Re: WIP: [[Parallel] Shared] Hash

On Sun, Mar 26, 2017 at 1:53 PM, Peter Geoghegan <pg@bowt.ie> wrote:
> ISTM that your patch now shares a quality with parallel tuplesort: You
> may now hold files open after an unlink() of the original link/path
> that they were opened using. As Robert pointed out when discussing
> parallel tuplesort earlier in the week, that comes with the risk,
> however small, that the vFD cache will close() the file out from under
> us during LRU maintenance, resulting in a subsequent open() (at the
> tail-end of the vFD's lifetime) that fails unexpectedly. It's probably
> fine to assume that we can sanely close() the file ourselves in fd.c
> error paths despite a concurrent unlink(), since we never operate on
> the link itself, and there probably isn't much pressure on each
> backend's vFD cache. But, is that good enough? I can't say, though I
> suspect that this particular risk is one that's best avoided.
>
> I haven't tested out how much of a problem this might be for your
> patch, but I do know that resowner.c will call your shared mem segment
> callback before closing any backend local vFDs, so I can't imagine how
> it could be that this risk doesn't exist.

I wouldn't have expected anything like that to be a problem, because
FileClose() doesn't call FileAccess().  So IIUC it wouldn't ever try
to reopen a kernel fd just to close it.

But... what you said above must be a problem for Windows.  I believe
it doesn't allow files to be unlinked if they are open, and I see that
DSM segments are cleaned up in resowner's phase ==
RESOURCE_RELEASE_BEFORE_LOCKS and files are closed in phase ==
RESOURCE_RELEASE_AFTER_LOCKS.

Hmm.

-- 
Thomas Munro
http://www.enterprisedb.com