BlockedIndefinitelyOnMVar thrown with live reference in unrelated finalizer
I am not sure whether this is a bug, but it is certainly unexpected behaviour. The following code throws a BlockedIndefinitelyOnMVar in the forked thread even though the MVar would be eventually written to by an unrelated finalizer:
import Control.Concurrent import Data.IORef main :: IO () main = do mvar <- newEmptyMVar -- _ <- forkIO $ threadDelay 9999999999999 >> isEmptyMVar mvar >> return () ref <- newIORef () -- unrelated IORef _ <- mkWeakIORef ref (putMVar mvar ()) -- register finalizer _ <- forkFinally (takeMVar mvar :: IO ()) print threadDelay 1000000
And indeed, if the forkIO line is uncommented no exception is thrown, as the new thread keeps another live reference to the MVar. Is this intended behaviour? Why does the MVar reference in the finalizer not count for BlockedIndefinitelyOnMVar?
(A similar thing happens with STM primitives.)