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Commit93519b0

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committed
Fix race condition in relcache init file invalidation.
The previous code tried to synchronize by unlinking the init file twice,but that doesn't actually work: it leaves a window wherein a third processcould read the already-stale init file but miss the SI messages that wouldtell it the data is stale. The result would be bizarre failures in catalogaccesses, typically "could not read block 0 in file ..." later duringstartup.Instead, hold RelCacheInitLock across both the unlink and the sending ofthe SI messages. This is more straightforward, and might even be a bitfaster since only one unlink call is needed.This has been wrong since it was put in (in 2002!), so back-patch to allsupported releases.
1 parentf239ec5 commit93519b0

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4 files changed

+57
-49
lines changed

4 files changed

+57
-49
lines changed

‎src/backend/access/transam/twophase.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1338,10 +1338,10 @@ FinishPreparedTransaction(const char *gid, bool isCommit)
13381338
* after we send the SI messages. See AtEOXact_Inval()
13391339
*/
13401340
if (hdr->initfileinval)
1341-
RelationCacheInitFileInvalidate(true);
1341+
RelationCacheInitFilePreInvalidate();
13421342
SendSharedInvalidMessages(invalmsgs,hdr->ninvalmsgs);
13431343
if (hdr->initfileinval)
1344-
RelationCacheInitFileInvalidate(false);
1344+
RelationCacheInitFilePostInvalidate();
13451345

13461346
/* And now do the callbacks */
13471347
if (isCommit)

‎src/backend/utils/cache/inval.c

Lines changed: 17 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -849,24 +849,12 @@ xactGetCommittedInvalidationMessages(SharedInvalidationMessage **msgs,
849849
returnnumSharedInvalidMessagesArray;
850850
}
851851

852-
#defineRecoveryRelationCacheInitFileInvalidate(dbo,tbo,tf) \
853-
{ \
854-
DatabasePath = GetDatabasePath(dbo, tbo); \
855-
elog(trace_recovery(DEBUG4), "removing relcache init file in %s", DatabasePath); \
856-
RelationCacheInitFileInvalidate(tf); \
857-
pfree(DatabasePath); \
858-
}
859-
860852
/*
861853
* ProcessCommittedInvalidationMessages is executed by xact_redo_commit()
862854
* to process invalidation messages added to commit records.
863855
*
864856
* Relcache init file invalidation requires processing both
865857
* before and after we send the SI messages. See AtEOXact_Inval()
866-
*
867-
* We deliberately avoid SetDatabasePath() since it is intended to be used
868-
* only once by normal backends, so we set DatabasePath directly then
869-
* pfree after use. See RecoveryRelationCacheInitFileInvalidate() macro.
870858
*/
871859
void
872860
ProcessCommittedInvalidationMessages(SharedInvalidationMessage*msgs,
@@ -880,12 +868,25 @@ ProcessCommittedInvalidationMessages(SharedInvalidationMessage *msgs,
880868
(RelcacheInitFileInval ?" and relcache file invalidation" :""));
881869

882870
if (RelcacheInitFileInval)
883-
RecoveryRelationCacheInitFileInvalidate(dbid,tsid, true);
871+
{
872+
/*
873+
* RelationCacheInitFilePreInvalidate requires DatabasePath to be set,
874+
* but we should not use SetDatabasePath during recovery, since it is
875+
* intended to be used only once by normal backends. Hence, a quick
876+
* hack: set DatabasePath directly then unset after use.
877+
*/
878+
DatabasePath=GetDatabasePath(dbid,tsid);
879+
elog(trace_recovery(DEBUG4),"removing relcache init file in \"%s\"",
880+
DatabasePath);
881+
RelationCacheInitFilePreInvalidate();
882+
pfree(DatabasePath);
883+
DatabasePath=NULL;
884+
}
884885

885886
SendSharedInvalidMessages(msgs,nmsgs);
886887

887888
if (RelcacheInitFileInval)
888-
RecoveryRelationCacheInitFileInvalidate(dbid,tsid, false);
889+
RelationCacheInitFilePostInvalidate();
889890
}
890891

891892
/*
@@ -926,7 +927,7 @@ AtEOXact_Inval(bool isCommit)
926927
* unless we committed.
927928
*/
928929
if (transInvalInfo->RelcacheInitFileInval)
929-
RelationCacheInitFileInvalidate(true);
930+
RelationCacheInitFilePreInvalidate();
930931

931932
AppendInvalidationMessages(&transInvalInfo->PriorCmdInvalidMsgs,
932933
&transInvalInfo->CurrentCmdInvalidMsgs);
@@ -935,7 +936,7 @@ AtEOXact_Inval(bool isCommit)
935936
SendSharedInvalidMessages);
936937

937938
if (transInvalInfo->RelcacheInitFileInval)
938-
RelationCacheInitFileInvalidate(false);
939+
RelationCacheInitFilePostInvalidate();
939940
}
940941
elseif (transInvalInfo!=NULL)
941942
{

‎src/backend/utils/cache/relcache.c

Lines changed: 36 additions & 30 deletions
Original file line numberDiff line numberDiff line change
@@ -4402,8 +4402,8 @@ write_relcache_init_file(bool shared)
44024402
* updated by SI message processing, but we can't be sure whether what we
44034403
* wrote out was up-to-date.)
44044404
*
4405-
* This mustn't run concurrently withRelationCacheInitFileInvalidate, so
4406-
* grab a serialization lock for the duration.
4405+
* This mustn't run concurrently withthe code that unlinks an init file
4406+
*and sends SI messages, sograb a serialization lock for the duration.
44074407
*/
44084408
LWLockAcquire(RelCacheInitLock,LW_EXCLUSIVE);
44094409

@@ -4467,19 +4467,22 @@ RelationIdIsInInitFile(Oid relationId)
44674467
* changed one or more of the relation cache entries that are kept in the
44684468
* local init file.
44694469
*
4470-
* We actually need to remove the init file twice: once just before sending
4471-
* the SI messages that include relcache inval for such relations, and once
4472-
* just after sending them. The unlink before ensures that a backend that's
4473-
* currently starting cannot read the now-obsolete init file and then miss
4474-
* the SI messages that will force it to update its relcache entries. (This
4475-
* works because the backend startup sequence gets into the PGPROC array before
4476-
* trying to load the init file.) The unlink after is to synchronize with a
4477-
* backend that may currently be trying to write an init file based on data
4478-
* that we've just rendered invalid. Such a backend will see the SI messages,
4479-
* but we can't leave the init file sitting around to fool later backends.
4470+
* To be safe against concurrent inspection or rewriting of the init file,
4471+
* we must take RelCacheInitLock, then remove the old init file, then send
4472+
* the SI messages that include relcache inval for such relations, and then
4473+
* release RelCacheInitLock. This serializes the whole affair against
4474+
* write_relcache_init_file, so that we can be sure that any other process
4475+
* that's concurrently trying to create a new init file won't move an
4476+
* already-stale version into place after we unlink. Also, because we unlink
4477+
* before sending the SI messages, a backend that's currently starting cannot
4478+
* read the now-obsolete init file and then miss the SI messages that will
4479+
* force it to update its relcache entries. (This works because the backend
4480+
* startup sequence gets into the sinval array before trying to load the init
4481+
* file.)
44804482
*
4481-
* Ignore any failure to unlink the file, since it might not be there if
4482-
* no backend has been started since the last removal.
4483+
* We take the lock and do the unlink in RelationCacheInitFilePreInvalidate,
4484+
* then release the lock in RelationCacheInitFilePostInvalidate. Caller must
4485+
* send any pending SI messages between those calls.
44834486
*
44844487
* Notice this deals only with the local init file, not the shared init file.
44854488
* The reason is that there can never be a "significant" change to the
@@ -4489,34 +4492,37 @@ RelationIdIsInInitFile(Oid relationId)
44894492
* be invalid enough to make it necessary to remove it.
44904493
*/
44914494
void
4492-
RelationCacheInitFileInvalidate(boolbeforeSend)
4495+
RelationCacheInitFilePreInvalidate(void)
44934496
{
44944497
charinitfilename[MAXPGPATH];
44954498

44964499
snprintf(initfilename,sizeof(initfilename),"%s/%s",
44974500
DatabasePath,RELCACHE_INIT_FILENAME);
44984501

4499-
if (beforeSend)
4500-
{
4501-
/* no interlock needed here */
4502-
unlink(initfilename);
4503-
}
4504-
else
4502+
LWLockAcquire(RelCacheInitLock,LW_EXCLUSIVE);
4503+
4504+
if (unlink(initfilename)<0)
45054505
{
45064506
/*
4507-
* We need to interlock this against write_relcache_init_file, to
4508-
* guard against possibility that someone renames a new-but-
4509-
* already-obsolete init file into place just after we unlink. With
4510-
* the interlock, it's certain that write_relcache_init_file will
4511-
* notice our SI inval message before renaming into place, or else
4512-
* that we will execute second and successfully unlink the file.
4507+
* The file might not be there if no backend has been started since
4508+
* the last removal. But complain about failures other than ENOENT.
4509+
* Fortunately, it's not too late to abort the transaction if we
4510+
* can't get rid of the would-be-obsolete init file.
45134511
*/
4514-
LWLockAcquire(RelCacheInitLock,LW_EXCLUSIVE);
4515-
unlink(initfilename);
4516-
LWLockRelease(RelCacheInitLock);
4512+
if (errno!=ENOENT)
4513+
ereport(ERROR,
4514+
(errcode_for_file_access(),
4515+
errmsg("could not remove cache file \"%s\": %m",
4516+
initfilename)));
45174517
}
45184518
}
45194519

4520+
void
4521+
RelationCacheInitFilePostInvalidate(void)
4522+
{
4523+
LWLockRelease(RelCacheInitLock);
4524+
}
4525+
45204526
/*
45214527
* Remove the init files during postmaster startup.
45224528
*

‎src/include/utils/relcache.h

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -96,7 +96,8 @@ extern void AtEOSubXact_RelationCache(bool isCommit, SubTransactionId mySubid,
9696
* Routines to help manage rebuilding of relcache init files
9797
*/
9898
externboolRelationIdIsInInitFile(OidrelationId);
99-
externvoidRelationCacheInitFileInvalidate(boolbeforeSend);
99+
externvoidRelationCacheInitFilePreInvalidate(void);
100+
externvoidRelationCacheInitFilePostInvalidate(void);
100101
externvoidRelationCacheInitFileRemove(void);
101102

102103
/* should be used only by relcache.c and catcache.c */

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