Enable logical slots to follow timeline switches

When decoding from a logical slot, it's necessary for xlog reading to be
able to read xlog from historical (i.e. not current) timelines;
otherwise, decoding fails after failover, because the archives are in
the historical timeline.  This is required to make "failover logical
slots" possible; it currently has no other use, although theoretically
it could be used by an extension that creates a slot on a standby and
continues to replay from the slot when the standby is promoted.

This commit includes a module in src/test/modules with functions to
manipulate the slots (which is not otherwise possible in SQL code) in
order to enable testing, and a new test in src/test/recovery to ensure
that the behavior is as expected.

Author: Craig Ringer
Reviewed-By: Oleksii Kliukin, Andres Freund, Petr Jelínek

1 files changed, 9 insertions, 8 deletions

diff --git a/src/backend/replication/logical/logicalfuncs.c b/src/backend/replication/logical/logicalfuncs.c
index 3853ab4cf5f..dd6cd62ccd1 100644
--- a/src/backend/replication/logical/logicalfuncs.c
+++ b/src/backend/replication/logical/logicalfuncs.c
@@ -231,12 +231,6 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 	rsinfo->setResult = p->tupstore;
 	rsinfo->setDesc = p->tupdesc;
 
-	/* compute the current end-of-wal */
-	if (!RecoveryInProgress())
-		end_of_wal = GetFlushRecPtr();
-	else
-		end_of_wal = GetXLogReplayRecPtr(NULL);
-
 	ReplicationSlotAcquire(NameStr(*name));
 
 	PG_TRY();
@@ -273,7 +267,8 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 		 * slot's confirmed_flush. This means we might read xlog we don't
 		 * actually decode rows from, but the snapshot builder might need it
 		 * to get to a consistent point. The point we start returning data to
-		 * *users* at is the candidate restart lsn from the decoding context.
+		 * *users* at is the confirmed_flush lsn set up in the decoding
+		 * context.
 		 */
 		startptr = MyReplicationSlot->data.restart_lsn;
 
@@ -282,8 +277,14 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 		/* invalidate non-timetravel entries */
 		InvalidateSystemCaches();
 
+		if (!RecoveryInProgress())
+			end_of_wal = GetFlushRecPtr();
+		else
+			end_of_wal = GetXLogReplayRecPtr(NULL);
+
+		/* Decode until we run out of records */
 		while ((startptr != InvalidXLogRecPtr && startptr < end_of_wal) ||
-			 (ctx->reader->EndRecPtr && ctx->reader->EndRecPtr < end_of_wal))
+			   (ctx->reader->EndRecPtr != InvalidXLogRecPtr && ctx->reader->EndRecPtr < end_of_wal))
 		{
 			XLogRecord *record;
 			char	   *errm = NULL;