Date: Mon, 13 Jul 2020 13:23:56 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgbf@twiska.com, Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	masao.fujii@oss.nttdata.com, pgsql-hackers@lists.postgresql.org
Subject: Re: min_safe_lsn column in pg_replication_slots view
Message-ID: <20200713172356.GA446@alvherre.pgsql>
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A much more sensible answer is to initialize the segno to the segment
currently being written, as in the attached.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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 filename="v3-0001-Fix-uninitialized-value-in-segno-calculation.patch"

From 699e3a25e0673353fcb10fa92577f7534e594227 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Mon, 13 Jul 2020 13:22:40 -0400
Subject: [PATCH v3] Fix uninitialized value in segno calculation

Remove previous hack in KeepLogSeg that added a case to deal with a
(badly represented) invalid segment number; to replace it, initialize
the segment number to be retreated to the currently being written
segment.

Per valgrind-running buildfarm member skink, and some sparc64 animals.

Discussion: https://postgr.es/m/1724648.1594230917@sss.pgh.pa.us
---
 src/backend/access/transam/xlog.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 28daf72a50..0a97b1d37f 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -9523,13 +9523,13 @@ GetWALAvailability(XLogRecPtr targetLSN)
 	if (XLogRecPtrIsInvalid(targetLSN))
 		return WALAVAIL_INVALID_LSN;
 
-	currpos = GetXLogWriteRecPtr();
-
 	/*
-	 * calculate the oldest segment currently reserved by all slots,
-	 * considering wal_keep_segments and max_slot_wal_keep_size
+	 * Calculate the oldest segment currently reserved by all slots,
+	 * considering wal_keep_segments and max_slot_wal_keep_size.  Initialize
+	 * oldestSlotSeg to the current segment.
 	 */
-	XLByteToSeg(targetLSN, targetSeg, wal_segment_size);
+	currpos = GetXLogWriteRecPtr();
+	XLByteToSeg(currpos, oldestSlotSeg, wal_segment_size);
 	KeepLogSeg(currpos, &oldestSlotSeg);
 
 	/*
@@ -9548,6 +9548,9 @@ GetWALAvailability(XLogRecPtr targetLSN)
 	else
 		oldestSegMaxWalSize = 1;
 
+	/* the segment we care about */
+	XLByteToSeg(targetLSN, targetSeg, wal_segment_size);
+
 	/*
 	 * No point in returning reserved or extended status values if the
 	 * targetSeg is known to be lost.
@@ -9624,7 +9627,7 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
 	}
 
 	/* don't delete WAL segments newer than the calculated segment */
-	if (XLogRecPtrIsInvalid(*logSegNo) || segno < *logSegNo)
+	if (segno < *logSegNo)
 		*logSegNo = segno;
 }
 
-- 
2.20.1


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