace record appearing later in
+			 * the WAL as already been replayed.  That means we are replaying
+			 * the create database record second time, as part of crash
+			 * recovery.  In that case, the tablespace directory has already
+			 * been removed and the create database operation cannot be
+			 * replayed.  We should skip the replay but remember the missing
+			 * tablespace directory, to be matched with a drop tablespace
+			 * record later.
+			 */
+			parent_path = pstrdup(dst_path);
+			get_parent_directory(parent_path);
+			if (!(stat(parent_path, &st) == 0 && S_ISDIR(st.st_mode)))
+			{
+				XLogReportMissingDir(xlrec->tablespace_id, InvalidOid, parent_path);
+				skip = true;
+				ereport(WARNING,
+						(errmsg("skipping create database WAL record"),
+						 errdetail("Target tablespace \"%s\" not found. We "
+								   "expect to encounter a WAL record that "
+								   "removes this directory before reaching "
+								   "consistent state.", parent_path)));
+			}
+			pfree(parent_path);
+		}
+
+		/*
+		 * Source directory may be missing.  E.g. the template database used
+		 * for creating this database may have been dropped, due to reasons
+		 * noted above.  Moving a database from one tablespace may also be a
+		 * partner in the crime.
+		 */
+		if (!(stat(src_path, &st) == 0 && S_ISDIR(st.st_mode)) &&
+			!reachedConsistency)
+		{
+			XLogReportMissingDir(xlrec->src_tablespace_id, xlrec->src_db_id, src_path);
+			skip = true;
+			ereport(WARNING,
+					(errmsg("skipping create database WAL record"),
+					 errdetail("Source database \"%s\" not found. We expect "
+							   "to encounter a WAL record that removes this "
+							   "directory before reaching consistent state.",
+							   src_path)));
+		}
+
+		if (skip)
+			return;
 
 		/*
 		 * Force dirty buffers out to disk, to ensure source database is
@@ -2218,6 +2269,10 @@ dbase_redo(XLogReaderState *record)
 				ereport(WARNING,
 						(errmsg("some useless files may be left behind in old database directory \"%s\"",
 								dst_path)));
+
+			if (!reachedConsistency)
+				XLogForgetMissingDir(xlrec->tablespace_ids[i], xlrec->db_id);
+
 			pfree(dst_path);
 		}
 
diff --git a/src/backend/commands/tablespace.c b/src/backend/commands/tablespace.c
index 4b96eec9df..0d5dfe007f 100644
--- a/src/backend/commands/tablespace.c
+++ b/src/backend/commands/tablespace.c
@@ -1527,6 +1527,11 @@ tblspc_redo(XLogReaderState *record)
 	{
 		xl_tblspc_drop_rec *xlrec = (xl_tblspc_drop_rec *) XLogRecGetData(record);
 
+		if (!reachedConsistency)
+			XLogForgetMissingDir(xlrec->ts_id, InvalidOid);
+
+		XLogFlush(record->EndRecPtr);
+
 		/*
 		 * If we issued a WAL record for a drop tablespace it implies that
 		 * there were no files in it at all when the DROP was done. That means
diff --git a/src/include/access/xlogutils.h b/src/include/access/xlogutils.h
index eebc91f3a5..3341efc052 100644
--- a/src/include/access/xlogutils.h
+++ b/src/include/access/xlogutils.h
@@ -65,6 +65,10 @@ extern void XLogDropDatabase(Oid dbid);
 extern void XLogTruncateRelation(RelFileNode rnode, ForkNumber forkNum,
 								 BlockNumber nblocks);
 
+extern void XLogReportMissingDir(Oid spcNode, Oid dbNode, char *path);
+extern void XLogForgetMissingDir(Oid spcNode, Oid dbNode);
+extern void XLogCheckMissingDirs(void);
+
 /* Result codes for XLogReadBufferForRedo[Extended] */
 typedef enum
 {
-- 
2.27.0

----Next_Part(Fri_Nov_12_16_43_27_2021_789)----