Handle interleavings between CREATE DATABASE steps and base backup.

Restoring a base backup taken in the middle of CreateDirAndVersionFile()
or write_relmap_file() would lose the function's effects.  The symptom
was absence of the database directory, PG_VERSION file, or
pg_filenode.map.  If missing the directory, recovery would fail.  Either
missing file would not fail recovery but would render the new database
unusable.  Fix CreateDirAndVersionFile() with the transam/README "action
first and then write a WAL entry" strategy.  That has a side benefit of
moving filesystem mutations out of a critical section, reducing the ways
to PANIC.  Fix the write_relmap_file() call with a lock acquisition, so
it interacts with checkpoints like non-CREATE DATABASE calls do.
Back-patch to v15, where commit 9c08aea
introduced STRATEGY=WAL_LOG and made it the default.

Discussion: https://postgr.es/m/20240130195003.0a.nmisch@google.com

src/backend/commands/dbcommands.c

@@ -462,35 +462,12 @@ CreateDirAndVersionFile(char *dbpath, Oid dbid, Oid tsid, bool isRedo)
 	char		buf[16];
 
 	/*
-	 * Prepare version data before starting a critical section.
-	 *
-	 * Note that we don't have to copy this from the source database; there's
-	 * only one legal value.
+	 * Note that we don't have to copy version data from the source database;
+	 * there's only one legal value.
 	 */
 	sprintf(buf, "%s\n", PG_MAJORVERSION);
 	nbytes = strlen(PG_MAJORVERSION) + 1;
 
-	/* If we are not in WAL replay then write the WAL. */
-	if (!isRedo)
-	{
-		xl_dbase_create_wal_log_rec xlrec;
-		XLogRecPtr	lsn;
-
-		START_CRIT_SECTION();
-
-		xlrec.db_id = dbid;
-		xlrec.tablespace_id = tsid;
-
-		XLogBeginInsert();
-		XLogRegisterData((char *) (&xlrec),
-						 sizeof(xl_dbase_create_wal_log_rec));
-
-		lsn = XLogInsert(RM_DBASE_ID, XLOG_DBASE_CREATE_WAL_LOG);
-
-		/* As always, WAL must hit the disk before the data update does. */
-		XLogFlush(lsn);
-	}
-
 	/* Create database directory. */
 	if (MakePGDirectory(dbpath) < 0)
 	{
@@ -534,9 +511,24 @@ CreateDirAndVersionFile(char *dbpath, Oid dbid, Oid tsid, bool isRedo)
 	/* Close the version file. */
 	CloseTransientFile(fd);
 
-	/* Critical section done. */
+	/* If we are not in WAL replay then write the WAL. */
 	if (!isRedo)
+	{
+		xl_dbase_create_wal_log_rec xlrec;
+
+		START_CRIT_SECTION();
+
+		xlrec.db_id = dbid;
+		xlrec.tablespace_id = tsid;
+
+		XLogBeginInsert();
+		XLogRegisterData((char *) (&xlrec),
+						 sizeof(xl_dbase_create_wal_log_rec));
+
+		(void) XLogInsert(RM_DBASE_ID, XLOG_DBASE_CREATE_WAL_LOG);
+
 		END_CRIT_SECTION();
+	}
 }
 
 /*

src/backend/utils/cache/relmapper.c

@@ -303,14 +303,15 @@ RelationMapCopy(Oid dbid, Oid tsid, char *srcdbpath, char *dstdbpath)
 	 * Write the same data into the destination database's relmap file.
 	 *
 	 * No sinval is needed because no one can be connected to the destination
-	 * database yet. For the same reason, there is no need to acquire
-	 * RelationMappingLock.
+	 * database yet.
 	 *
 	 * There's no point in trying to preserve files here. The new database
 	 * isn't usable yet anyway, and won't ever be if we can't install a relmap
 	 * file.
 	 */
+	LWLockAcquire(RelationMappingLock, LW_EXCLUSIVE);
 	write_relmap_file(&map, true, false, false, dbid, tsid, dstdbpath);
+	LWLockRelease(RelationMappingLock);
 }
 
 /*
@@ -633,10 +634,12 @@ RelationMapFinishBootstrap(void)
 	Assert(pending_local_updates.num_mappings == 0);
 
 	/* Write the files; no WAL or sinval needed */
+	LWLockAcquire(RelationMappingLock, LW_EXCLUSIVE);
 	write_relmap_file(&shared_map, false, false, false,
 					  InvalidOid, GLOBALTABLESPACE_OID, "global");
 	write_relmap_file(&local_map, false, false, false,
 					  MyDatabaseId, MyDatabaseTableSpace, DatabasePath);
+	LWLockRelease(RelationMappingLock);
 }
 
 /*
@@ -891,6 +894,15 @@ write_relmap_file(RelMapFile *newmap, bool write_wal, bool send_sinval,
 	char		mapfilename[MAXPGPATH];
 	char		maptempfilename[MAXPGPATH];
 
+	/*
+	 * Even without concurrent use of this map, CheckPointRelationMap() relies
+	 * on this locking.  Without it, a restore of a base backup taken after
+	 * this function's XLogInsert() and before its durable_rename() would not
+	 * have the changes.  wal_level=minimal doesn't need the lock, but this
+	 * isn't performance-critical enough for such a micro-optimization.
+	 */
+	Assert(LWLockHeldByMeInMode(RelationMappingLock, LW_EXCLUSIVE));
+
 	/*
 	 * Fill in the overhead fields and update CRC.
 	 */