Re: Parallel tuplesort (for parallel B-Tree index creation)

On Thu, Feb 9, 2017 at 7:10 PM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> At the risk of stating the obvious, ISTM that the right way to do
> this, at a high level, is to err on the side of unneeded extra
> unlink() calls, not leaking files. And, to make the window for problem
> ("remaining hole that you haven't quite managed to plug") practically
> indistinguishable from no hole at all, in a way that's kind of baked
> into the API.

I do not think there should be any reason why we can't get the
resource accounting exactly correct here. If a single backend manages
to remove every temporary file that it creates exactly once (and
that's currently true, modulo system crashes), a group of cooperating
backends ought to be able to manage to remove every temporary file
that any of them create exactly once (again, modulo system crashes).

I do agree that a duplicate unlink() call isn't as bad as a missing
unlink() call, at least if there's no possibility that the filename
could have been reused by some other process, or some other part of
our own process, which doesn't want that new file unlinked. But it's
messy. If the seatbelts in your car were to randomly unbuckle, that
would be a safety hazard. If they were to randomly refuse to
unbuckle, you wouldn't say "that's OK because it's not a safety
hazard", you'd say "these seatbelts are badly designed". And I think
the same is true of this mechanism.

The way to make this 100% reliable is to set things up so that there
is joint ownership from the beginning and shared state that lets you
know whether the work has already been done.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company