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Commitd493bed

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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 symptomwas absence of the database directory, PG_VERSION file, orpg_filenode.map. If missing the directory, recovery would fail. Eithermissing file would not fail recovery but would render the new databaseunusable. Fix CreateDirAndVersionFile() with the transam/README "actionfirst and then write a WAL entry" strategy. That has a side benefit ofmoving filesystem mutations out of a critical section, reducing the waysto PANIC. Fix the write_relmap_file() call with a lock acquisition, soit interacts with checkpoints like non-CREATE DATABASE calls do.Back-patch to v15, where commit9c08aeaintroduced STRATEGY=WAL_LOG and made it the default.Discussion:https://postgr.es/m/20240130195003.0a.nmisch@google.com
1 parent970b1ae commitd493bed

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2 files changed

+32
-28
lines changed

2 files changed

+32
-28
lines changed

‎src/backend/commands/dbcommands.c‎

Lines changed: 18 additions & 26 deletions
Original file line numberDiff line numberDiff line change
@@ -461,35 +461,12 @@ CreateDirAndVersionFile(char *dbpath, Oid dbid, Oid tsid, bool isRedo)
461461
charbuf[16];
462462

463463
/*
464-
* Prepare version data before starting a critical section.
465-
*
466-
* Note that we don't have to copy this from the source database; there's
467-
* only one legal value.
464+
* Note that we don't have to copy version data from the source database;
465+
* there's only one legal value.
468466
*/
469467
sprintf(buf,"%s\n",PG_MAJORVERSION);
470468
nbytes=strlen(PG_MAJORVERSION)+1;
471469

472-
/* If we are not in WAL replay then write the WAL. */
473-
if (!isRedo)
474-
{
475-
xl_dbase_create_wal_log_recxlrec;
476-
XLogRecPtrlsn;
477-
478-
START_CRIT_SECTION();
479-
480-
xlrec.db_id=dbid;
481-
xlrec.tablespace_id=tsid;
482-
483-
XLogBeginInsert();
484-
XLogRegisterData((char*) (&xlrec),
485-
sizeof(xl_dbase_create_wal_log_rec));
486-
487-
lsn=XLogInsert(RM_DBASE_ID,XLOG_DBASE_CREATE_WAL_LOG);
488-
489-
/* As always, WAL must hit the disk before the data update does. */
490-
XLogFlush(lsn);
491-
}
492-
493470
/* Create database directory. */
494471
if (MakePGDirectory(dbpath)<0)
495472
{
@@ -533,9 +510,24 @@ CreateDirAndVersionFile(char *dbpath, Oid dbid, Oid tsid, bool isRedo)
533510
/* Close the version file. */
534511
CloseTransientFile(fd);
535512

536-
/*Critical section done. */
513+
/*If we are not in WAL replay then write the WAL. */
537514
if (!isRedo)
515+
{
516+
xl_dbase_create_wal_log_recxlrec;
517+
518+
START_CRIT_SECTION();
519+
520+
xlrec.db_id=dbid;
521+
xlrec.tablespace_id=tsid;
522+
523+
XLogBeginInsert();
524+
XLogRegisterData((char*) (&xlrec),
525+
sizeof(xl_dbase_create_wal_log_rec));
526+
527+
(void)XLogInsert(RM_DBASE_ID,XLOG_DBASE_CREATE_WAL_LOG);
528+
538529
END_CRIT_SECTION();
530+
}
539531
}
540532

541533
/*

‎src/backend/utils/cache/relmapper.c‎

Lines changed: 14 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -298,14 +298,15 @@ RelationMapCopy(Oid dbid, Oid tsid, char *srcdbpath, char *dstdbpath)
298298
* Write the same data into the destination database's relmap file.
299299
*
300300
* No sinval is needed because no one can be connected to the destination
301-
* database yet. For the same reason, there is no need to acquire
302-
* RelationMappingLock.
301+
* database yet.
303302
*
304303
* There's no point in trying to preserve files here. The new database
305304
* isn't usable yet anyway, and won't ever be if we can't install a relmap
306305
* file.
307306
*/
307+
LWLockAcquire(RelationMappingLock,LW_EXCLUSIVE);
308308
write_relmap_file(&map, true, false, false,dbid,tsid,dstdbpath);
309+
LWLockRelease(RelationMappingLock);
309310
}
310311

311312
/*
@@ -627,10 +628,12 @@ RelationMapFinishBootstrap(void)
627628
Assert(pending_local_updates.num_mappings==0);
628629

629630
/* Write the files; no WAL or sinval needed */
631+
LWLockAcquire(RelationMappingLock,LW_EXCLUSIVE);
630632
write_relmap_file(&shared_map, false, false, false,
631633
InvalidOid,GLOBALTABLESPACE_OID,"global");
632634
write_relmap_file(&local_map, false, false, false,
633635
MyDatabaseId,MyDatabaseTableSpace,DatabasePath);
636+
LWLockRelease(RelationMappingLock);
634637
}
635638

636639
/*
@@ -876,6 +879,15 @@ write_relmap_file(RelMapFile *newmap, bool write_wal, bool send_sinval,
876879
intfd;
877880
charmapfilename[MAXPGPATH];
878881

882+
/*
883+
* Even without concurrent use of this map, CheckPointRelationMap() relies
884+
* on this locking. Without it, a restore of a base backup taken after
885+
* this function's XLogInsert() and before its durable_rename() would not
886+
* have the changes. wal_level=minimal doesn't need the lock, but this
887+
* isn't performance-critical enough for such a micro-optimization.
888+
*/
889+
Assert(LWLockHeldByMeInMode(RelationMappingLock,LW_EXCLUSIVE));
890+
879891
/*
880892
* Fill in the overhead fields and update CRC.
881893
*/

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