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Re: [PATCH] config: use a static lock_file struct




On Wed, Aug 30, 2017 at 05:25:18AM +0200, Michael Haggerty wrote:

> >>> In the long run we may want to drop the "tempfiles must remain forever"
> >>> rule. This is certainly not the first time it has caused confusion or
> >>> leaks. And I don't think it's a fundamental issue, just the way the code
> >>> is written. But in the interim, this fix is probably worth doing.
> > 
> > The main issue is that the caller needs to make sure they're removed
> > from the list (via commit or rollback) before being freed.
> > 
> > As far as I know anyway. This restriction dates back to the very early
> > days of the lockfile code and has been carried through the various
> > tempfile-cleanup refactorings over the years (mostly because each was
> > afraid to make functional changes).
> > 
> > +cc Michael, who did most comprehensive cleanup of that code.
> 
> It was surprisingly hard trying to get that code to do the right thing,
> non-racily, in every error path. Since `remove_tempfiles()` can be
> called any time (even from a signal handler), the linked list of
> `tempfile` objects has to be kept valid at all times but can't use
> mutexes. I didn't have the energy to keep going and make the lock
> objects freeable.
> 
> I suppose the task could be made a bit easier by using `sigprocmask(2)`
> or `pthread_sigmask(3)` to make its running environment a little bit
> less hostile.

I think there are really two levels of carefulness here:

  1. Avoiding complicated things during a signal handler that may rely
     on having a sane state from the rest of the program (e.g.,
     half-formed entries, stdio locks, etc).

  2. Being truly race-free in the face of a signal arriving while we're
     running arbitrary code that might have a tempfile struct in a funny
     state.

I feel like right now we meet (1) and not (2). But I think if we keep to
that lower bar of (1), it might not be that bad. We're assuming now that
there's no race on the tempfile->active flag, for instance. We could
probably make a similar assumption about putting items onto or taking
them off of a linked list (it's not really atomic, but a single pointer
assignment is probably "atomic enough" for our purposes).

Or I dunno. There's a lot of "volatile" modifiers sprinkled around.
Maybe those are enough to give us (2) as well (though in that case, I
think we'd still be as OK with the list manipulation as we are with the
active flag manipulation).

-Peff