Allow logical decoding on standbys

Unsurprisingly, this requires wal_level = logical to be set on the primary and
standby. The infrastructure added in 26669757b6a ensures that slots are
invalidated if the primary's wal_level is lowered.

Creating a slot on a standby waits for a xl_running_xact record to be
processed. If the primary is idle (and thus not emitting xl_running_xact
records), that can take a while.  To make that faster, this commit also
introduces the pg_log_standby_snapshot() function. By executing it on the
primary, completion of slot creation on the standby can be accelerated.

Note that logical decoding on a standby does not itself enforce that required
catalog rows are not removed. The user has to use physical replication slots +
hot_standby_feedback or other measures to prevent that. If catalog rows
required for a slot are removed, the slot is invalidated.

See 6af1793954e for an overall design of logical decoding on a standby.

Bumps catversion, for the addition of the pg_log_standby_snapshot() function.

Author: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Author: Andres Freund <andres@anarazel.de> (in an older version)
Author: Amit Khandekar <amitdkhan.pg@gmail.com> (in an older version)
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: FabrÌzio de Royes Mello <fabriziomello@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>

1 files changed, 32 insertions, 16 deletions

diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 5423cf0a171..45b8b3684f6 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -906,23 +906,34 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 	int			count;
 	WALReadError errinfo;
 	XLogSegNo	segno;
-	TimeLineID	currTLI = GetWALInsertionTimeLine();
+	TimeLineID	currTLI;
+
+	/*
+	 * Make sure we have enough WAL available before retrieving the current
+	 * timeline. This is needed to determine am_cascading_walsender accurately
+	 * which is needed to determine the current timeline.
+	 */
+	flushptr = WalSndWaitForWal(targetPagePtr + reqLen);
 
 	/*
-	 * Since logical decoding is only permitted on a primary server, we know
-	 * that the current timeline ID can't be changing any more. If we did this
-	 * on a standby, we'd have to worry about the values we compute here
-	 * becoming invalid due to a promotion or timeline change.
+	 * Since logical decoding is also permitted on a standby server, we need
+	 * to check if the server is in recovery to decide how to get the current
+	 * timeline ID (so that it also cover the promotion or timeline change
+	 * cases).
 	 */
+	am_cascading_walsender = RecoveryInProgress();
+
+	if (am_cascading_walsender)
+		GetXLogReplayRecPtr(&currTLI);
+	else
+		currTLI = GetWALInsertionTimeLine();
+
 	XLogReadDetermineTimeline(state, targetPagePtr, reqLen, currTLI);
 	sendTimeLineIsHistoric = (state->currTLI != currTLI);
 	sendTimeLine = state->currTLI;
 	sendTimeLineValidUpto = state->currTLIValidUntil;
 	sendTimeLineNextTLI = state->nextTLI;
 
-	/* make sure we have enough WAL available */
-	flushptr = WalSndWaitForWal(targetPagePtr + reqLen);
-
 	/* fail if not (implies we are going to shut down) */
 	if (flushptr < targetPagePtr + reqLen)
 		return -1;
@@ -937,9 +948,9 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 				 cur_page,
 				 targetPagePtr,
 				 XLOG_BLCKSZ,
-				 state->seg.ws_tli, /* Pass the current TLI because only
-									 * WalSndSegmentOpen controls whether new
-									 * TLI is needed. */
+				 currTLI,		/* Pass the current TLI because only
+								 * WalSndSegmentOpen controls whether new TLI
+								 * is needed. */
 				 &errinfo))
 		WALReadRaiseError(&errinfo);
 
@@ -3076,10 +3087,14 @@ XLogSendLogical(void)
 	 * If first time through in this session, initialize flushPtr.  Otherwise,
 	 * we only need to update flushPtr if EndRecPtr is past it.
 	 */
-	if (flushPtr == InvalidXLogRecPtr)
-		flushPtr = GetFlushRecPtr(NULL);
-	else if (logical_decoding_ctx->reader->EndRecPtr >= flushPtr)
-		flushPtr = GetFlushRecPtr(NULL);
+	if (flushPtr == InvalidXLogRecPtr ||
+		logical_decoding_ctx->reader->EndRecPtr >= flushPtr)
+	{
+		if (am_cascading_walsender)
+			flushPtr = GetStandbyFlushRecPtr(NULL);
+		else
+			flushPtr = GetFlushRecPtr(NULL);
+	}
 
 	/* If EndRecPtr is still past our flushPtr, it means we caught up. */
 	if (logical_decoding_ctx->reader->EndRecPtr >= flushPtr)
@@ -3170,7 +3185,8 @@ GetStandbyFlushRecPtr(TimeLineID *tli)
 	receivePtr = GetWalRcvFlushRecPtr(NULL, &receiveTLI);
 	replayPtr = GetXLogReplayRecPtr(&replayTLI);
 
-	*tli = replayTLI;
+	if (tli)
+		*tli = replayTLI;
 
 	result = replayPtr;
 	if (receiveTLI == replayTLI && receivePtr > replayPtr)