From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 17 Jun 2026 07:05:11 +0000 (+0900)
Subject: Fix pgstat_count_io_op_time() calls passing incorrect information
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;ds=inline;p=thirdparty%2Fpostgresql.git

Fix pgstat_count_io_op_time() calls passing incorrect information

Several calls of pgstat_count_io_op_time() have been used as data to
count negative values returned by pg_pread() or pg_pwrite(), leading to
an incorrect count reported, casting them back to uint64.

Most of the problematic calls updated here are adjusted so as we do not
report buggy negative numbers anymore.  In xlogrecovery.c, the spot
updated still counts short reads.  In xlog.c, after a WAL segment
initialization, I/O numbers are aggregated only after checking that the
operation has succeeded.

issues introduced by a051e71e28a1.

Reported-by: Peter Eisentraut <peter@eisentraut.org>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Discussion: https://postgr.es/m/0db864e6-4477-4eba-b2be-d3523cc86564@eisentraut.org
Backpatch-through: 18
---

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 6c2304fef33..a81912b7441 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2455,9 +2455,6 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 				written = pg_pwrite(openLogFile, from, nleft, startoffset);
 				pgstat_report_wait_end();
 
-				pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL,
-										IOOP_WRITE, start, 1, written);
-
 				if (written <= 0)
 				{
 					char		xlogfname[MAXFNAMELEN];
@@ -2475,6 +2472,9 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 							 errmsg("could not write to log file \"%s\" at offset %u, length %zu: %m",
 									xlogfname, startoffset, nleft)));
 				}
+
+				pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL,
+										IOOP_WRITE, start, 1, written);
 				nleft -= written;
 				from += written;
 				startoffset += written;
@@ -3331,14 +3331,6 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
 	}
 	pgstat_report_wait_end();
 
-	/*
-	 * A full segment worth of data is written when using wal_init_zero. One
-	 * byte is written when not using it.
-	 */
-	pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_INIT, IOOP_WRITE,
-							io_start, 1,
-							wal_init_zero ? wal_segment_size : 1);
-
 	if (save_errno)
 	{
 		/*
@@ -3355,6 +3347,14 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
 				 errmsg("could not write to file \"%s\": %m", tmppath)));
 	}
 
+	/*
+	 * A full segment worth of data is written when using wal_init_zero. One
+	 * byte is written when not using it.
+	 */
+	pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_INIT, IOOP_WRITE,
+							io_start, 1,
+							wal_init_zero ? wal_segment_size : 1);
+
 	/* Measure I/O timing when flushing segment */
 	io_start = pgstat_prepare_io_time(track_wal_io_timing);
 
diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index 3145c58a9b1..9d64ae34932 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -1597,9 +1597,6 @@ WALRead(XLogReaderState *state,
 
 #ifndef FRONTEND
 		pgstat_report_wait_end();
-
-		pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL, IOOP_READ,
-								io_start, 1, readbytes);
 #endif
 
 		if (readbytes <= 0)
@@ -1612,6 +1609,11 @@ WALRead(XLogReaderState *state,
 			return false;
 		}
 
+#ifndef FRONTEND
+		pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL, IOOP_READ,
+								io_start, 1, readbytes);
+#endif
+
 		/* Update state for read */
 		recptr += readbytes;
 		nbytes -= readbytes;
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index 73b78a83fa7..4d61795b483 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -3390,8 +3390,10 @@ retry:
 
 		pgstat_report_wait_end();
 
-		pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL, IOOP_READ,
-								io_start, 1, r);
+		/* Count I/O stats only for successful short reads */
+		if (r > 0)
+			pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL, IOOP_READ,
+									io_start, 1, r);
 
 		XLogFileName(fname, curFileTLI, readSegNo, wal_segment_size);
 		if (r < 0)
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index d19317703c1..05e2f690fa7 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -954,9 +954,6 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr, TimeLineID tli)
 		byteswritten = pg_pwrite(recvFile, buf, segbytes, (pgoff_t) startoff);
 		pgstat_report_wait_end();
 
-		pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL,
-								IOOP_WRITE, start, 1, byteswritten);
-
 		if (byteswritten <= 0)
 		{
 			char		xlogfname[MAXFNAMELEN];
@@ -976,6 +973,9 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr, TimeLineID tli)
 							xlogfname, startoff, segbytes)));
 		}
 
+		pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL,
+								IOOP_WRITE, start, 1, byteswritten);
+
 		/* Update state for write */
 		recptr += byteswritten;