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

src/backend/replication/logical/logical.c

@@ -746,6 +746,7 @@ 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);
@@ -773,6 +774,7 @@ 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);
@@ -808,6 +810,7 @@ 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);
@@ -839,6 +842,7 @@ 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);
@@ -879,6 +883,7 @@ begin_prepare_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;
 
 	/*
 	 * If the plugin supports two-phase commits then begin prepare callback is
@@ -923,6 +928,7 @@ prepare_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;
 
 	/*
 	 * If the plugin supports two-phase commits then prepare callback is
@@ -967,6 +973,7 @@ commit_prepared_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;
 
 	/*
 	 * If the plugin support two-phase commits then commit prepared callback
@@ -1012,6 +1019,7 @@ rollback_prepared_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;
 
 	/*
 	 * If the plugin support two-phase commits then rollback prepared callback
@@ -1062,6 +1070,8 @@ 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 */
@@ -1102,6 +1112,8 @@ 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 */
@@ -1129,6 +1141,7 @@ filter_prepare_cb_wrapper(LogicalDecodingContext *ctx, TransactionId xid,
 
 	/* set output state */
 	ctx->accept_writes = false;
+	ctx->end_xact = false;
 
 	/* do the actual work: call callback */
 	ret = ctx->callbacks.filter_prepare_cb(ctx, xid, gid);
@@ -1159,6 +1172,7 @@ 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);
@@ -1196,6 +1210,7 @@ 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,
@@ -1239,6 +1254,8 @@ stream_start_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	 */
 	ctx->write_location = first_lsn;
 
+	ctx->end_xact = false;
+
 	/* in streaming mode, stream_start_cb is required */
 	if (ctx->callbacks.stream_start_cb == NULL)
 		ereport(ERROR,
@@ -1286,6 +1303,8 @@ stream_stop_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	 */
 	ctx->write_location = last_lsn;
 
+	ctx->end_xact = false;
+
 	/* in streaming mode, stream_stop_cb is required */
 	if (ctx->callbacks.stream_stop_cb == NULL)
 		ereport(ERROR,
@@ -1325,6 +1344,7 @@ stream_abort_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	ctx->accept_writes = true;
 	ctx->write_xid = txn->xid;
 	ctx->write_location = abort_lsn;
+	ctx->end_xact = true;
 
 	/* in streaming mode, stream_abort_cb is required */
 	if (ctx->callbacks.stream_abort_cb == NULL)
@@ -1369,6 +1389,7 @@ stream_prepare_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	ctx->accept_writes = true;
 	ctx->write_xid = txn->xid;
 	ctx->write_location = txn->end_lsn;
+	ctx->end_xact = true;
 
 	/* in streaming mode with two-phase commits, stream_prepare_cb is required */
 	if (ctx->callbacks.stream_prepare_cb == NULL)
@@ -1409,6 +1430,7 @@ stream_commit_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	ctx->accept_writes = true;
 	ctx->write_xid = txn->xid;
 	ctx->write_location = txn->end_lsn;
+	ctx->end_xact = true;
 
 	/* in streaming mode, stream_commit_cb is required */
 	if (ctx->callbacks.stream_commit_cb == NULL)
@@ -1457,6 +1479,8 @@ stream_change_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	 */
 	ctx->write_location = change->lsn;
 
+	ctx->end_xact = false;
+
 	/* in streaming mode, stream_change_cb is required */
 	if (ctx->callbacks.stream_change_cb == NULL)
 		ereport(ERROR,
@@ -1501,6 +1525,7 @@ stream_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.stream_message_cb(ctx, txn, message_lsn, transactional, prefix,
@@ -1549,6 +1574,8 @@ stream_truncate_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 	 */
 	ctx->write_location = change->lsn;
 
+	ctx->end_xact = false;
+
 	ctx->callbacks.stream_truncate_cb(ctx, txn, nrelations, relations, change);
 
 	/* Pop the error context stack */

src/backend/replication/pgoutput/pgoutput.c

@@ -91,6 +91,8 @@ static void send_relation_and_attrs(Relation relation, TransactionId xid,
 static void send_repl_origin(LogicalDecodingContext *ctx,
 							 RepOriginId origin_id, XLogRecPtr origin_lsn,
 							 bool send_origin);
+static void update_replication_progress(LogicalDecodingContext *ctx,
+										bool skipped_xact);
 
 /*
  * Only 3 publication actions are used for row filtering ("insert", "update",
@@ -558,7 +560,7 @@ pgoutput_commit_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	 * from this transaction has been sent to the downstream.
 	 */
 	sent_begin_txn = txndata->sent_begin_txn;
-	OutputPluginUpdateProgress(ctx, !sent_begin_txn);
+	update_replication_progress(ctx, !sent_begin_txn);
 	pfree(txndata);
 	txn->output_plugin_private = NULL;
 
@@ -597,7 +599,7 @@ static void
 pgoutput_prepare_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 					 XLogRecPtr prepare_lsn)
 {
-	OutputPluginUpdateProgress(ctx, false);
+	update_replication_progress(ctx, false);
 
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_prepare(ctx->out, txn, prepare_lsn);
@@ -611,7 +613,7 @@ static void
 pgoutput_commit_prepared_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 							 XLogRecPtr commit_lsn)
 {
-	OutputPluginUpdateProgress(ctx, false);
+	update_replication_progress(ctx, false);
 
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_commit_prepared(ctx->out, txn, commit_lsn);
@@ -627,7 +629,7 @@ pgoutput_rollback_prepared_txn(LogicalDecodingContext *ctx,
 							   XLogRecPtr prepare_end_lsn,
 							   TimestampTz prepare_time)
 {
-	OutputPluginUpdateProgress(ctx, false);
+	update_replication_progress(ctx, false);
 
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_rollback_prepared(ctx->out, txn, prepare_end_lsn,
@@ -1360,6 +1362,8 @@ pgoutput_change(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	TupleTableSlot *old_slot = NULL;
 	TupleTableSlot *new_slot = NULL;
 
+	update_replication_progress(ctx, false);
+
 	if (!is_publishable_relation(relation))
 		return;
 
@@ -1592,6 +1596,8 @@ pgoutput_truncate(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	Oid		   *relids;
 	TransactionId xid = InvalidTransactionId;
 
+	update_replication_progress(ctx, false);
+
 	/* Remember the xid for the change in streaming mode. See pgoutput_change. */
 	if (in_streaming)
 		xid = change->txn->xid;
@@ -1655,6 +1661,8 @@ pgoutput_message(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	PGOutputData *data = (PGOutputData *) ctx->output_plugin_private;
 	TransactionId xid = InvalidTransactionId;
 
+	update_replication_progress(ctx, false);
+
 	if (!data->messages)
 		return;
 
@@ -1847,7 +1855,7 @@ pgoutput_stream_commit(struct LogicalDecodingContext *ctx,
 	Assert(!in_streaming);
 	Assert(rbtxn_is_streamed(txn));
 
-	OutputPluginUpdateProgress(ctx, false);
+	update_replication_progress(ctx, false);
 
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_stream_commit(ctx->out, txn, commit_lsn);
@@ -1868,7 +1876,7 @@ pgoutput_stream_prepare_txn(LogicalDecodingContext *ctx,
 {
 	Assert(rbtxn_is_streamed(txn));
 
-	OutputPluginUpdateProgress(ctx, false);
+	update_replication_progress(ctx, false);
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_stream_prepare(ctx->out, txn, prepare_lsn);
 	OutputPluginWrite(ctx, true);
@@ -2361,3 +2369,37 @@ send_repl_origin(LogicalDecodingContext *ctx, RepOriginId origin_id,
 		}
 	}
 }
+
+/*
+ * 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 either not published or
+ * got filtered out.
+ */
+static void
+update_replication_progress(LogicalDecodingContext *ctx, bool skipped_xact)
+{
+	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, skipped_xact);
+		changes_count = 0;
+	}
+}

src/backend/replication/walsender.c

@@ -1482,14 +1482,20 @@ WalSndUpdateProgress(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId
 {
 	static TimestampTz sendTime = 0;
 	TimestampTz now = GetCurrentTimestamp();
+	bool		pending_writes = false;
+	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))
+	if (end_xact && TimestampDifferenceExceeds(sendTime, now,
+											   WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS))
 	{
 		LagTrackerWrite(lsn, now);
 		sendTime = now;
@@ -1515,8 +1521,20 @@ WalSndUpdateProgress(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId
 
 		/* If we have pending write here, make sure it's actually flushed */
 		if (pq_is_send_pending())
-			ProcessPendingWrites();
+			pending_writes = true;
 	}
+
+	/*
+	 * Process pending writes if any or 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 (pending_writes || (!end_xact &&
+						   now >= TimestampTzPlusMilliseconds(last_reply_timestamp,
+															  wal_sender_timeout / 2)))
+		ProcessPendingWrites();
 }
 
 /*

src/include/replication/logical.h

@@ -107,6 +107,8 @@ typedef struct LogicalDecodingContext
 	bool		prepared_write;
 	XLogRecPtr	write_location;
 	TransactionId write_xid;
+	/* Are we processing the end LSN of a transaction? */
+	bool		end_xact;
 } LogicalDecodingContext;