Proposal: Use uninterruptibleMask for cleanup actions in Control.Exception

Eyal Lotem eyal.lotem at
Thu Sep 4 07:29:08 UTC 2014

In addition to hClose, the majority of cleanup handlers in my programs
turned out to be interruptible. Moreover, whether something is
interruptible is unclear. You can easily add a putStrLn to a cleanup
handler and now it is accidentally interruptible.

I'd love to see examples of some code where interruptible cleanup handlers
are not a bug.

Every single one in my programs that I examined was a bug.

Is withMVar also a primop? Because it's buggy in the same way as withFile
currently is.

The current situation is that virtually all uses of bracket in the entire
Haskell ecosystem are subtle bugs.
On Sep 4, 2014 3:01 AM, "Gregory Collins" <greg at> wrote:

> Unless I'm mistaken, here the "mask" call inside bracket already makes
> sure you don't receive asynchronous exceptions unless you call a function
> that is interruptible (i.e. goes back into the runtime system). The hClose
> example you give doesn't fall in this category, as something inside the RTS
> needs to call "allowInterrupt" (or otherwise unmask exceptions) in order
> for async exceptions to be delivered. The "readMVar" example you give
> *does* have this issue (because putMVar does an implicit allowInterrupt)
> but in recent GHC readMVar has been redefined as a primop.
> The danger of deadlock is *not* minimal here, doing what you suggest will
> transform many valid programs (i.e. if you block on a "takeMVar" in the
> cleanup action) into ones that have unkillable orphan threads.
> G
> On Wed, Sep 3, 2014 at 1:56 PM, Eyal Lotem <eyal.lotem at> wrote:
>> I'd like to propose a change in the behavior of Control.Exception to help
>> guarantee cleanups are not accidentally lost.
>> For example, bracket is defined as:
>> bracket before after thing =  mask $ \restore -> do    a <- before    r <- restore (thing a) `onException` after a    _ <- after a    return r
>> This definition has a serious problem: "after a" (in either the exception
>> handling case, or the ordinary case) can include interruptible actions
>> which abort the cleanup.
>> This means bracket does not in fact guarantee the cleanup occurs.
>> For example:
>> readMVar = bracket takeMVar putMVar -- If async exception occurs during
>> putMVar, MVar is broken!
>> withFile .. = bracket (openFile ..) hClose -- Async exception during
>> hClose leaks the file handle!
>> Interruptible actions during "before" are fine (as long as "before"
>> handles them properly).  Interruptible actions during "after" are virtually
>> always a bug -- at best leaking a resource, and at worst breaking the
>> program's invariants.
>> I propose changing all the cleanup handlers to run under
>> uninterruptibleMask, specifically:
>> *bracket, bracketOnError, bracket_, catch, catchJust, finally, handle,
>> handleJust, onException*
>> should all simply wrap their exception/cancellation handler with
>> uninterruptibleMask.
>> The danger of a deadlock is minimal when compared with the virtually
>> guaranteed buggy incorrect handling of async exceptions during cleanup.
>> --
>> Eyal
>> _______________________________________________
>> Libraries mailing list
>> Libraries at
> --
> Gregory Collins <greg at>
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