3 files changed, 16 insertions, 6 deletions
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 6f614e4fad0..84409ea9562 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -2137,7 +2137,7 @@ CommitTransaction(void)
 	AtEOXact_ComboCid();
 	AtEOXact_HashTables(true);
 	AtEOXact_PgStat(true);
-	AtEOXact_Snapshot(true);
+	AtEOXact_Snapshot(true, false);
 	AtCommit_ApplyLauncher();
 	pgstat_report_xact_timestamp(0);
 
@@ -2409,7 +2409,7 @@ PrepareTransaction(void)
 	AtEOXact_ComboCid();
 	AtEOXact_HashTables(true);
 	/* don't call AtEOXact_PgStat here; we fixed pgstat state above */
-	AtEOXact_Snapshot(true);
+	AtEOXact_Snapshot(true, true);
 	pgstat_report_xact_timestamp(0);
 
 	CurrentResourceOwner = NULL;
@@ -2640,7 +2640,7 @@ CleanupTransaction(void)
 	 * do abort cleanup processing
 	 */
 	AtCleanup_Portals();		/* now safe to release portal memory */
-	AtEOXact_Snapshot(false);	/* and release the transaction's snapshots */
+	AtEOXact_Snapshot(false, false); /* and release the transaction's snapshots */
 
 	CurrentResourceOwner = NULL;	/* and resource owner */
 	if (TopTransactionResourceOwner)
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index ecc32cb3fe7..dff3d51ec3c 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -1051,7 +1051,7 @@ AtSubAbort_Snapshot(int level)
  *		Snapshot manager's cleanup function for end of transaction
  */
 void
-AtEOXact_Snapshot(bool isCommit)
+AtEOXact_Snapshot(bool isCommit, bool isPrepare)
 {
 	/*
 	 * In transaction-snapshot mode we must release our privately-managed
@@ -1136,7 +1136,17 @@ AtEOXact_Snapshot(bool isCommit)
 
 	FirstSnapshotSet = false;
 
-	SnapshotResetXmin();
+	/*
+	 * During normal commit and abort processing, we call
+	 * ProcArrayEndTransaction() or ProcArrayClearTransaction() to
+	 * reset the PgXact->xmin. That call happens prior to the call to
+	 * AtEOXact_Snapshot(), so we need not touch xmin here at all,
+	 * accept when we are preparing a transaction.
+	 */
+	if (isPrepare)
+		SnapshotResetXmin();
+
+	Assert(isPrepare || MyPgXact->xmin == 0);
 }
 
 
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index 86c48d9c8ef..2f8d4fd4a0c 100644
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -85,7 +85,7 @@ extern void UnregisterSnapshotFromOwner(Snapshot snapshot, ResourceOwner owner);
 
 extern void AtSubCommit_Snapshot(int level);
 extern void AtSubAbort_Snapshot(int level);
-extern void AtEOXact_Snapshot(bool isCommit);
+extern void AtEOXact_Snapshot(bool isCommit, bool isPrepare);
 
 extern void ImportSnapshot(const char *idstr);
 extern bool XactHasExportedSnapshots(void);