Refactor confusing code in _mdfd_openseg().

macdice · macdice · commit f37ff03478ae · 2020-01-27T09:12:56.000+13:00
As reported independently by a couple of people, _mdfd_openseg() is coded in a way that seems to imply that the segments could be opened in an order that isn't strictly sequential. Even if that were true, it's also using the wrong comparison. It's not an active bug, since the condition is always true anyway, but it's confusing, so replace it with an assertion. Author: Thomas Munro Reviewed-by: Andres Freund, Kyotaro Horiguchi, Noah Misch Discussion: https://postgr.es/m/CA%2BhUKG%2BNBw%2BuSzxF1os-SO6gUuw%3DcqO5DAybk6KnHKzgGvxhxA%40mail.gmail.com Discussion: https://postgr.es/m/20191222091930.GA1280238%40rfd.leadboat.com
diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
@@ -1100,8 +1100,13 @@ _mdfd_openseg(SMgrRelation reln, ForkNumber forknum, BlockNumber segno,
 	if (fd < 0)
 		return NULL;
 
-	if (segno <= reln->md_num_open_segs[forknum])
-		_fdvec_resize(reln, forknum, segno + 1);
+	/*
+	 * Segments are always opened in order from lowest to highest, so we must
+	 * be adding a new one at the end.
+	 */
+	Assert(segno == reln->md_num_open_segs[forknum]);
+
+	_fdvec_resize(reln, forknum, segno + 1);
 
 	/* fill the entry */
 	v = &reln->md_seg_fds[forknum][segno];
