Fix the logical replication timeout during large transactions.

The problem is that we don't send keep-alive messages for a long time
while processing large transactions during logical replication where we
don't send any data of such transactions. This can happen when the table
modified in the transaction is not published or because all the changes
got filtered. We do try to send the keep_alive if necessary at the end of
the transaction (via WalSndWriteData()) but by that time the
subscriber-side can timeout and exit.

To fix this we try to send the keepalive message if required after
processing certain threshold of changes.

Reported-by: Fabrice Chapuis
Author: Wang wei and Amit Kapila
Reviewed By: Masahiko Sawada, Euler Taveira, Hou Zhijie, Hayato Kuroda
Backpatch-through: 10
Discussion: https://postgr.es/m/CAA5-nLARN7-3SLU_QUxfy510pmrYK6JJb=bk3hcgemAM_pAv+w@mail.gmail.com

Author: Amit Kapila

src/backend/replication/logical/logical.c

@@ -622,6 +622,7 @@ startup_cb_wrapper(LogicalDecodingContext *ctx, OutputPluginOptions *opt, bool i
 
 	/* set output state */
 	ctx->accept_writes = false;
+	ctx->end_xact = false;
 
 	/* do the actual work: call callback */
 	ctx->callbacks.startup_cb(ctx, opt, is_init);
@@ -649,6 +650,7 @@ shutdown_cb_wrapper(LogicalDecodingContext *ctx)
 
 	/* set output state */
 	ctx->accept_writes = false;
+	ctx->end_xact = false;
 
 	/* do the actual work: call callback */
 	ctx->callbacks.shutdown_cb(ctx);
@@ -684,6 +686,7 @@ begin_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn)
 	ctx->accept_writes = true;
 	ctx->write_xid = txn->xid;
 	ctx->write_location = txn->first_lsn;
+	ctx->end_xact = false;
 
 	/* do the actual work: call callback */
 	ctx->callbacks.begin_cb(ctx, txn);
@@ -715,6 +718,7 @@ commit_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	ctx->accept_writes = true;
 	ctx->write_xid = txn->xid;
 	ctx->write_location = txn->end_lsn; /* points to the end of the record */
+	ctx->end_xact = true;
 
 	/* do the actual work: call callback */
 	ctx->callbacks.commit_cb(ctx, txn, commit_lsn);
@@ -754,6 +758,8 @@ change_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	 */
 	ctx->write_location = change->lsn;
 
+	ctx->end_xact = false;
+
 	ctx->callbacks.change_cb(ctx, txn, relation, change);
 
 	/* Pop the error context stack */
@@ -794,6 +800,8 @@ truncate_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	 */
 	ctx->write_location = change->lsn;
 
+	ctx->end_xact = false;
+
 	ctx->callbacks.truncate_cb(ctx, txn, nrelations, relations, change);
 
 	/* Pop the error context stack */
@@ -820,6 +828,7 @@ filter_by_origin_cb_wrapper(LogicalDecodingContext *ctx, RepOriginId origin_id)
 
 	/* set output state */
 	ctx->accept_writes = false;
+	ctx->end_xact = false;
 
 	/* do the actual work: call callback */
 	ret = ctx->callbacks.filter_by_origin_cb(ctx, origin_id);
@@ -857,6 +866,7 @@ message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	ctx->accept_writes = true;
 	ctx->write_xid = txn != NULL ? txn->xid : InvalidTransactionId;
 	ctx->write_location = message_lsn;
+	ctx->end_xact = false;
 
 	/* do the actual work: call callback */
 	ctx->callbacks.message_cb(ctx, txn, message_lsn, transactional, prefix,

src/backend/replication/pgoutput/pgoutput.c

@@ -53,6 +53,7 @@ static List *LoadPublications(List *pubnames);
 static void publication_invalidation_cb(Datum arg, int cacheid,
 										uint32 hashvalue);
 static void send_relation_and_attrs(Relation relation, LogicalDecodingContext *ctx);
+static void update_replication_progress(LogicalDecodingContext *ctx);
 
 /*
  * Entry in the map used to remember which relation schemas we sent.
@@ -277,7 +278,7 @@ static void
 pgoutput_commit_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 					XLogRecPtr commit_lsn)
 {
-	OutputPluginUpdateProgress(ctx);
+	update_replication_progress(ctx);
 
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_commit(ctx->out, txn, commit_lsn);
@@ -385,6 +386,8 @@ pgoutput_change(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	RelationSyncEntry *relentry;
 	Relation	ancestor = NULL;
 
+	update_replication_progress(ctx);
+
 	if (!is_publishable_relation(relation))
 		return;
 
@@ -514,6 +517,8 @@ pgoutput_truncate(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	int			nrelids;
 	Oid		   *relids;
 
+	update_replication_progress(ctx);
+
 	old = MemoryContextSwitchTo(data->context);
 
 	relids = palloc0(nrelations * sizeof(Oid));
@@ -912,3 +917,36 @@ rel_sync_cache_publication_cb(Datum arg, int cacheid, uint32 hashvalue)
 	while ((entry = (RelationSyncEntry *) hash_seq_search(&status)) != NULL)
 		entry->replicate_valid = false;
 }
+
+/*
+ * Try to update progress and send a keepalive message if too many changes were
+ * processed.
+ *
+ * For a large transaction, if we don't send any change to the downstream for a
+ * long time (exceeds the wal_receiver_timeout of standby) then it can timeout.
+ * This can happen when all or most of the changes are not published.
+ */
+static void
+update_replication_progress(LogicalDecodingContext *ctx)
+{
+	static int	changes_count = 0;
+
+	/*
+	 * We don't want to try sending a keepalive message after processing each
+	 * change as that can have overhead. Tests revealed that there is no
+	 * noticeable overhead in doing it after continuously processing 100 or so
+	 * changes.
+	 */
+#define CHANGES_THRESHOLD 100
+
+	/*
+	 * If we are at the end of transaction LSN, update progress tracking.
+	 * Otherwise, after continuously processing CHANGES_THRESHOLD changes, we
+	 * try to send a keepalive message if required.
+	 */
+	if (ctx->end_xact || ++changes_count >= CHANGES_THRESHOLD)
+	{
+		OutputPluginUpdateProgress(ctx);
+		changes_count = 0;
+	}
+}

src/backend/replication/walsender.c

@@ -240,6 +240,7 @@ static void ProcessStandbyMessage(void);
 static void ProcessStandbyReplyMessage(void);
 static void ProcessStandbyHSFeedbackMessage(void);
 static void ProcessRepliesIfAny(void);
+static void ProcessPendingWrites(void);
 static void WalSndKeepalive(bool requestReply);
 static void WalSndKeepaliveIfNecessary(void);
 static void WalSndCheckTimeOut(void);
@@ -1295,6 +1296,16 @@ WalSndWriteData(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
 	}
 
 	/* If we have pending write here, go to slow path */
+	ProcessPendingWrites();
+}
+
+/*
+ * Wait until there is no pending write. Also process replies from the other
+ * side and check timeouts during that.
+ */
+static void
+ProcessPendingWrites(void)
+{
 	for (;;)
 	{
 		int			wakeEvents;
@@ -1354,18 +1365,35 @@ WalSndUpdateProgress(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId
 {
 	static TimestampTz sendTime = 0;
 	TimestampTz now = GetCurrentTimestamp();
+	bool		end_xact = ctx->end_xact;
 
 	/*
 	 * Track lag no more than once per WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS to
 	 * avoid flooding the lag tracker when we commit frequently.
+	 *
+	 * We don't have a mechanism to get the ack for any LSN other than end
+	 * xact LSN from the downstream. So, we track lag only for end of
+	 * transaction LSN.
 	 */
 #define WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS	1000
-	if (!TimestampDifferenceExceeds(sendTime, now,
-									WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS))
-		return;
+	if (end_xact && TimestampDifferenceExceeds(sendTime, now,
+											   WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS))
+	{
+		LagTrackerWrite(lsn, now);
+		sendTime = now;
+	}
 
-	LagTrackerWrite(lsn, now);
-	sendTime = now;
+	/*
+	 * Try to send a keepalive if required. We don't need to try sending keep
+	 * alive messages at the transaction end as that will be done at a later
+	 * point in time. This is required only for large transactions where we
+	 * don't send any changes to the downstream and the receiver can timeout
+	 * due to that.
+	 */
+	if (!end_xact &&
+		now >= TimestampTzPlusMilliseconds(last_reply_timestamp,
+										   wal_sender_timeout / 2))
+		ProcessPendingWrites();
 }
 
 /*

src/include/replication/logical.h

@@ -49,6 +49,9 @@ typedef struct LogicalDecodingContext
 	 */
 	bool		fast_forward;
 
+	/* Are we processing the end LSN of a transaction? */
+	bool		end_xact;
+
 	OutputPluginCallbacks callbacks;
 	OutputPluginOptions options;