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Reset properly snapshot export state during transaction abort

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During a replication slot creation, an ERROR generated in the same
transaction as the one creating a to-be-exported snapshot would have
left the backend in an inconsistent state, as the associated static
export snapshot state was not being reset on transaction abort, but only
on the follow-up command received by the WAL sender that created this
snapshot on replication slot creation.  This would trigger inconsistency
failures if this session tried to export again a snapshot, like during
the creation of a replication slot.

Note that a snapshot export cannot happen in a transaction block, so
there is no need to worry resetting this state for subtransaction
aborts.  Also, this inconsistent state would very unlikely show up to
users.  For example, one case where this could happen is an
out-of-memory error when building the initial snapshot to-be-exported.
Dilip found this problem while poking at a different patch, that caused
an error in this code path for reasons unrelated to HEAD.

Author: Dilip Kumar
Reviewed-by: Michael Paquier, Zhihong Yu
Discussion: https://postgr.es/m/CAFiTN-s0zA1Kj0ozGHwkYkHwa5U0zUE94RSc_g81WrpcETB5=w@mail.gmail.com
Backpatch-through: 9.6
This commit is contained in:
Michael Paquier 2021-10-18 11:55:42 +09:00
parent 384f1abdb9
commit 409f9ca447
3 changed files with 28 additions and 2 deletions
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9
src/backend/access/transam/xact.c
View File

@ -46,6 +46,7 @@
#include "replication/logical.h"
#include "replication/logicallauncher.h"
#include "replication/origin.h"
#include "replication/snapbuild.h"
#include "replication/syncrep.h"
#include "replication/walsender.h"
#include "storage/condition_variable.h"
@ -2698,6 +2699,9 @@ AbortTransaction(void)
/* Reset logical streaming state. */
ResetLogicalStreamingState();
/* Reset snapshot export state. */
SnapBuildResetExportedSnapshotState();
/* If in parallel mode, clean up workers and exit parallel mode. */
if (IsInParallelMode())
{
@ -5010,6 +5014,11 @@ AbortSubTransaction(void)
/* Reset logical streaming state. */
ResetLogicalStreamingState();
/*
* No need for SnapBuildResetExportedSnapshotState() here, snapshot
* exports are not supported in subtransactions.
*/
/* Exit from parallel mode, if necessary. */
if (IsInParallelMode())
{

20
src/backend/replication/logical/snapbuild.c
View File

@ -682,6 +682,8 @@ SnapBuildGetOrBuildSnapshot(SnapBuild *builder, TransactionId xid)
void
SnapBuildClearExportedSnapshot(void)
{
ResourceOwner tmpResOwner;
/* nothing exported, that is the usual case */
if (!ExportInProgress)
return;
@ -689,10 +691,24 @@ SnapBuildClearExportedSnapshot(void)
if (!IsTransactionState())
elog(ERROR, "clearing exported snapshot in wrong transaction state");
/* make sure nothing could have ever happened */
/*
* AbortCurrentTransaction() takes care of resetting the snapshot state,
* so remember SavedResourceOwnerDuringExport.
*/
tmpResOwner = SavedResourceOwnerDuringExport;
/* make sure nothing could have ever happened */
AbortCurrentTransaction();
CurrentResourceOwner = SavedResourceOwnerDuringExport;
CurrentResourceOwner = tmpResOwner;
}
/*
* Clear snapshot export state during transaction abort.
*/
void
SnapBuildResetExportedSnapshotState(void)
{
SavedResourceOwnerDuringExport = NULL;
ExportInProgress = false;
}

1
src/include/replication/snapbuild.h
View File

@ -70,6 +70,7 @@ extern void SnapBuildSnapDecRefcount(Snapshot snap);
extern Snapshot SnapBuildInitialSnapshot(SnapBuild *builder);
extern const char *SnapBuildExportSnapshot(SnapBuild *snapstate);
extern void SnapBuildClearExportedSnapshot(void);
extern void SnapBuildResetExportedSnapshotState(void);
extern SnapBuildState SnapBuildCurrentState(SnapBuild *snapstate);
extern Snapshot SnapBuildGetOrBuildSnapshot(SnapBuild *builder,