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Commit2ada677

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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 parent1bb6924 commit2ada677

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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
@@ -1356,10 +1356,10 @@ FinishPreparedTransaction(const char *gid, bool isCommit)
13561356
* after we send the SI messages. See AtEOXact_Inval()
13571357
*/
13581358
if (hdr->initfileinval)
1359-
RelationCacheInitFileInvalidate(true);
1359+
RelationCacheInitFilePreInvalidate();
13601360
SendSharedInvalidMessages(invalmsgs,hdr->ninvalmsgs);
13611361
if (hdr->initfileinval)
1362-
RelationCacheInitFileInvalidate(false);
1362+
RelationCacheInitFilePostInvalidate();
13631363

13641364
/* And now do the callbacks */
13651365
if (isCommit)

‎src/backend/utils/cache/inval.c

Lines changed: 17 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -854,24 +854,12 @@ xactGetCommittedInvalidationMessages(SharedInvalidationMessage **msgs,
854854
returnnumSharedInvalidMessagesArray;
855855
}
856856

857-
#defineRecoveryRelationCacheInitFileInvalidate(dbo,tbo,tf) \
858-
{ \
859-
DatabasePath = GetDatabasePath(dbo, tbo); \
860-
elog(trace_recovery(DEBUG4), "removing relcache init file in %s", DatabasePath); \
861-
RelationCacheInitFileInvalidate(tf); \
862-
pfree(DatabasePath); \
863-
}
864-
865857
/*
866858
* ProcessCommittedInvalidationMessages is executed by xact_redo_commit()
867859
* to process invalidation messages added to commit records.
868860
*
869861
* Relcache init file invalidation requires processing both
870862
* before and after we send the SI messages. See AtEOXact_Inval()
871-
*
872-
* We deliberately avoid SetDatabasePath() since it is intended to be used
873-
* only once by normal backends, so we set DatabasePath directly then
874-
* pfree after use. See RecoveryRelationCacheInitFileInvalidate() macro.
875863
*/
876864
void
877865
ProcessCommittedInvalidationMessages(SharedInvalidationMessage*msgs,
@@ -885,12 +873,25 @@ ProcessCommittedInvalidationMessages(SharedInvalidationMessage *msgs,
885873
(RelcacheInitFileInval ?" and relcache file invalidation" :""));
886874

887875
if (RelcacheInitFileInval)
888-
RecoveryRelationCacheInitFileInvalidate(dbid,tsid, true);
876+
{
877+
/*
878+
* RelationCacheInitFilePreInvalidate requires DatabasePath to be set,
879+
* but we should not use SetDatabasePath during recovery, since it is
880+
* intended to be used only once by normal backends. Hence, a quick
881+
* hack: set DatabasePath directly then unset after use.
882+
*/
883+
DatabasePath=GetDatabasePath(dbid,tsid);
884+
elog(trace_recovery(DEBUG4),"removing relcache init file in \"%s\"",
885+
DatabasePath);
886+
RelationCacheInitFilePreInvalidate();
887+
pfree(DatabasePath);
888+
DatabasePath=NULL;
889+
}
889890

890891
SendSharedInvalidMessages(msgs,nmsgs);
891892

892893
if (RelcacheInitFileInval)
893-
RecoveryRelationCacheInitFileInvalidate(dbid,tsid, false);
894+
RelationCacheInitFilePostInvalidate();
894895
}
895896

896897
/*
@@ -931,7 +932,7 @@ AtEOXact_Inval(bool isCommit)
931932
* unless we committed.
932933
*/
933934
if (transInvalInfo->RelcacheInitFileInval)
934-
RelationCacheInitFileInvalidate(true);
935+
RelationCacheInitFilePreInvalidate();
935936

936937
AppendInvalidationMessages(&transInvalInfo->PriorCmdInvalidMsgs,
937938
&transInvalInfo->CurrentCmdInvalidMsgs);
@@ -940,7 +941,7 @@ AtEOXact_Inval(bool isCommit)
940941
SendSharedInvalidMessages);
941942

942943
if (transInvalInfo->RelcacheInitFileInval)
943-
RelationCacheInitFileInvalidate(false);
944+
RelationCacheInitFilePostInvalidate();
944945
}
945946
elseif (transInvalInfo!=NULL)
946947
{

‎src/backend/utils/cache/relcache.c

Lines changed: 36 additions & 30 deletions
Original file line numberDiff line numberDiff line change
@@ -4377,8 +4377,8 @@ write_relcache_init_file(bool shared)
43774377
* updated by SI message processing, but we can't be sure whether what we
43784378
* wrote out was up-to-date.)
43794379
*
4380-
* This mustn't run concurrently withRelationCacheInitFileInvalidate, so
4381-
* grab a serialization lock for the duration.
4380+
* This mustn't run concurrently withthe code that unlinks an init file
4381+
*and sends SI messages, sograb a serialization lock for the duration.
43824382
*/
43834383
LWLockAcquire(RelCacheInitLock,LW_EXCLUSIVE);
43844384

@@ -4442,19 +4442,22 @@ RelationIdIsInInitFile(Oid relationId)
44424442
* changed one or more of the relation cache entries that are kept in the
44434443
* local init file.
44444444
*
4445-
* We actually need to remove the init file twice: once just before sending
4446-
* the SI messages that include relcache inval for such relations, and once
4447-
* just after sending them. The unlink before ensures that a backend that's
4448-
* currently starting cannot read the now-obsolete init file and then miss
4449-
* the SI messages that will force it to update its relcache entries. (This
4450-
* works because the backend startup sequence gets into the PGPROC array before
4451-
* trying to load the init file.) The unlink after is to synchronize with a
4452-
* backend that may currently be trying to write an init file based on data
4453-
* that we've just rendered invalid. Such a backend will see the SI messages,
4454-
* but we can't leave the init file sitting around to fool later backends.
4445+
* To be safe against concurrent inspection or rewriting of the init file,
4446+
* we must take RelCacheInitLock, then remove the old init file, then send
4447+
* the SI messages that include relcache inval for such relations, and then
4448+
* release RelCacheInitLock. This serializes the whole affair against
4449+
* write_relcache_init_file, so that we can be sure that any other process
4450+
* that's concurrently trying to create a new init file won't move an
4451+
* already-stale version into place after we unlink. Also, because we unlink
4452+
* before sending the SI messages, a backend that's currently starting cannot
4453+
* read the now-obsolete init file and then miss the SI messages that will
4454+
* force it to update its relcache entries. (This works because the backend
4455+
* startup sequence gets into the sinval array before trying to load the init
4456+
* file.)
44554457
*
4456-
* Ignore any failure to unlink the file, since it might not be there if
4457-
* no backend has been started since the last removal.
4458+
* We take the lock and do the unlink in RelationCacheInitFilePreInvalidate,
4459+
* then release the lock in RelationCacheInitFilePostInvalidate. Caller must
4460+
* send any pending SI messages between those calls.
44584461
*
44594462
* Notice this deals only with the local init file, not the shared init file.
44604463
* The reason is that there can never be a "significant" change to the
@@ -4464,34 +4467,37 @@ RelationIdIsInInitFile(Oid relationId)
44644467
* be invalid enough to make it necessary to remove it.
44654468
*/
44664469
void
4467-
RelationCacheInitFileInvalidate(boolbeforeSend)
4470+
RelationCacheInitFilePreInvalidate(void)
44684471
{
44694472
charinitfilename[MAXPGPATH];
44704473

44714474
snprintf(initfilename,sizeof(initfilename),"%s/%s",
44724475
DatabasePath,RELCACHE_INIT_FILENAME);
44734476

4474-
if (beforeSend)
4475-
{
4476-
/* no interlock needed here */
4477-
unlink(initfilename);
4478-
}
4479-
else
4477+
LWLockAcquire(RelCacheInitLock,LW_EXCLUSIVE);
4478+
4479+
if (unlink(initfilename)<0)
44804480
{
44814481
/*
4482-
* We need to interlock this against write_relcache_init_file, to
4483-
* guard against possibility that someone renames a new-but-
4484-
* already-obsolete init file into place just after we unlink. With
4485-
* the interlock, it's certain that write_relcache_init_file will
4486-
* notice our SI inval message before renaming into place, or else
4487-
* that we will execute second and successfully unlink the file.
4482+
* The file might not be there if no backend has been started since
4483+
* the last removal. But complain about failures other than ENOENT.
4484+
* Fortunately, it's not too late to abort the transaction if we
4485+
* can't get rid of the would-be-obsolete init file.
44884486
*/
4489-
LWLockAcquire(RelCacheInitLock,LW_EXCLUSIVE);
4490-
unlink(initfilename);
4491-
LWLockRelease(RelCacheInitLock);
4487+
if (errno!=ENOENT)
4488+
ereport(ERROR,
4489+
(errcode_for_file_access(),
4490+
errmsg("could not remove cache file \"%s\": %m",
4491+
initfilename)));
44924492
}
44934493
}
44944494

4495+
void
4496+
RelationCacheInitFilePostInvalidate(void)
4497+
{
4498+
LWLockRelease(RelCacheInitLock);
4499+
}
4500+
44954501
/*
44964502
* Remove the init files during postmaster startup.
44974503
*

‎src/include/utils/relcache.h

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -98,7 +98,8 @@ extern void AtEOSubXact_RelationCache(bool isCommit, SubTransactionId mySubid,
9898
* Routines to help manage rebuilding of relcache init files
9999
*/
100100
externboolRelationIdIsInInitFile(OidrelationId);
101-
externvoidRelationCacheInitFileInvalidate(boolbeforeSend);
101+
externvoidRelationCacheInitFilePreInvalidate(void);
102+
externvoidRelationCacheInitFilePostInvalidate(void);
102103
externvoidRelationCacheInitFileRemove(void);
103104

104105
/* should be used only by relcache.c and catcache.c */

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