Subject: [PATCH] Fix offset of pg_walfile_name_offset.

From: Kyotaro Horiguchi <[email-masked]>
Date: Fri, 26 Jul 2019 17:12:24 +0900

The function returns the name of the previous segment with the offset
of the given location at segment boundaries. For example, it returns
("...16", 0) returned for '0/17000000'. That is inconsistent, or at
least confusing. This patch changes the function to return the given
LSN - 1 as offset.
---
 src/backend/access/transam/xlogfuncs.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/backend/access/transam/xlogfuncs.c b/src/backend/access/transam/xlogfuncs.c
index b35043bf71..79ea0495b4 100644
--- a/src/backend/access/transam/xlogfuncs.c
+++ b/src/backend/access/transam/xlogfuncs.c
@@ -447,6 +447,8 @@ pg_last_wal_replay_lsn(PG_FUNCTION_ARGS)
  * Note that a location exactly at a segment boundary is taken to be in
  * the previous segment.  This is usually the right thing, since the
  * expected usage is to determine which xlog file(s) are ready to archive.
+ * To be consistent to filename, returns the offset one byte before the given
+ * location as offset.
  */
 Datum
 pg_walfile_name_offset(PG_FUNCTION_ARGS)
@@ -480,10 +482,16 @@ pg_walfile_name_offset(PG_FUNCTION_ARGS)
 
 	resultTupleDesc = BlessTupleDesc(resultTupleDesc);
 
+	/*
+	 * We assume the given location point as one-byte after the location
+	 * really wanted.
+	 */
+	locationpoint--;
+
 	/*
 	 * xlogfilename
 	 */
-	XLByteToPrevSeg(locationpoint, xlogsegno, wal_segment_size);
+	XLByteToSeg(locationpoint, xlogsegno, wal_segment_size);
 	XLogFileName(xlogfilename, ThisTimeLineID, xlogsegno, wal_segment_size);
 
 	values[0] = CStringGetTextDatum(xlogfilename);
-- 
2.16.3

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