Retry short writes when flushing WAL.

We don't normally bother retrying when the number of bytes written by
write() is short of what was requested. It is generally assumed that a
write() to disk doesn't return short, unless you run out of disk space.
While writing the WAL, however, it seems prudent to try a bit harder,
because a failure leads to PANIC. The write() is also much larger than most
write()s in the backend (up to wal_buffers), so there's more room for
surprises.

Also retry on EINTR. All signals used in the backend are flagged SA_RESTART
nowadays, so it shouldn't happen, but better to be defensive.

src/backend/access/transam/xlog.c

@@ -1606,6 +1606,8 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
 		{
 			char	   *from;
 			Size		nbytes;
+			Size		nleft;
+			int			written;
 
 			/* Need to seek in the file? */
 			if (openLogOff != startoffset)
@@ -1622,12 +1624,15 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
 			/* OK to write the page(s) */
 			from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
 			nbytes = npages * (Size) XLOG_BLCKSZ;
+			nleft = nbytes;
+			do
+			{
 				errno = 0;
-			if (write(openLogFile, from, nbytes) != nbytes)
+				written  = write(openLogFile, from, nleft);
+				if (written <= 0)
 				{
-				/* if write didn't set errno, assume no disk space */
-				if (errno == 0)
-					errno = ENOSPC;
+					if (errno == EINTR)
+						continue;
 					ereport(PANIC,
 							(errcode_for_file_access(),
 							 errmsg("could not write to log file %s "
@@ -1635,6 +1640,9 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
 									XLogFileNameP(ThisTimeLineID, openLogSegNo),
 									openLogOff, (unsigned long) nbytes)));
 				}
+				nleft -= written;
+				from += written;
+			} while (nleft > 0);
 
 			/* Update state for write */
 			openLogOff += nbytes;