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Handle interleavings between CREATE DATABASE steps and base backup.

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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 9c08aea6a3090a396be334cc58c511edab05776a
introduced STRATEGY=WAL_LOG and made it the default.

Discussion: https://postgr.es/m/20240130195003.0a.nmisch@google.com
This commit is contained in:
Noah Misch 2024-02-01 13:44:19 -08:00
parent 970b1aeeba
commit d493bed28f
2 changed files with 32 additions and 28 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

44
src/backend/commands/dbcommands.c
View File

@ -461,35 +461,12 @@ 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)
{
@ -533,10 +510,25 @@ 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();
}
}
/*
* Create a new database using the FILE_COPY strategy.

16
src/backend/utils/cache/relmapper.c vendored
View File

@ -298,14 +298,15 @@ 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);
}
/*
@ -627,10 +628,12 @@ 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);
}
/*
@ -876,6 +879,15 @@ write_relmap_file(RelMapFile *newmap, bool write_wal, bool send_sinval,
int fd;
char mapfilename[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.
*/