Reset properly snapshot export state during transaction abort

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

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 4cc38f0d85e652e94f5f731727752ae3062676a5..d96881c0de553f86ece6e458564bc3cb62830a03 100644 (file)
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -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())
    {

diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index a5333349a8a3a6c176af59af2e0afd742e57a912..dbdc172a2bdc7f89698ce724f36bdfc2dbd07da6 100644 (file)
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -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;
 }

diff --git a/src/include/replication/snapbuild.h b/src/include/replication/snapbuild.h
index de7212464af55796c06a4e76e8a2e4334cced1f3..82aa86125b92680d9a26b806c2b635a8da62bd1c 100644 (file)
--- a/src/include/replication/snapbuild.h
+++ b/src/include/replication/snapbuild.h
@@ -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,