XLogReader general code cleanup

Some minor tweaks and comment additions, for cleanliness sake and to
avoid having the upcoming timeline-following patch be polluted with
unrelated cleanup.

Extracted from a larger patch by Craig Ringer, reviewed by Andres
Freund, with some additions by myself.

1 files changed, 17 insertions, 1 deletions

diff --git a/src/backend/replication/logical/logicalfuncs.c b/src/backend/replication/logical/logicalfuncs.c
index f789fc127d0..3853ab4cf5f 100644
--- a/src/backend/replication/logical/logicalfuncs.c
+++ b/src/backend/replication/logical/logicalfuncs.c
@@ -115,7 +115,7 @@ logical_read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
 	int reqLen, XLogRecPtr targetRecPtr, char *cur_page, TimeLineID *pageTLI)
 {
 	return read_local_xlog_page(state, targetPagePtr, reqLen,
-						 targetRecPtr, cur_page, pageTLI);
+								targetRecPtr, cur_page, pageTLI);
 }
 
 /*
@@ -241,6 +241,10 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 
 	PG_TRY();
 	{
+		/*
+		 * Passing InvalidXLogRecPtr here causes replay to start at the slot's
+		 * confirmed_flush.
+		 */
 		ctx = CreateDecodingContext(InvalidXLogRecPtr,
 									options,
 									logical_read_local_xlog_page,
@@ -263,6 +267,14 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 
 		ctx->output_writer_private = p;
 
+		/*
+		 * We start reading xlog from the restart lsn, even though in
+		 * CreateDecodingContext we set the snapshot builder up using the
+		 * 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.
+		 */
 		startptr = MyReplicationSlot->data.restart_lsn;
 
 		CurrentResourceOwner = ResourceOwnerCreate(CurrentResourceOwner, "logical decoding");
@@ -280,6 +292,10 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 			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"
+			 */
 			startptr = InvalidXLogRecPtr;
 
 			/*