Address set of issues with errno handling

System calls mixed up in error code paths are causing two issues which
several code paths have not correctly handled:
1) For write() calls, sometimes the system may return less bytes than
what has been written without errno being set.  Some paths were careful
enough to consider that case, and assumed that errno should be set to
ENOSPC, other calls missed that.
2) errno generated by a system call is overwritten by other system calls
which may succeed once an error code path is taken, causing what is
reported to the user to be incorrect.

This patch uses the brute-force approach of correcting all those code
paths.  Some refactoring could happen in the future, but this is let as
future work, which is not targeted for back-branches anyway.

Author: Michael Paquier
Reviewed-by: Ashutosh Sharma
Discussion: https://postgr.es/m/20180622061535.GD5215@paquier.xyz

3 files changed, 34 insertions, 0 deletions

diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index cdd0091a5f0..c7cde25f835 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -1171,12 +1171,17 @@ ReadTwoPhaseFile(TransactionId xid, bool give_warnings)
 	 */
 	if (fstat(fd, &stat))
 	{
+		int			save_errno = errno;
+
 		CloseTransientFile(fd);
 		if (give_warnings)
+		{
+			errno = save_errno;
 			ereport(WARNING,
 					(errcode_for_file_access(),
 					 errmsg("could not stat two-phase state file \"%s\": %m",
 							path)));
+		}
 		return NULL;
 	}
 
@@ -1203,12 +1208,17 @@ ReadTwoPhaseFile(TransactionId xid, bool give_warnings)
 
 	if (read(fd, buf, stat.st_size) != stat.st_size)
 	{
+		int			save_errno = errno;
+
 		CloseTransientFile(fd);
 		if (give_warnings)
+		{
+			errno = save_errno;
 			ereport(WARNING,
 					(errcode_for_file_access(),
 					 errmsg("could not read two-phase state file \"%s\": %m",
 							path)));
+		}
 		pfree(buf);
 		return NULL;
 	}
@@ -1550,14 +1560,24 @@ RecreateTwoPhaseFile(TransactionId xid, void *content, int len)
 	/* Write content and CRC */
 	if (write(fd, content, len) != len)
 	{
+		int			save_errno = errno;
+
 		CloseTransientFile(fd);
+
+		/* if write didn't set errno, assume problem is no disk space */
+		errno = save_errno ? save_errno : ENOSPC;
 		ereport(ERROR,
 				(errcode_for_file_access(),
 				 errmsg("could not write two-phase state file: %m")));
 	}
 	if (write(fd, &statefile_crc, sizeof(pg_crc32c)) != sizeof(pg_crc32c))
 	{
+		int			save_errno = errno;
+
 		CloseTransientFile(fd);
+
+		/* if write didn't set errno, assume problem is no disk space */
+		errno = save_errno ? save_errno : ENOSPC;
 		ereport(ERROR,
 				(errcode_for_file_access(),
 				 errmsg("could not write two-phase state file: %m")));
@@ -1569,7 +1589,10 @@ RecreateTwoPhaseFile(TransactionId xid, void *content, int len)
 	 */
 	if (pg_fsync(fd) != 0)
 	{
+		int			save_errno = errno;
+
 		CloseTransientFile(fd);
+		errno = save_errno;
 		ereport(ERROR,
 				(errcode_for_file_access(),
 				 errmsg("could not fsync two-phase state file: %m")));
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 98abc4961f0..248ea9a9766 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -3087,7 +3087,10 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
 
 	if (pg_fsync(fd) != 0)
 	{
+		int			save_errno = errno;
+
 		close(fd);
+		errno = save_errno;
 		ereport(ERROR,
 				(errcode_for_file_access(),
 				 errmsg("could not fsync file \"%s\": %m", tmppath)));
@@ -11255,8 +11258,10 @@ retry:
 	if (lseek(readFile, (off_t) readOff, SEEK_SET) < 0)
 	{
 		char		fname[MAXFNAMELEN];
+		int			save_errno = errno;
 
 		XLogFileName(fname, curFileTLI, readSegNo);
+		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",
@@ -11267,8 +11272,10 @@ retry:
 	if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
 	{
 		char		fname[MAXFNAMELEN];
+		int			save_errno = errno;
 
 		XLogFileName(fname, curFileTLI, readSegNo);
+		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",
diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index 51a8e8ddb2e..a82ccd45697 100644
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -710,9 +710,11 @@ XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
 			if (lseek(sendFile, (off_t) startoff, SEEK_SET) < 0)
 			{
 				char		path[MAXPGPATH];
+				int			save_errno = errno;
 
 				XLogFilePath(path, tli, sendSegNo);
 
+				errno = save_errno;
 				ereport(ERROR,
 						(errcode_for_file_access(),
 				  errmsg("could not seek in log segment %s to offset %u: %m",
@@ -731,9 +733,11 @@ XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
 		if (readbytes <= 0)
 		{
 			char		path[MAXPGPATH];
+			int			save_errno = errno;
 
 			XLogFilePath(path, tli, sendSegNo);
 
+			errno = save_errno;
 			ereport(ERROR,
 					(errcode_for_file_access(),
 					 errmsg("could not read from log segment %s, offset %u, length %lu: %m",