Make safeguard against incorrect flags for fsync more portable.

The existing code assumed that O_RDONLY is defined as 0, but this is
not required by POSIX and is not true on GNU Hurd.  We can avoid
the assumption by relying on O_ACCMODE to mask the fcntl() result.
(Hopefully, all supported platforms define that.)

Author: Michael Banck <mbanck@gmx.net>
Co-authored-by: Samuel Thibault
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/6862e8d1.050a0220.194b8d.76fa@mx.google.com
Discussion: https://postgr.es/m/68480868.5d0a0220.1e214d.68a6@mx.google.com
Backpatch-through: 13

src/backend/storage/file/fd.c

@@ -400,25 +400,22 @@ pg_fsync(int fd)
 	 * portable, even if it runs ok on the current system.
 	 *
 	 * We assert here that a descriptor for a file was opened with write
-	 * permissions (either O_RDWR or O_WRONLY) and for a directory without
-	 * write permissions (O_RDONLY).
+	 * permissions (i.e., not O_RDONLY) and for a directory without write
+	 * permissions (O_RDONLY).  Notice that the assertion check is made even
+	 * if fsync() is disabled.
 	 *
-	 * Ignore any fstat errors and let the follow-up fsync() do its work.
-	 * Doing this sanity check here counts for the case where fsync() is
-	 * disabled.
+	 * If fstat() fails, ignore it and let the follow-up fsync() complain.
 	 */
 	if (fstat(fd, &st) == 0)
 	{
 		int			desc_flags = fcntl(fd, F_GETFL);
 
-		/*
-		 * O_RDONLY is historically 0, so just make sure that for directories
-		 * no write flags are used.
-		 */
+		desc_flags &= O_ACCMODE;
+
 		if (S_ISDIR(st.st_mode))
-			Assert((desc_flags & (O_RDWR | O_WRONLY)) == 0);
+			Assert(desc_flags == O_RDONLY);
 		else
-			Assert((desc_flags & (O_RDWR | O_WRONLY)) != 0);
+			Assert(desc_flags != O_RDONLY);
 	}
 	errno = 0;
 #endif