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, 29 insertions, 1 deletions

diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c
index 8fe7bb65f1f..5508cc21777 100644
--- a/src/backend/replication/logical/decode.c
+++ b/src/backend/replication/logical/decode.c
@@ -152,11 +152,39 @@ xlog_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
 			 * can restart from there.
 			 */
 			break;
+		case XLOG_PARAMETER_CHANGE:
+			{
+				xl_parameter_change *xlrec =
+				(xl_parameter_change *) XLogRecGetData(buf->record);
+
+				/*
+				 * If wal_level on the primary is reduced to less than
+				 * logical, we want to prevent existing logical slots from
+				 * being used.  Existing logical slots on the standby get
+				 * invalidated when this WAL record is replayed; and further,
+				 * slot creation fails when wal_level is not sufficient; but
+				 * all these operations are not synchronized, so a logical
+				 * slot may creep in while the wal_level is being
+				 * reduced. Hence this extra check.
+				 */
+				if (xlrec->wal_level < WAL_LEVEL_LOGICAL)
+				{
+					/*
+					 * This can occur only on a standby, as a primary would
+					 * not allow to restart after changing wal_level < logical
+					 * if there is pre-existing logical slot.
+					 */
+					Assert(RecoveryInProgress());
+					ereport(ERROR,
+							(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+							 errmsg("logical decoding on a standby requires wal_level to be at least logical on the primary")));
+				}
+				break;
+			}
 		case XLOG_NOOP:
 		case XLOG_NEXTOID:
 		case XLOG_SWITCH:
 		case XLOG_BACKUP_END:
-		case XLOG_PARAMETER_CHANGE:
 		case XLOG_RESTORE_POINT:
 		case XLOG_FPW_CHANGE:
 		case XLOG_FPI_FOR_HINT: