Final bikeshedding call: Fixing Control.Exception.bracket

[Eyal Lotem]

On Thu, Nov 13, 2014 at 10:48 AM, Simon Marlow <marlowsd at gmail.com> wrote:

> I think the bigger objection to using uninterruptibleMask for the
> allocation phase of bracket is that it breaks this:
>
> withMVar m io = bracket (takeMVar m) (putMVar m) io
>
> Now withMVar will be uninterruptible while it is blocked, which will make
> a lot of common idioms unresponsive to async exceptions.  This was the
> whole motivation behind the idea of interruptible operations.
>

But do you really prefer the alternative to this unresponsiveness?

The alternative here is responsiveness, but it surely breaking the
invariants of this MVar (the number of takes/puts will no longer match).

I think it is far more important to maintain these kinds of invariants than
to be responsive to async exceptions at any price.

Also, if this is not responsive -- it means that someone is holding up the
MVar, and responsiveness can still be attained by making sure that the
holder stops hogging the mvar.  That way you can maintain your invariants
and be responsive, and this is the kind of approach I've used in my
programs after using uninterruptibleMask for cleanups.


> Cheers,
> Simon
>
>
> On 11/11/2014 20:17, 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".
>>
>> 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.
>>
>> 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.
>>
>> 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
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>>>>
>>>
>>>
>>>
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-- 
Eyal
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