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

src/include/replication/logical.h

@@ -109,6 +109,9 @@ typedef struct LogicalDecodingContext
 	TransactionId write_xid;
 	/* Are we processing the end LSN of a transaction? */
 	bool		end_xact;
+
+	/* Do we need to process any change in fast_forward mode? */
+	bool		processing_required;
 } LogicalDecodingContext;
 
 
@@ -145,4 +148,6 @@ extern bool filter_by_origin_cb_wrapper(LogicalDecodingContext *ctx, RepOriginId
 extern void ResetLogicalStreamingState(void);
 extern void UpdateDecodingStats(LogicalDecodingContext *ctx);
 
+extern bool LogicalReplicationSlotHasPendingWal(XLogRecPtr end_of_wal);
+
 #endif