Migrate logical slots to the new node during an upgrade.

While reading information from the old cluster, a list of logical
slots is fetched. At the later part of upgrading, pg_upgrade revisits the
list and restores slots by executing pg_create_logical_replication_slot()
on the new cluster. Migration of logical replication slots is only
supported when the old cluster is version 17.0 or later.

If the old node has invalid slots or slots with unconsumed WAL records,
the pg_upgrade fails. These checks are needed to prevent data loss.

The significant advantage of this commit is that it makes it easy to
continue logical replication even after upgrading the publisher node.
Previously, pg_upgrade allowed copying publications to a new node. With
this patch, adjusting the connection string to the new publisher will
cause the apply worker on the subscriber to connect to the new publisher
automatically. This enables seamless continuation of logical replication,
even after an upgrade.

Author: Hayato Kuroda, Hou Zhijie
Reviewed-by: Peter Smith, Bharath Rupireddy, Dilip Kumar, Vignesh C, Shlok Kyal
Discussion: http://postgr.es/m/TYAPR01MB58664C81887B3AF2EB6B16E3F5939@TYAPR01MB5866.jpnprd01.prod.outlook.com
Discussion: http://postgr.es/m/CAA4eK1+t7xYcfa0rEQw839=b2MzsfvYDPz3xbD+ZqOdP3zpKYg@mail.gmail.com

1 files changed, 39 insertions, 9 deletions

diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c
index 24b712aa667..1237118e84f 100644
--- a/src/backend/replication/logical/decode.c
+++ b/src/backend/replication/logical/decode.c
@@ -600,12 +600,8 @@ logicalmsg_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
 
 	ReorderBufferProcessXid(ctx->reorder, XLogRecGetXid(r), buf->origptr);
 
-	/*
-	 * If we don't have snapshot or we are just fast-forwarding, there is no
-	 * point in decoding messages.
-	 */
-	if (SnapBuildCurrentState(builder) < SNAPBUILD_FULL_SNAPSHOT ||
-		ctx->fast_forward)
+	/* If we don't have snapshot, there is no point in decoding messages */
+	if (SnapBuildCurrentState(builder) < SNAPBUILD_FULL_SNAPSHOT)
 		return;
 
 	message = (xl_logical_message *) XLogRecGetData(r);
@@ -623,6 +619,26 @@ logicalmsg_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
 		return;
 
 	/*
+	 * We also skip decoding in fast_forward mode. This check must be last
+	 * because we don't want to set the processing_required flag unless we
+	 * have a decodable message.
+	 */
+	if (ctx->fast_forward)
+	{
+		/*
+		 * We need to set processing_required flag to notify the message's
+		 * existence to the caller. Usually, the flag is set when either the
+		 * COMMIT or ABORT records are decoded, but this must be turned on
+		 * here because the non-transactional logical message is decoded
+		 * without waiting for these records.
+		 */
+		if (!message->transactional)
+			ctx->processing_required = true;
+
+		return;
+	}
+
+	/*
 	 * If this is a non-transactional change, get the snapshot we're expected
 	 * to use. We only get here when the snapshot is consistent, and the
 	 * change is not meant to be skipped.
@@ -1286,7 +1302,21 @@ static bool
 DecodeTXNNeedSkip(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
 				  Oid txn_dbid, RepOriginId origin_id)
 {
-	return (SnapBuildXactNeedsSkip(ctx->snapshot_builder, buf->origptr) ||
-			(txn_dbid != InvalidOid && txn_dbid != ctx->slot->data.database) ||
-			ctx->fast_forward || FilterByOrigin(ctx, origin_id));
+	if (SnapBuildXactNeedsSkip(ctx->snapshot_builder, buf->origptr) ||
+		(txn_dbid != InvalidOid && txn_dbid != ctx->slot->data.database) ||
+		FilterByOrigin(ctx, origin_id))
+		return true;
+
+	/*
+	 * We also skip decoding in fast_forward mode. In passing set the
+	 * processing_required flag to indicate that if it were not for
+	 * fast_forward mode, processing would have been required.
+	 */
+	if (ctx->fast_forward)
+	{
+		ctx->processing_required = true;
+		return true;
+	}
+
+	return false;
 }