read-errno-handling.patch

diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
index 87942b4cca..d487347cc6 100644
--- a/src/backend/access/transam/slru.c
+++ b/src/backend/access/transam/slru.c
@@ -683,6 +683,11 @@ SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno)
 	pgstat_report_wait_start(WAIT_EVENT_SLRU_READ);
 	if (read(fd, shared->page_buffer[slotno], BLCKSZ) != BLCKSZ)
 	{
+		/*
+		 * XXX: Note that this may actually report sucess if the number
+		 * of bytes read is positive, but lacking data so that errno is
+		 * not set.
+		 */
 		pgstat_report_wait_end();
 		slru_errcause = SLRU_READ_FAILED;
 		slru_errno = errno;
diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index 65194db70e..52f634e706 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -1219,6 +1219,7 @@ ReadTwoPhaseFile(TransactionId xid, bool give_warnings)
 	uint32		crc_offset;
 	pg_crc32c	calc_crc,
 				file_crc;
+	int			r;
 
 	TwoPhaseFilePath(path, xid);
 
@@ -1272,15 +1273,28 @@ ReadTwoPhaseFile(TransactionId xid, bool give_warnings)
 	buf = (char *) palloc(stat.st_size);
 
 	pgstat_report_wait_start(WAIT_EVENT_TWOPHASE_FILE_READ);
-	if (read(fd, buf, stat.st_size) != stat.st_size)
+	r = read(fd, buf, stat.st_size);
+	if (r != stat.st_size)
 	{
+		int		save_errno = errno;
+
 		pgstat_report_wait_end();
 		CloseTransientFile(fd);
 		if (give_warnings)
-			ereport(WARNING,
-					(errcode_for_file_access(),
-					 errmsg("could not read two-phase state file \"%s\": %m",
-							path)));
+		{
+			if (r < 0)
+			{
+				errno = save_errno;
+				ereport(WARNING,
+						(errcode_for_file_access(),
+						 errmsg("could not read two-phase state file \"%s\": %m",
+								path)));
+			}
+			else
+				ereport(WARNING,
+						(errmsg("could not read two-phase state file \"%s\": %d read, expected %zu",
+								path, r, stat.st_size)));
+		}
 		pfree(buf);
 		return NULL;
 	}
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index adbd6a2126..72fd800646 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -3395,21 +3395,24 @@ XLogFileCopy(XLogSegNo destsegno, TimeLineID srcTLI, XLogSegNo srcsegno,
 
 		if (nread > 0)
 		{
+			int		r;
+
 			if (nread > sizeof(buffer))
 				nread = sizeof(buffer);
 			errno = 0;
 			pgstat_report_wait_start(WAIT_EVENT_WAL_COPY_READ);
-			if (read(srcfd, buffer, nread) != nread)
+			r = read(srcfd, buffer, nread);
+			if (r != nread)
 			{
-				if (errno != 0)
+				if (r < 0)
 					ereport(ERROR,
 							(errcode_for_file_access(),
 							 errmsg("could not read file \"%s\": %m",
 									path)));
 				else
 					ereport(ERROR,
-							(errmsg("not enough data in file \"%s\"",
-									path)));
+							(errmsg("not enough data in file \"%s\": read %d, expected %d",
+									path, r, nread)));
 			}
 			pgstat_report_wait_end();
 		}
@@ -11594,6 +11597,7 @@ XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr, int reqLen,
 	int			emode = private->emode;
 	uint32		targetPageOff;
 	XLogSegNo	targetSegNo PG_USED_FOR_ASSERTS_ONLY;
+	int			r;
 
 	XLByteToSeg(targetPagePtr, targetSegNo, wal_segment_size);
 	targetPageOff = XLogSegmentOffset(targetPagePtr, wal_segment_size);
@@ -11675,8 +11679,10 @@ retry:
 	if (lseek(readFile, (off_t) readOff, SEEK_SET) < 0)
 	{
 		char		fname[MAXFNAMELEN];
+		int			save_errno = errno;
 
 		XLogFileName(fname, curFileTLI, readSegNo, wal_segment_size);
+		errno = save_errno;
 		ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
 				(errcode_for_file_access(),
 				 errmsg("could not seek in log segment %s to offset %u: %m",
@@ -11685,16 +11691,29 @@ retry:
 	}
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
-	if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
+	r = read(readFile, readBuf, XLOG_BLCKSZ);
+	if (r != XLOG_BLCKSZ)
 	{
 		char		fname[MAXFNAMELEN];
+		int			save_errno = errno;
 
 		pgstat_report_wait_end();
 		XLogFileName(fname, curFileTLI, readSegNo, wal_segment_size);
-		ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
-				(errcode_for_file_access(),
-				 errmsg("could not read from log segment %s, offset %u: %m",
-						fname, readOff)));
+
+		if (r < 0)
+		{
+			errno = save_errno;
+			ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
+					(errcode_for_file_access(),
+					 errmsg("could not read from log segment %s, offset %u: %m",
+							fname, readOff)));
+		}
+		else
+			ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
+					(errcode_for_file_access(),
+					 errmsg("could not read from log segment %s, offset %u: read %d, expected %d",
+							fname, readOff, r, XLOG_BLCKSZ)));
+
 		goto next_record_is_invalid;
 	}
 	pgstat_report_wait_end();
diff --git a/src/backend/replication/logical/origin.c b/src/backend/replication/logical/origin.c
index 963878a5d8..c39d023d22 100644
--- a/src/backend/replication/logical/origin.c
+++ b/src/backend/replication/logical/origin.c
@@ -697,9 +697,16 @@ StartupReplicationOrigin(void)
 	/* verify magic, that is written even if nothing was active */
 	readBytes = read(fd, &magic, sizeof(magic));
 	if (readBytes != sizeof(magic))
-		ereport(PANIC,
-				(errmsg("could not read file \"%s\": %m",
-						path)));
+	{
+		if (readBytes < 0)
+			ereport(PANIC,
+					(errmsg("could not read file \"%s\": %m",
+							path)));
+		else
+			ereport(PANIC,
+					(errmsg("could not read file \"%s\": %d read of %zu",
+							path, readBytes, sizeof(magic))));
+	}
 	COMP_CRC32C(crc, &magic, sizeof(magic));
 
 	if (magic != REPLICATION_STATE_MAGIC)
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 056628fe8e..cc64ff22ac 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -1394,11 +1394,15 @@ RestoreSlotFromDisk(const char *name)
 
 		CloseTransientFile(fd);
 		errno = saved_errno;
-		ereport(PANIC,
-				(errcode_for_file_access(),
-				 errmsg("could not read file \"%s\", read %d of %u: %m",
-						path, readBytes,
-						(uint32) ReplicationSlotOnDiskConstantSize)));
+		if (readBytes < 0)
+			ereport(PANIC,
+					(errcode_for_file_access(),
+					 errmsg("could not read file \"%s\": %m", path)));
+		else
+			ereport(PANIC,
+					(errmsg("could not read file \"%s\": read %d of %u",
+							path, readBytes,
+							(uint32) ReplicationSlotOnDiskConstantSize)));
 	}
 
 	/* verify magic */
@@ -1434,10 +1438,14 @@ RestoreSlotFromDisk(const char *name)
 
 		CloseTransientFile(fd);
 		errno = saved_errno;
-		ereport(PANIC,
-				(errcode_for_file_access(),
-				 errmsg("could not read file \"%s\", read %d of %u: %m",
-						path, readBytes, cp.length)));
+		if (readBytes < 0)
+			ereport(PANIC,
+					(errcode_for_file_access(),
+					 errmsg("could not read file \"%s\": %m", path)));
+		else
+			ereport(PANIC,
+					(errmsg("could not read file \"%s\": read %d of %u",
+							path, readBytes, cp.length)));
 	}
 
 	CloseTransientFile(fd);
diff --git a/src/backend/utils/cache/relmapper.c b/src/backend/utils/cache/relmapper.c
index 99d095f2df..c4839b11f6 100644
--- a/src/backend/utils/cache/relmapper.c
+++ b/src/backend/utils/cache/relmapper.c
@@ -629,6 +629,7 @@ load_relmap_file(bool shared)
 	char		mapfilename[MAXPGPATH];
 	pg_crc32c	crc;
 	int			fd;
+	int			r;
 
 	if (shared)
 	{
@@ -659,11 +660,19 @@ load_relmap_file(bool shared)
 	 * are able to access any relation that's affected by the change.
 	 */
 	pgstat_report_wait_start(WAIT_EVENT_RELATION_MAP_READ);
-	if (read(fd, map, sizeof(RelMapFile)) != sizeof(RelMapFile))
-		ereport(FATAL,
-				(errcode_for_file_access(),
-				 errmsg("could not read relation mapping file \"%s\": %m",
-						mapfilename)));
+	r = read(fd, map, sizeof(RelMapFile));
+	if (r != sizeof(RelMapFile))
+	{
+		if (r < 0)
+			ereport(FATAL,
+					(errcode_for_file_access(),
+					 errmsg("could not read relation mapping file \"%s\": %m",
+							mapfilename)));
+		else
+			ereport(FATAL,
+					(errmsg("could not read relation mapping file \"%s\": %d read, expected %zu",
+							mapfilename, r, sizeof(RelMapFile))));
+	}
 	pgstat_report_wait_end();
 
 	CloseTransientFile(fd);
diff --git a/src/bin/pg_rewind/file_ops.c b/src/bin/pg_rewind/file_ops.c
index 94bcc13ae8..174ca7216d 100644
--- a/src/bin/pg_rewind/file_ops.c
+++ b/src/bin/pg_rewind/file_ops.c
@@ -289,6 +289,7 @@ slurpFile(const char *datadir, const char *path, size_t *filesize)
 	struct stat statbuf;
 	char		fullpath[MAXPGPATH];
 	int			len;
+	int			r;
 
 	snprintf(fullpath, sizeof(fullpath), "%s/%s", datadir, path);
 
@@ -304,9 +305,17 @@ slurpFile(const char *datadir, const char *path, size_t *filesize)
 
 	buffer = pg_malloc(len + 1);
 
-	if (read(fd, buffer, len) != len)
-		pg_fatal("could not read file \"%s\": %s\n",
-				 fullpath, strerror(errno));
+	r = read(fd, buffer, len);
+	if (r != len)
+	{
+		if (r < 0)
+			pg_fatal("could not read file \"%s\": %s\n",
+					 fullpath, strerror(errno));
+		else
+			pg_fatal("could not read file \"%s\": read %d, expected %d\n",
+					 fullpath, r, len);
+
+	}
 	close(fd);
 
 	/* Zero-terminate the buffer. */
diff --git a/src/bin/pg_rewind/parsexlog.c b/src/bin/pg_rewind/parsexlog.c
index b4c1f827a6..4f27cfe42b 100644
--- a/src/bin/pg_rewind/parsexlog.c
+++ b/src/bin/pg_rewind/parsexlog.c
@@ -246,6 +246,7 @@ SimpleXLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
 	uint32		targetPageOff;
 	XLogRecPtr	targetSegEnd;
 	XLogSegNo	targetSegNo;
+	int			r;
 
 	XLByteToSeg(targetPagePtr, targetSegNo, WalSegSz);
 	XLogSegNoOffsetToRecPtr(targetSegNo + 1, 0, targetSegEnd, WalSegSz);
@@ -309,10 +310,17 @@ SimpleXLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
 		return -1;
 	}
 
-	if (read(xlogreadfd, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
+
+	r = read(xlogreadfd, readBuf, XLOG_BLCKSZ);
+	if (r != XLOG_BLCKSZ)
 	{
-		printf(_("could not read from file \"%s\": %s\n"), xlogfpath,
-			   strerror(errno));
+		if (r < 0)
+			printf(_("could not read from file \"%s\": %s\n"), xlogfpath,
+				   strerror(errno));
+		else
+			printf(_("could not read from file \"%s\": read %d, expected %d\n"),
+				   xlogfpath, r, XLOG_BLCKSZ);
+
 		return -1;
 	}
 
diff --git a/src/bin/pg_waldump/pg_waldump.c b/src/bin/pg_waldump/pg_waldump.c
index 5c4f38e597..64391a7e5c 100644
--- a/src/bin/pg_waldump/pg_waldump.c
+++ b/src/bin/pg_waldump/pg_waldump.c
@@ -210,8 +210,10 @@ search_directory(const char *directory, const char *fname)
 	if (fd >= 0)
 	{
 		char		buf[XLOG_BLCKSZ];
+		int			r;
 
-		if (read(fd, buf, XLOG_BLCKSZ) == XLOG_BLCKSZ)
+		r = read(fd, buf, XLOG_BLCKSZ);
+		if (r == XLOG_BLCKSZ)
 		{
 			XLogLongPageHeader longhdr = (XLogLongPageHeader) buf;
 
@@ -229,7 +231,8 @@ search_directory(const char *directory, const char *fname)
 				fatal_error("could not read file \"%s\": %s",
 							fname, strerror(errno));
 			else
-				fatal_error("not enough data in file \"%s\"", fname);
+				fatal_error("not enough data in file \"%s\": %d read, %d expected",
+							fname, r, XLOG_BLCKSZ);
 		}
 		close(fd);
 		return true;