Re: Proposal: Use uninterruptibleMask for cleanup actions in Control.Exception

It's hasn't been noted or I haven't noticed that putMVar is only interruptible when the MVar is full. This means that there isn't a problem in cases like withMVar because the MVar is empty due to the establishing takeMVar, and there is no leak. For hClose if you are closing the handle, presumably the contained MVar is full and there is no block and therefore no interruption. Only if another thread is actively using the handle and you are trying to close the handle out from under it is there potential for failure. In the exotic cases like this there is already a way to make an interruptible operation uninterruptible. I'm -1 until it becomes clear that it is actually an issue in common code. On Sep 4, 2014 12:47 AM, "John Lato" wrote:

Being a primop or not has nothing to do with whether an MVar operation is interruptible. What matters is whether or not the operation will block. withMVar (or readMVar in any incarnation) on an empty MVar is interruptible. If the MVar happened to be full, it's not interruptible.

I agree this is a problem. I don't think the proposed solution is perfect, but I do think it's possibly better than the status quo. Perhaps the user should be required to use uninterruptibleMask_ on the cleanup action if necessary? I've long thought that using an interruptible operation in a cleanup handler is a programming error.

John L.

On Thu, Sep 4, 2014 at 12:29 AM, Eyal Lotem wrote:

...

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" 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 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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-- Gregory Collins

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