Final bikeshedding call: Fixing Control.Exception.bracket

Tue Nov 11 21:31:31 UTC 2014

The flaw in your argument is that you're assuming that the author of a
cleanup action is a single library author, rather than a random user
using bracket with a random action from library X as cleanup. Now the
user has to investigate whether random action X can block. There's not a
single library that properly documents "this action can potentially
block" because in many cases even the library authors themselves aren't
sure. Not to mention, even if library authors *did* document this, then
this would be incredibly brittle as the correctness of my code now
relies on my recursive dependencies all properly documenting blocking
behaviour AND following the PVP to avoid accidental breakage.

This is just not realistic, the only sane option is to fix it
unambiguously in bracket so I as a user can safely assume that my
cleanup is never interrupted.

Unless you can provide me with a concrete, *real*, actual library that
requires cleanup to be interruptible, I will not believe this line of
arguing.

-- 
Merijn

On Tue, Nov 11, 2014, at 13:00, Yuras Shumovich wrote:
> On Tue, 2014-11-11 at 12:17 -0800, Merijn Verstraaten wrote:
> > Allocation should not use uninterruptibleMask as it is possible to handle async exceptions during allocation by nesting bracketOnError
> > 
> > Example:
> > someFun mvar1 mvar2 = do
> >     (val1, val2) <- bracketOnError 
> >         (takeMVar mvar1)
> >         (putMVar mvar1)
> >         (\x -> takeMVar mvar2 >>= \y -> return (x, y)))
> > 
> > This can be made nicer using the Cont monad to hide the marching to the left. The same cannot be done for cleanup, as there's no sane thing as "half a cleanup".
> 
> It definitely can be done for cleanups too. E.g. using
> uninterruptibleMask as the last resort.
> 
> And "half a cleanup" is valid thing. E.g. database that uses two files:
> 
> data DB = DB Handle Handle
> 
> The following cleanup action is buggy:
> 
> close :: DB -> IO ()
> close (DB h1 h2) = hClose h1 >> hClose h2
> 
> Note: it is broken regardless async exceptions(!)
> You can get half cleanup even under uninterruptibleMask
> 
> The code is either exception-safe or buggy. You can't magically fix
> buddy code using uninterruptibleMask.
> 
> > 
> > I disagree that it should be left to the author of allocation operation to ensure uninterruptibility as it is impossible to know whether a given IO blocks internally and thus should be masked without inspecting the *entire* code path potentially called by the cleanup handler.
> 
> (You probably mean "the author of *cleanup* operation"? I'll assume
> that)
> 
> Hm... You have to inspect code if you expect it to be buggy. Otherwise
> you should assume it is interruptible (unless listed in Cotrol.Exception
> module in "Interruptible operations" section or explicitly stated in
> other way), but all cleanup actions do full cleanup even if they throw
> exception. If the code is buggy -- it should be fixed, not hidden.
> 
> > 
> > Both Eyal and me have had trouble with this where we had to entire half of base and part of the runtime, to figure out whether our code was async exception safe. Auditing half the ecosystem to be able to write a safe cleanup handler is *NOT* a viable option.
> 
> You need to audit half of ecosystem anyway to ensure allocating actions
> are not buggy.
> And as the example above shows, even uninterruptibleMask doesn't
> guaranty anything.
> 
> 
> I agree that current situation is bad. I have two drafts in my mailbox
> where I propose to use uninterruptibleMask in bracket, but I didn't send
> them -- every time I found that it doesn't fix anything actually. I
> don't know better solution, but the proposal is not even a solution.
> 
> Thanks,
> Yuras
> 
> > 
> > Cheers,
> > Merijn
> > 
> > > On 11 Nov 2014, at 11:58, Yuras Shumovich <shumovichy at gmail.com> wrote:
> > > 
> > > Hello,
> > > 
> > > Should we use `uninterrubtibleMask` for allocating action too?
> > > 
> > > 
> > > I'm not sure my voice will be counted, but anyway,
> > > I'm strong -1 because it fixes wrong issue.
> > > 
> > > `hClose` is interruptible, but it closes the handle in any case. I'm
> > > pretty sure. I ask that question (see
> > > http://haskell.1045720.n5.nabble.com/Control-Exception-bracket-is-broken-td5752251.html ) but didn't get any answer, so I read code and made experiments. IIRC `hClose` wraps internal interruptible action into `try` and handles everything correctly.
> > > 
> > > I argue that cleanup action can be interruptible, but should ensure
> > > cleanup is done. As the last resort, it should use `uninterrubtibleMask`
> > > internally.
> > > 
> > > Other issue is that a lot of allocating action are broken because they
> > > perform interruptible actions after allocating resource without handling
> > > async exceptions. So my point is that masking async exceptions solves
> > > only one half of the issue while masking the other.
> > > 
> > > Handling async exceptions is hard, and we can't make is easy using
> > > `uninterrubtibleMask`. Instead we should educate ourselves to do it
> > > correctly from the very beginning. There is only one alternative --
> > > remove async exceptions from haskell.
> > > 
> > > To summarize,
> > > - allocating action should either allocate resource or throw exception;
> > > it is a bug to allocate resource *and* throw exception
> > > - cleanup action should release resource even if it throws an exception
> > > Developer should ensure both properties holds.
> > > 
> > > Sorry my poor English.
> > > 
> > > Thanks,
> > > Yuras
> > > 
> > > On Tue, 2014-11-11 at 10:09 -0800, Merijn Verstraaten wrote:
> > >> Ola!
> > >> 
> > >> In September Eyal Lotem raised the issue of bracket's cleanup handler not being uninterruptible [1]. This is a final bikeshedding email before I submit a patch.
> > >> 
> > >> The problem, summarised:
> > >> Blocking cleanup actions can be interrupted, causing cleanup not to happen and potentially leaking resources.
> > >> 
> > >> Main objection to making the cleanup handler uninterruptible:
> > >> Could cause deadlock if the code relies on async exceptions to interrupt a blocked thread.
> > >> 
> > >> I count only two objections in the previous thread, 1 on the grounds that "deadlocks are NOT unlikely" and 1 that is conditioned on "I don't believe this is a problem".
> > >> 
> > >> The rest seems either +1, or at least agrees that the status quo is *worse* than the proposed solution.
> > >> 
> > >> My counter to these objections is:
> > >> 1) No one has yet shown me any code that relies on the cleanup handler being interruptible
> > >> 
> > >> 2) There are plenty of examples of current code being broken, for example every single 'bracket' using file handles is broken due to handle operations using a potentially blocking MVar operation internally, potentially leaking file descriptors/handles.
> > >> 
> > >> 3) Even GHC-HQ can't use bracket correctly (see Simon's emails)
> > >> 
> > >> Potential solution #1:
> > >> Leave bracket as-is, add bracketUninterruptible with an uninterruptible cleanup handler.
> > >> 
> > >> Potential solution #2:
> > >> Change bracket to use uninterruptible cleanup handler, add bracketInterruptible for interruptible cleanups.
> > >> 
> > >> Trade-offs:
> > >> Solution 1 won't change the semantics of any existing code, however this also means that any currently broken uses of bracket will remain broken, possibly indefinitely.
> > >> 
> > >> Solution 2 will change the semantics of bracket, which means any currently broken uses of bracket will be fixed, at the cost of creating potential deadlocks in code that relies on the interruptibility of cleanup.
> > >> 
> > >> I will argue that solution #2 is preferable, since I have yet to see any code that uses the interruptibility of the cleanup handler. Whereas there's many broken assumption assuming the cleanup handler is not interruptible.
> > >> 
> > >> Secondly, it is easier to detect deadlocks caused by this problem than it is to detect resource leaks which only happen in unlucky timings of async exceptions. Especially since any deadlock caused by the change can be fixed by replacing bracket with bracketInterruptible.
> > >> 
> > >> [1] - https://www.haskell.org/pipermail/libraries/2014-September/023675.html
> > >> 
> > >> Cheers,
> > >> Merijn
> > >> _______________________________________________
> > >> Libraries mailing list
> > >> Libraries at haskell.org
> > >> http://www.haskell.org/mailman/listinfo/libraries
> > > 
> > > 
> > 
> 
> 