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Commit63ecdaf

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Fix race conditions in synchronous standby management.
We have repeatedly seen the buildfarm reach the Assert(false) inSyncRepGetSyncStandbysPriority. This apparently is due to failing toconsider the possibility that the sync_standby_priority values inshared memory might be inconsistent; but they will be whenever onlysome of the walsenders have updated their values after a change inthe synchronous_standby_names setting. That function is vastly toocomplex for what it does, anyway, so rewriting it seems better thantrying to apply a band-aid fix.Furthermore, the API of SyncRepGetSyncStandbys is broken by design:it returns a list of WalSnd array indexes, but there is nothingguaranteeing that the contents of the WalSnd array remain stable.Thus, if some walsender exits and then a new walsender processtakes over that WalSnd array slot, a caller might make use ofWAL position data that it should not, potentially leading toincorrect decisions about whether to release transactions thatare waiting for synchronous commit.To fix, replace SyncRepGetSyncStandbys with a new functionSyncRepGetCandidateStandbys that copies all the required datafrom shared memory while holding the relevant mutexes. If theassociated walsender process then exits, this data is still safe tomake release decisions with, since we know that that much WAL *was*sent to a valid standby server. This incidentally means that we nolonger need to treat sync_standby_priority as protected by theSyncRepLock rather than the per-walsender mutex.SyncRepGetSyncStandbys is no longer used by the core code, so removeit entirely in HEAD. However, it seems possible that external code isrelying on that function, so do not remove it from the back branches.Instead, just remove the known-incorrect Assert. When the bug occurs,the function will return a too-short list, which callers should treatas meaning there are not enough sync standbys, which seems like areasonably safe fallback until the inconsistent state is resolved.Moreover it's bug-compatible with what has been happening in non-assertbuilds. We cannot do anything about the walsender-replacement racecondition without an API/ABI break.The bogus assertion exists back to 9.6, but 9.6 is sufficientlydifferent from the later branches that the patch doesn't apply at all.I chose to just remove the bogus assertion in 9.6, feeling that theprobability of a bad outcome from the walsender-replacement racecondition is too low to justify rewriting the whole patch for 9.6.Discussion:https://postgr.es/m/21519.1585272409@sss.pgh.pa.us
1 parentddac64f commit63ecdaf

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4 files changed

+243
-66
lines changed

4 files changed

+243
-66
lines changed

‎src/backend/replication/syncrep.c‎

Lines changed: 191 additions & 54 deletions
Original file line numberDiff line numberDiff line change
@@ -108,14 +108,18 @@ static bool SyncRepGetSyncRecPtr(XLogRecPtr *writePtr,
108108
staticvoidSyncRepGetOldestSyncRecPtr(XLogRecPtr*writePtr,
109109
XLogRecPtr*flushPtr,
110110
XLogRecPtr*applyPtr,
111-
List*sync_standbys);
111+
SyncRepStandbyData*sync_standbys,
112+
intnum_standbys);
112113
staticvoidSyncRepGetNthLatestSyncRecPtr(XLogRecPtr*writePtr,
113114
XLogRecPtr*flushPtr,
114115
XLogRecPtr*applyPtr,
115-
List*sync_standbys,uint8nth);
116+
SyncRepStandbyData*sync_standbys,
117+
intnum_standbys,
118+
uint8nth);
116119
staticintSyncRepGetStandbyPriority(void);
117120
staticList*SyncRepGetSyncStandbysPriority(bool*am_sync);
118121
staticList*SyncRepGetSyncStandbysQuorum(bool*am_sync);
122+
staticintstandby_priority_comparator(constvoid*a,constvoid*b);
119123
staticintcmp_lsn(constvoid*a,constvoid*b);
120124

121125
#ifdefUSE_ASSERT_CHECKING
@@ -400,9 +404,10 @@ SyncRepInitConfig(void)
400404
priority=SyncRepGetStandbyPriority();
401405
if (MyWalSnd->sync_standby_priority!=priority)
402406
{
403-
LWLockAcquire(SyncRepLock,LW_EXCLUSIVE);
407+
SpinLockAcquire(&MyWalSnd->mutex);
404408
MyWalSnd->sync_standby_priority=priority;
405-
LWLockRelease(SyncRepLock);
409+
SpinLockRelease(&MyWalSnd->mutex);
410+
406411
ereport(DEBUG1,
407412
(errmsg("standby \"%s\" now has synchronous standby priority %u",
408413
application_name,priority)));
@@ -453,7 +458,11 @@ SyncRepReleaseWaiters(void)
453458

454459
/*
455460
* Check whether we are a sync standby or not, and calculate the synced
456-
* positions among all sync standbys.
461+
* positions among all sync standbys. (Note: although this step does not
462+
* of itself require holding SyncRepLock, it seems like a good idea to do
463+
* it after acquiring the lock. This ensures that the WAL pointers we use
464+
* to release waiters are newer than any previous execution of this
465+
* routine used.)
457466
*/
458467
got_recptr=SyncRepGetSyncRecPtr(&writePtr,&flushPtr,&applyPtr,&am_sync);
459468

@@ -528,25 +537,41 @@ static bool
528537
SyncRepGetSyncRecPtr(XLogRecPtr*writePtr,XLogRecPtr*flushPtr,
529538
XLogRecPtr*applyPtr,bool*am_sync)
530539
{
531-
List*sync_standbys;
540+
SyncRepStandbyData*sync_standbys;
541+
intnum_standbys;
542+
inti;
532543

544+
/* Initialize default results */
533545
*writePtr=InvalidXLogRecPtr;
534546
*flushPtr=InvalidXLogRecPtr;
535547
*applyPtr=InvalidXLogRecPtr;
536548
*am_sync= false;
537549

550+
/* Quick out if not even configured to be synchronous */
551+
if (SyncRepConfig==NULL)
552+
return false;
553+
538554
/* Get standbys that are considered as synchronous at this moment */
539-
sync_standbys=SyncRepGetSyncStandbys(am_sync);
555+
num_standbys=SyncRepGetCandidateStandbys(&sync_standbys);
556+
557+
/* Am I among the candidate sync standbys? */
558+
for (i=0;i<num_standbys;i++)
559+
{
560+
if (sync_standbys[i].is_me)
561+
{
562+
*am_sync= true;
563+
break;
564+
}
565+
}
540566

541567
/*
542-
*Quick exitif we are not managing a sync standby or there are not
543-
* enough synchronous standbys.
568+
*Nothing more to doif we are not managing a sync standby or there are
569+
*notenough synchronous standbys.
544570
*/
545571
if (!(*am_sync)||
546-
SyncRepConfig==NULL||
547-
list_length(sync_standbys)<SyncRepConfig->num_sync)
572+
num_standbys<SyncRepConfig->num_sync)
548573
{
549-
list_free(sync_standbys);
574+
pfree(sync_standbys);
550575
return false;
551576
}
552577

@@ -566,43 +591,41 @@ SyncRepGetSyncRecPtr(XLogRecPtr *writePtr, XLogRecPtr *flushPtr,
566591
if (SyncRepConfig->syncrep_method==SYNC_REP_PRIORITY)
567592
{
568593
SyncRepGetOldestSyncRecPtr(writePtr,flushPtr,applyPtr,
569-
sync_standbys);
594+
sync_standbys,num_standbys);
570595
}
571596
else
572597
{
573598
SyncRepGetNthLatestSyncRecPtr(writePtr,flushPtr,applyPtr,
574-
sync_standbys,SyncRepConfig->num_sync);
599+
sync_standbys,num_standbys,
600+
SyncRepConfig->num_sync);
575601
}
576602

577-
list_free(sync_standbys);
603+
pfree(sync_standbys);
578604
return true;
579605
}
580606

581607
/*
582608
* Calculate the oldest Write, Flush and Apply positions among sync standbys.
583609
*/
584610
staticvoid
585-
SyncRepGetOldestSyncRecPtr(XLogRecPtr*writePtr,XLogRecPtr*flushPtr,
586-
XLogRecPtr*applyPtr,List*sync_standbys)
611+
SyncRepGetOldestSyncRecPtr(XLogRecPtr*writePtr,
612+
XLogRecPtr*flushPtr,
613+
XLogRecPtr*applyPtr,
614+
SyncRepStandbyData*sync_standbys,
615+
intnum_standbys)
587616
{
588-
ListCell*cell;
617+
inti;
589618

590619
/*
591620
* Scan through all sync standbys and calculate the oldest Write, Flush
592-
* and Apply positions.
621+
* and Apply positions. We assume *writePtr et al were initialized to
622+
* InvalidXLogRecPtr.
593623
*/
594-
foreach(cell,sync_standbys)
624+
for (i=0;i<num_standbys;i++)
595625
{
596-
WalSnd*walsnd=&WalSndCtl->walsnds[lfirst_int(cell)];
597-
XLogRecPtrwrite;
598-
XLogRecPtrflush;
599-
XLogRecPtrapply;
600-
601-
SpinLockAcquire(&walsnd->mutex);
602-
write=walsnd->write;
603-
flush=walsnd->flush;
604-
apply=walsnd->apply;
605-
SpinLockRelease(&walsnd->mutex);
626+
XLogRecPtrwrite=sync_standbys[i].write;
627+
XLogRecPtrflush=sync_standbys[i].flush;
628+
XLogRecPtrapply=sync_standbys[i].apply;
606629

607630
if (XLogRecPtrIsInvalid(*writePtr)||*writePtr>write)
608631
*writePtr=write;
@@ -618,38 +641,36 @@ SyncRepGetOldestSyncRecPtr(XLogRecPtr *writePtr, XLogRecPtr *flushPtr,
618641
* standbys.
619642
*/
620643
staticvoid
621-
SyncRepGetNthLatestSyncRecPtr(XLogRecPtr*writePtr,XLogRecPtr*flushPtr,
622-
XLogRecPtr*applyPtr,List*sync_standbys,uint8nth)
644+
SyncRepGetNthLatestSyncRecPtr(XLogRecPtr*writePtr,
645+
XLogRecPtr*flushPtr,
646+
XLogRecPtr*applyPtr,
647+
SyncRepStandbyData*sync_standbys,
648+
intnum_standbys,
649+
uint8nth)
623650
{
624-
ListCell*cell;
625651
XLogRecPtr*write_array;
626652
XLogRecPtr*flush_array;
627653
XLogRecPtr*apply_array;
628-
intlen;
629-
inti=0;
654+
inti;
630655

631-
len=list_length(sync_standbys);
632-
write_array= (XLogRecPtr*)palloc(sizeof(XLogRecPtr)*len);
633-
flush_array= (XLogRecPtr*)palloc(sizeof(XLogRecPtr)*len);
634-
apply_array= (XLogRecPtr*)palloc(sizeof(XLogRecPtr)*len);
656+
/* Should have enough candidates, or somebody messed up */
657+
Assert(nth>0&&nth <=num_standbys);
635658

636-
foreach(cell,sync_standbys)
637-
{
638-
WalSnd*walsnd=&WalSndCtl->walsnds[lfirst_int(cell)];
659+
write_array= (XLogRecPtr*)palloc(sizeof(XLogRecPtr)*num_standbys);
660+
flush_array= (XLogRecPtr*)palloc(sizeof(XLogRecPtr)*num_standbys);
661+
apply_array= (XLogRecPtr*)palloc(sizeof(XLogRecPtr)*num_standbys);
639662

640-
SpinLockAcquire(&walsnd->mutex);
641-
write_array[i]=walsnd->write;
642-
flush_array[i]=walsnd->flush;
643-
apply_array[i]=walsnd->apply;
644-
SpinLockRelease(&walsnd->mutex);
645-
646-
i++;
663+
for (i=0;i<num_standbys;i++)
664+
{
665+
write_array[i]=sync_standbys[i].write;
666+
flush_array[i]=sync_standbys[i].flush;
667+
apply_array[i]=sync_standbys[i].apply;
647668
}
648669

649670
/* Sort each array in descending order */
650-
qsort(write_array,len,sizeof(XLogRecPtr),cmp_lsn);
651-
qsort(flush_array,len,sizeof(XLogRecPtr),cmp_lsn);
652-
qsort(apply_array,len,sizeof(XLogRecPtr),cmp_lsn);
671+
qsort(write_array,num_standbys,sizeof(XLogRecPtr),cmp_lsn);
672+
qsort(flush_array,num_standbys,sizeof(XLogRecPtr),cmp_lsn);
673+
qsort(apply_array,num_standbys,sizeof(XLogRecPtr),cmp_lsn);
653674

654675
/* Get Nth latest Write, Flush, Apply positions */
655676
*writePtr=write_array[nth-1];
@@ -678,13 +699,122 @@ cmp_lsn(const void *a, const void *b)
678699
return1;
679700
}
680701

702+
/*
703+
* Return data about walsenders that are candidates to be sync standbys.
704+
*
705+
* *standbys is set to a palloc'd array of structs of per-walsender data,
706+
* and the number of valid entries (candidate sync senders) is returned.
707+
* (This might be more or fewer than num_sync; caller must check.)
708+
*/
709+
int
710+
SyncRepGetCandidateStandbys(SyncRepStandbyData**standbys)
711+
{
712+
inti;
713+
intn;
714+
715+
/* Create result array */
716+
*standbys= (SyncRepStandbyData*)
717+
palloc(max_wal_senders*sizeof(SyncRepStandbyData));
718+
719+
/* Quick exit if sync replication is not requested */
720+
if (SyncRepConfig==NULL)
721+
return0;
722+
723+
/* Collect raw data from shared memory */
724+
n=0;
725+
for (i=0;i<max_wal_senders;i++)
726+
{
727+
volatileWalSnd*walsnd;/* Use volatile pointer to prevent code
728+
* rearrangement */
729+
SyncRepStandbyData*stby;
730+
WalSndStatestate;/* not included in SyncRepStandbyData */
731+
732+
walsnd=&WalSndCtl->walsnds[i];
733+
stby=*standbys+n;
734+
735+
SpinLockAcquire(&walsnd->mutex);
736+
stby->pid=walsnd->pid;
737+
state=walsnd->state;
738+
stby->write=walsnd->write;
739+
stby->flush=walsnd->flush;
740+
stby->apply=walsnd->apply;
741+
stby->sync_standby_priority=walsnd->sync_standby_priority;
742+
SpinLockRelease(&walsnd->mutex);
743+
744+
/* Must be active */
745+
if (stby->pid==0)
746+
continue;
747+
748+
/* Must be streaming or stopping */
749+
if (state!=WALSNDSTATE_STREAMING&&
750+
state!=WALSNDSTATE_STOPPING)
751+
continue;
752+
753+
/* Must be synchronous */
754+
if (stby->sync_standby_priority==0)
755+
continue;
756+
757+
/* Must have a valid flush position */
758+
if (XLogRecPtrIsInvalid(stby->flush))
759+
continue;
760+
761+
/* OK, it's a candidate */
762+
stby->walsnd_index=i;
763+
stby->is_me= (walsnd==MyWalSnd);
764+
n++;
765+
}
766+
767+
/*
768+
* In quorum mode, we return all the candidates. In priority mode, if we
769+
* have too many candidates then return only the num_sync ones of highest
770+
* priority.
771+
*/
772+
if (SyncRepConfig->syncrep_method==SYNC_REP_PRIORITY&&
773+
n>SyncRepConfig->num_sync)
774+
{
775+
/* Sort by priority ... */
776+
qsort(*standbys,n,sizeof(SyncRepStandbyData),
777+
standby_priority_comparator);
778+
/* ... then report just the first num_sync ones */
779+
n=SyncRepConfig->num_sync;
780+
}
781+
782+
returnn;
783+
}
784+
785+
/*
786+
* qsort comparator to sort SyncRepStandbyData entries by priority
787+
*/
788+
staticint
789+
standby_priority_comparator(constvoid*a,constvoid*b)
790+
{
791+
constSyncRepStandbyData*sa= (constSyncRepStandbyData*)a;
792+
constSyncRepStandbyData*sb= (constSyncRepStandbyData*)b;
793+
794+
/* First, sort by increasing priority value */
795+
if (sa->sync_standby_priority!=sb->sync_standby_priority)
796+
returnsa->sync_standby_priority-sb->sync_standby_priority;
797+
798+
/*
799+
* We might have equal priority values; arbitrarily break ties by position
800+
* in the WALSnd array. (This is utterly bogus, since that is arrival
801+
* order dependent, but there are regression tests that rely on it.)
802+
*/
803+
returnsa->walsnd_index-sb->walsnd_index;
804+
}
805+
806+
681807
/*
682808
* Return the list of sync standbys, or NIL if no sync standby is connected.
683809
*
684810
* The caller must hold SyncRepLock.
685811
*
686812
* On return, *am_sync is set to true if this walsender is connecting to
687813
* sync standby. Otherwise it's set to false.
814+
*
815+
* XXX This function is BROKEN and should not be used in new code. It has
816+
* an inherent race condition, since the returned list of integer indexes
817+
* might no longer correspond to reality.
688818
*/
689819
List*
690820
SyncRepGetSyncStandbys(bool*am_sync)
@@ -944,8 +1074,15 @@ SyncRepGetSyncStandbysPriority(bool *am_sync)
9441074
priority=next_highest_priority;
9451075
}
9461076

947-
/* never reached, but keep compiler quiet */
948-
Assert(false);
1077+
/*
1078+
* We might get here if the set of sync_standby_priority values in shared
1079+
* memory is inconsistent, as can happen transiently after a change in the
1080+
* synchronous_standby_names setting. In that case, just return the
1081+
* incomplete list we have so far. That will cause the caller to decide
1082+
* there aren't enough synchronous candidates, which should be a safe
1083+
* choice until the priority values become consistent again.
1084+
*/
1085+
list_free(pending);
9491086
returnresult;
9501087
}
9511088

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