Reset properly errno before calling write()

6cb33724 enforces errno to ENOSPC when less bytes than what is expected
have been written when it is unset, though it forgot to properly reset
errno before doing a system call to write(), causing errno to
potentially come from a previous system call.

Reported-by: Tom Lane
Author: Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/31797.1533326676@sss.pgh.pa.us

src/backend/access/transam/twophase.c

@@ -1565,6 +1565,7 @@ RecreateTwoPhaseFile(TransactionId xid, void *content, int len)
 						path)));
 
 	/* Write content and CRC */
+	errno = 0;
 	if (write(fd, content, len) != len)
 	{
 		int			save_errno = errno;

src/bin/pg_basebackup/receivelog.c

@@ -138,6 +138,7 @@ open_walfile(XLogRecPtr startpoint, uint32 timeline, char *basedir,
 	zerobuf = pg_malloc0(XLOG_BLCKSZ);
 	for (bytes = 0; bytes < XLogSegSize; bytes += XLOG_BLCKSZ)
 	{
+		errno = 0;
 		if (write(f, zerobuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
 		{
 			/* if write didn't set errno, assume problem is no disk space */
@@ -1129,6 +1130,7 @@ HandleCopyStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline,
 					}
 				}
 
+				errno = 0;
 				if (write(walfile,
 						  copybuf + hdr_len + bytes_written,
 						  bytes_to_write) != bytes_to_write)