[GHC] #11001: BlockedIndefinitelyOnMVar thrown with live reference in unrelated finalizer

[GHC]

#11001: BlockedIndefinitelyOnMVar thrown with live reference in unrelated finalizer
-------------------------------------+-------------------------------------
           Reporter:  exFalso        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Runtime        |           Version:  7.10.2
  System                             |
           Keywords:                 |  Operating System:  Linux
  BlockedIndefinitelyOnMVar          |
  finalize                           |
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 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:

 {{{#!hs
 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.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11001>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler