Preserve required !catalog tuples while computing initial decoding snapshot.

The logical decoding machinery already preserved all the required
catalog tuples, which is sufficient in the course of normal logical
decoding, but did not guarantee that non-catalog tuples were preserved
during computation of the initial snapshot when creating a slot over
the replication protocol.

This could cause a corrupted initial snapshot being exported.  The
time window for issues is usually not terribly large, but on a busy
server it's perfectly possible to it hit it.  Ongoing decoding is not
affected by this bug.

To avoid increased overhead for the SQL API, only retain additional
tuples when a logical slot is being created over the replication
protocol.  To do so this commit changes the signature of
CreateInitDecodingContext(), but it seems unlikely that it's being
used in an extension, so that's probably ok.

In a drive-by fix, fix handling of
ReplicationSlotsComputeRequiredXmin's already_locked argument, which
should only apply to ProcArrayLock, not ReplicationSlotControlLock.

Reported-By: Erik Rijkers
Analyzed-By: Petr Jelinek
Author: Petr Jelinek, heavily editorialized by Andres Freund
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/9a897b86-46e1-9915-ee4c-da02e4ff6a95@2ndquadrant.com
Backport: 9.4, where logical decoding was introduced.

src/backend/replication/logical/logical.c

@@ -210,6 +210,7 @@ StartupDecodingContext(List *output_plugin_options,
 LogicalDecodingContext *
 CreateInitDecodingContext(char *plugin,
 						  List *output_plugin_options,
+						  bool need_full_snapshot,
 						  XLogPageReadCB read_page,
 						  LogicalOutputPluginWriterPrepareWrite prepare_write,
 						  LogicalOutputPluginWriterWrite do_write)
@@ -267,23 +268,31 @@ CreateInitDecodingContext(char *plugin,
 	 * the slot machinery about the new limit. Once that's done the
 	 * ProcArrayLock can be released as the slot machinery now is
 	 * protecting against vacuum.
+	 *
+	 * Note that, temporarily, the data, not just the catalog, xmin has to be
+	 * reserved if a data snapshot is to be exported.  Otherwise the initial
+	 * data snapshot created here is not guaranteed to be valid. After that
+	 * the data xmin doesn't need to be managed anymore and the global xmin
+	 * should be recomputed. As we are fine with losing the pegged data xmin
+	 * after crash - no chance a snapshot would get exported anymore - we can
+	 * get away with just setting the slot's
+	 * effective_xmin. ReplicationSlotRelease will reset it again.
+	 *
 	 * ----
 	 */
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
-	slot->effective_catalog_xmin = GetOldestSafeDecodingTransactionId();
-	slot->data.catalog_xmin = slot->effective_catalog_xmin;
+	xmin_horizon = GetOldestSafeDecodingTransactionId(need_full_snapshot);
+
+	slot->effective_catalog_xmin = xmin_horizon;
+	slot->data.catalog_xmin = xmin_horizon;
+	if (need_full_snapshot)
+		slot->effective_xmin = xmin_horizon;
 
 	ReplicationSlotsComputeRequiredXmin(true);
 
 	LWLockRelease(ProcArrayLock);
 
-	/*
-	 * tell the snapshot builder to only assemble snapshot once reaching the
-	 * running_xact's record with the respective xmin.
-	 */
-	xmin_horizon = slot->data.catalog_xmin;
-
 	ReplicationSlotMarkDirty();
 	ReplicationSlotSave();
 

src/backend/replication/logical/snapbuild.c

@@ -533,6 +533,18 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
 	 * mechanism. Due to that we can do this without locks, we're only
 	 * changing our own value.
 	 */
+#ifdef USE_ASSERT_CHECKING
+	{
+		TransactionId safeXid;
+
+		LWLockAcquire(ProcArrayLock, LW_SHARED);
+		safeXid = GetOldestSafeDecodingTransactionId(true);
+		LWLockRelease(ProcArrayLock);
+
+		Assert(TransactionIdPrecedesOrEquals(safeXid, snap->xmin));
+	}
+#endif
+
 	MyPgXact->xmin = snap->xmin;
 
 	/* allocate in transaction context */

src/backend/replication/slot.c

@@ -398,6 +398,22 @@ ReplicationSlotRelease(void)
 		SpinLockRelease(&slot->mutex);
 	}
 
+
+	/*
+	 * If slot needed to temporarily restrain both data and catalog xmin to
+	 * create the catalog snapshot, remove that temporary constraint.
+	 * Snapshots can only be exported while the initial snapshot is still
+	 * acquired.
+	 */
+	if (!TransactionIdIsValid(slot->data.xmin) &&
+		TransactionIdIsValid(slot->effective_xmin))
+	{
+		SpinLockAcquire(&slot->mutex);
+		slot->effective_xmin = InvalidTransactionId;
+		SpinLockRelease(&slot->mutex);
+		ReplicationSlotsComputeRequiredXmin(false);
+	}
+
 	MyReplicationSlot = NULL;
 
 	/* might not have been set when we've been a plain slot */
@@ -612,6 +628,9 @@ ReplicationSlotPersist(void)
 
 /*
  * Compute the oldest xmin across all slots and store it in the ProcArray.
+ *
+ * If already_locked is true, ProcArrayLock has already been acquired
+ * exclusively.
  */
 void
 ReplicationSlotsComputeRequiredXmin(bool already_locked)
@@ -622,8 +641,7 @@ ReplicationSlotsComputeRequiredXmin(bool already_locked)
 
 	Assert(ReplicationSlotCtl != NULL);
 
-	if (!already_locked)
-		LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
+	LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
 
 	for (i = 0; i < max_replication_slots; i++)
 	{
@@ -652,8 +670,7 @@ ReplicationSlotsComputeRequiredXmin(bool already_locked)
 			agg_catalog_xmin = effective_catalog_xmin;
 	}
 
-	if (!already_locked)
-		LWLockRelease(ReplicationSlotControlLock);
+	LWLockRelease(ReplicationSlotControlLock);
 
 	ProcArraySetReplicationSlotXmin(agg_xmin, agg_catalog_xmin, already_locked);
 }

src/backend/replication/slotfuncs.c

@@ -131,8 +131,8 @@ pg_create_logical_replication_slot(PG_FUNCTION_ARGS)
 	/*
 	 * Create logical decoding context, to build the initial snapshot.
 	 */
-	ctx = CreateInitDecodingContext(
-									NameStr(*plugin), NIL,
+	ctx = CreateInitDecodingContext(NameStr(*plugin), NIL,
+									false, /* do not build snapshot */
 									logical_read_local_xlog_page, NULL, NULL);
 
 	/* build initial snapshot, might take a while */

src/backend/replication/walsender.c

@@ -909,6 +909,7 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
 		}
 
 		ctx = CreateInitDecodingContext(cmd->plugin, NIL,
+										true, /* build snapshot */
 										logical_read_xlog_page,
 										WalSndPrepareWrite, WalSndWriteData);
 

src/backend/storage/ipc/procarray.c

@@ -2151,7 +2151,7 @@ GetOldestActiveTransactionId(void)
  * that the caller will immediately use the xid to peg the xmin horizon.
  */
 TransactionId
-GetOldestSafeDecodingTransactionId(void)
+GetOldestSafeDecodingTransactionId(bool catalogOnly)
 {
 	ProcArrayStruct *arrayP = procArray;
 	TransactionId oldestSafeXid;
@@ -2174,9 +2174,17 @@ GetOldestSafeDecodingTransactionId(void)
 	/*
 	 * If there's already a slot pegging the xmin horizon, we can start with
 	 * that value, it's guaranteed to be safe since it's computed by this
-	 * routine initially and has been enforced since.
+	 * routine initially and has been enforced since.  We can always use the
+	 * slot's general xmin horizon, but the catalog horizon is only usable
+	 * when we only catalog data is going to be looked at.
 	 */
-	if (TransactionIdIsValid(procArray->replication_slot_catalog_xmin) &&
+	if (TransactionIdIsValid(procArray->replication_slot_xmin) &&
+		TransactionIdPrecedes(procArray->replication_slot_xmin,
+							  oldestSafeXid))
+		oldestSafeXid = procArray->replication_slot_xmin;
+
+	if (catalogOnly &&
+		TransactionIdIsValid(procArray->replication_slot_catalog_xmin) &&
 		TransactionIdPrecedes(procArray->replication_slot_catalog_xmin,
 							  oldestSafeXid))
 		oldestSafeXid = procArray->replication_slot_catalog_xmin;

src/include/replication/logical.h

@@ -82,6 +82,7 @@ extern void CheckLogicalDecodingRequirements(void);
 
 extern LogicalDecodingContext *CreateInitDecodingContext(char *plugin,
 						  List *output_plugin_options,
+						  bool need_full_snapshot,
 						  XLogPageReadCB read_page,
 						  LogicalOutputPluginWriterPrepareWrite prepare_write,
 						  LogicalOutputPluginWriterWrite do_write);

src/include/storage/procarray.h

@@ -88,7 +88,7 @@ extern bool TransactionIdIsInProgress(TransactionId xid);
 extern bool TransactionIdIsActive(TransactionId xid);
 extern TransactionId GetOldestXmin(Relation rel, int flags);
 extern TransactionId GetOldestActiveTransactionId(void);
-extern TransactionId GetOldestSafeDecodingTransactionId(void);
+extern TransactionId GetOldestSafeDecodingTransactionId(bool catalogOnly);
 
 extern VirtualTransactionId *GetVirtualXIDsDelayingChkpt(int *nvxids);
 extern bool HaveVirtualXIDsDelayingChkpt(VirtualTransactionId *vxids, int nvxids);