Final bikeshedding call: Fixing Control.Exception.bracket

[Merijn Verstraaten]

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