Incorrect blocked on MVar detection

Given the code:

import Control.Concurrent

main = do
    v <- newEmptyMVar
    forkFinally (putStrLn $ let x = x in x) (putMVar v)
    print =<< takeMVar v

The spawned thread should raise a NonTermination exception (assuming idle GC), then put it in the MVar v, which should cause the main thread to abort printing Left <<loop>>.

However, in reality, the main thread aborts with Exception: thread blocked indefinitely in an MVar operation.

My debugging shows that the spawned thread raises NonTermination and at the same time the main thread raises BlockedIndefinitelyOnMVar. Of course, the main thread is not really blocked on the exception, and after the finally bit runs, the MVar is filled. Adding tracing shows that the putMVar is called.

Hypothesis is that the GC notices it can raise the non-termination exception, and then opportunistically excludes that thread from further GC scanning, and detects the MVar is unreferenced, raising the subsequent exception. Of course, given you can catch the exception, that isn't safe. Roman Leshchinskiy worries that in the wrong circumstances the MVar might really be being GC'd and thus could corrupt memory. This issue was detected while debugging an error in the Shake build system for GHC itself.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ndmitchell@gmail.com
Operating system
Architecture