Rewrite comments in replication slot advance implementation

The code added by 9c7d06d6 was a bit obscure; clarify that by
rewriting the comments.  Lack of clarity has already caused bugs, so
it's a worthy goal.
Co-authored-by: Arseny Sher <a.sher@postgrespro.ru>
Co-authored-by: Michaël Paquier <michael@paquier.xyz>
Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Petr Jelínek <petr.jelinek@2ndquadrant.com>
Discussion: https://postgr.es/m/87y3fgoyrn.fsf@ars-thinkpad

src/backend/replication/logical/logical.c

@@ -338,7 +338,10 @@ CreateInitDecodingContext(char *plugin,
  *		that, see below).
  *
  * output_plugin_options
- *		contains options passed to the output plugin.
+ *		options passed to the output plugin.
+ *
+ * fast_forward
+ *		bypass the generation of logical changes.
  *
  * read_page, prepare_write, do_write, update_progress
  *		callbacks that have to be filled to perform the use-case dependent,

src/backend/replication/slotfuncs.c

@@ -317,10 +317,11 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 }
 
 /*
- * Helper function for advancing physical replication slot forward.
- * The LSN position to move to is compared simply to the slot's
- * restart_lsn, knowing that any position older than that would be
- * removed by successive checkpoints.
+ * Helper function for advancing our physical replication slot forward.
+ *
+ * The LSN position to move to is compared simply to the slot's restart_lsn,
+ * knowing that any position older than that would be removed by successive
+ * checkpoints.
  */
 static XLogRecPtr
 pg_physical_replication_slot_advance(XLogRecPtr moveto)
@@ -340,59 +341,78 @@ pg_physical_replication_slot_advance(XLogRecPtr moveto)
 }
 
 /*
- * Helper function for advancing logical replication slot forward.
+ * Helper function for advancing our logical replication slot forward.
+ *
  * The slot's restart_lsn is used as start point for reading records,
  * while confirmed_lsn is used as base point for the decoding context.
- * The LSN position to move to is checked by doing a per-record scan and
- * logical decoding which makes sure that confirmed_lsn is updated to a
- * LSN which allows the future slot consumer to get consistent logical
- * changes.
+ *
+ * We cannot just do LogicalConfirmReceivedLocation to update confirmed_flush,
+ * because we need to digest WAL to advance restart_lsn allowing to recycle
+ * WAL and removal of old catalog tuples.  As decoding is done in fast_forward
+ * mode, no changes are generated anyway.
  */
 static XLogRecPtr
 pg_logical_replication_slot_advance(XLogRecPtr moveto)
 {
 	LogicalDecodingContext *ctx;
 	ResourceOwner old_resowner = CurrentResourceOwner;
-	XLogRecPtr	startlsn = MyReplicationSlot->data.restart_lsn;
-	XLogRecPtr	retlsn = MyReplicationSlot->data.confirmed_flush;
+	XLogRecPtr	startlsn;
+	XLogRecPtr	retlsn;
 
 	PG_TRY();
 	{
-		/* restart at slot's confirmed_flush */
+		/*
+		 * Create our decoding context in fast_forward mode, passing start_lsn
+		 * as InvalidXLogRecPtr, so that we start processing from my slot's
+		 * confirmed_flush.
+		 */
 		ctx = CreateDecodingContext(InvalidXLogRecPtr,
 									NIL,
-									true,
+									true,	/* fast_forward */
 									logical_read_local_xlog_page,
 									NULL, NULL, NULL);
 
+		/*
+		 * Start reading at the slot's restart_lsn, which we know to point to
+		 * a valid record.
+		 */
+		startlsn = MyReplicationSlot->data.restart_lsn;
+
+		/* Initialize our return value in case we don't do anything */
+		retlsn = MyReplicationSlot->data.confirmed_flush;
+
 		/* invalidate non-timetravel entries */
 		InvalidateSystemCaches();
 
-		/* Decode until we run out of records */
-		while ((startlsn != InvalidXLogRecPtr && startlsn < moveto) ||
-			   (ctx->reader->EndRecPtr != InvalidXLogRecPtr && ctx->reader->EndRecPtr < moveto))
+		/* Decode at least one record, until we run out of records */
+		while ((!XLogRecPtrIsInvalid(startlsn) &&
+				startlsn < moveto) ||
+			   (!XLogRecPtrIsInvalid(ctx->reader->EndRecPtr) &&
+				ctx->reader->EndRecPtr < moveto))
 		{
-			XLogRecord *record;
 			char	   *errm = NULL;
+			XLogRecord *record;
 
+			/*
+			 * Read records.  No changes are generated in fast_forward mode,
+			 * but snapbuilder/slot statuses are updated properly.
+			 */
 			record = XLogReadRecord(ctx->reader, startlsn, &errm);
 			if (errm)
 				elog(ERROR, "%s", errm);
 
-			/*
-			 * Now that we've set up the xlog reader state, subsequent calls
-			 * pass InvalidXLogRecPtr to say "continue from last record"
-			 */
+			/* Read sequentially from now on */
 			startlsn = InvalidXLogRecPtr;
 
 			/*
-			 * The {begin_txn,change,commit_txn}_wrapper callbacks above will
-			 * store the description into our tuplestore.
+			 * Process the record.  Storage-level changes are ignored in
+			 * fast_forward mode, but other modules (such as snapbuilder)
+			 * might still have critical updates to do.
 			 */
-			if (record != NULL)
+			if (record)
 				LogicalDecodingProcessRecord(ctx, ctx->reader);
 
-			/* Stop once the moving point wanted by caller has been reached */
+			/* Stop once the requested target has been reached */
 			if (moveto <= ctx->reader->EndRecPtr)
 				break;
 
@@ -411,7 +431,7 @@ pg_logical_replication_slot_advance(XLogRecPtr moveto)
 			LogicalConfirmReceivedLocation(moveto);
 
 			/*
-			 * If only the confirmed_flush_lsn has changed the slot won't get
+			 * If only the confirmed_flush LSN has changed the slot won't get
 			 * marked as dirty by the above. Callers on the walsender
 			 * interface are expected to keep track of their own progress and
 			 * don't need it written out. But SQL-interface users cannot