Proposal: Use uninterruptibleMask for cleanup actions in Control.Exception

Gregory Collins greg at
Thu Sep 4 00:01:43 UTC 2014

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.


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
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> Libraries mailing list
> Libraries at

Gregory Collins <greg at>
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