mdclose() does not cope w/ FileClose() failure

Forking thread "WAL logging problem in 9.4.3?" for this tangent:

On Mon, Dec 09, 2019 at 06:04:06PM +0900, Kyotaro Horiguchi wrote:
> I don't understand why mdclose checks for (v->mdfd_vfd >= 0) of open
> segment but anyway mdimmedsync is believing that that won't happen and
> I follow the assumption. (I suspect that the if condition in mdclose
> should be an assertion..)

That check helps when data_sync_retry=on and FileClose() raised an error in a
previous mdclose() invocation. However, the check is not sufficient to make
that case work; the attached test case (not for commit) gets an assertion
failure or SIGSEGV.

I am inclined to fix this by decrementing md_num_open_segs before modifying
md_seg_fds (second attachment). An alternative would be to call
_fdvec_resize() after every FileClose(), like mdtruncate() does; however, the
repalloc() overhead could be noticeable. (mdclose() is called much more
frequently than mdtruncate().)

Incidentally, _mdfd_openseg() has this:

if (segno <= reln->md_num_open_segs[forknum])
_fdvec_resize(reln, forknum, segno + 1);

That should be >=, not <=. If the less-than case happened, this would delete
the record of a vfd for a higher-numbered segno. There's no live bug, because
only segno == reln->md_num_open_segs[forknum] actually happens. I am inclined
to make an assertion of that and remove the condition:

Assert(segno == reln->md_num_open_segs[forknum]);
_fdvec_resize(reln, forknum, segno + 1);