Rework WAL-reading supporting structs

The state-tracking of WAL reading in various places was pretty messy,
mostly because the ancient physical-replication WAL reading code wasn't
using the XLogReader abstraction.  This led to some untidy code.  Make
it prettier by creating two additional supporting structs,
WALSegmentContext and WALOpenSegment which keep track of WAL-reading
state.  This makes code cleaner, as well as supports more future
cleanup.

Author: Antonin Houska
Reviewed-by: Álvaro Herrera and (older versions) Robert Haas
Discussion: https://postgr.es/m/14984.1554998742@spoje.net

1 files changed, 14 insertions, 16 deletions

diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index 1fc39333f15..5f1e5ba75d5 100644
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -802,8 +802,8 @@ XLogRead(char *buf, int segsize, TimeLineID tli, XLogRecPtr startptr,
 void
 XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wantLength)
 {
-	const XLogRecPtr lastReadPage = state->readSegNo *
-	state->wal_segment_size + state->readOff;
+	const XLogRecPtr lastReadPage = state->seg.ws_segno *
+	state->segcxt.ws_segsize + state->seg.ws_off;
 
 	Assert(wantPage != InvalidXLogRecPtr && wantPage % XLOG_BLCKSZ == 0);
 	Assert(wantLength <= XLOG_BLCKSZ);
@@ -847,8 +847,8 @@ XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wa
 	if (state->currTLIValidUntil != InvalidXLogRecPtr &&
 		state->currTLI != ThisTimeLineID &&
 		state->currTLI != 0 &&
-		((wantPage + wantLength) / state->wal_segment_size) <
-		(state->currTLIValidUntil / state->wal_segment_size))
+		((wantPage + wantLength) / state->segcxt.ws_segsize) <
+		(state->currTLIValidUntil / state->segcxt.ws_segsize))
 		return;
 
 	/*
@@ -869,12 +869,12 @@ XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wa
 		 * by a promotion or replay from a cascaded replica.
 		 */
 		List	   *timelineHistory = readTimeLineHistory(ThisTimeLineID);
+		XLogRecPtr	endOfSegment;
 
-		XLogRecPtr	endOfSegment = (((wantPage / state->wal_segment_size) + 1)
-									* state->wal_segment_size) - 1;
-
-		Assert(wantPage / state->wal_segment_size ==
-			   endOfSegment / state->wal_segment_size);
+		endOfSegment = ((wantPage / state->segcxt.ws_segsize) + 1) *
+			state->segcxt.ws_segsize - 1;
+		Assert(wantPage / state->segcxt.ws_segsize ==
+			   endOfSegment / state->segcxt.ws_segsize);
 
 		/*
 		 * Find the timeline of the last LSN on the segment containing
@@ -909,8 +909,7 @@ XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wa
  */
 int
 read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
-					 int reqLen, XLogRecPtr targetRecPtr, char *cur_page,
-					 TimeLineID *pageTLI)
+					 int reqLen, XLogRecPtr targetRecPtr, char *cur_page)
 {
 	XLogRecPtr	read_upto,
 				loc;
@@ -933,8 +932,7 @@ read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
 			read_upto = GetFlushRecPtr();
 		else
 			read_upto = GetXLogReplayRecPtr(&ThisTimeLineID);
-
-		*pageTLI = ThisTimeLineID;
+		state->seg.ws_tli = ThisTimeLineID;
 
 		/*
 		 * Check which timeline to get the record from.
@@ -984,14 +982,14 @@ read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
 			read_upto = state->currTLIValidUntil;
 
 			/*
-			 * Setting pageTLI to our wanted record's TLI is slightly wrong;
+			 * Setting ws_tli to our wanted record's TLI is slightly wrong;
 			 * the page might begin on an older timeline if it contains a
 			 * timeline switch, since its xlog segment will have been copied
 			 * from the prior timeline. This is pretty harmless though, as
 			 * nothing cares so long as the timeline doesn't go backwards.  We
 			 * should read the page header instead; FIXME someday.
 			 */
-			*pageTLI = state->currTLI;
+			state->seg.ws_tli = state->currTLI;
 
 			/* No need to wait on a historical timeline */
 			break;
@@ -1022,7 +1020,7 @@ read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
 	 * as 'count', read the whole page anyway. It's guaranteed to be
 	 * zero-padded up to the page boundary if it's incomplete.
 	 */
-	XLogRead(cur_page, state->wal_segment_size, *pageTLI, targetPagePtr,
+	XLogRead(cur_page, state->segcxt.ws_segsize, state->seg.ws_tli, targetPagePtr,
 			 XLOG_BLCKSZ);
 
 	/* number of valid bytes in the buffer */