[GHC] #8804: BlockedIndefinitelyOnMVar thrown for an MVar which is still weakly accessible from another thread

19 Feb 2014

      #8804: BlockedIndefinitelyOnMVar thrown for an MVar which is still weakly
accessible from another thread
--------------------------+------------------------------------------------
       Reporter:  bholst  |             Owner:  simonmar
           Type:  bug     |            Status:  new
       Priority:  normal  |         Milestone:
      Component:          |           Version:  7.6.3
  Runtime System          |  Operating System:  Unknown/Multiple
       Keywords:          |   Type of failure:  Incorrect result at runtime
   Architecture:          |         Test Case:
  Unknown/Multiple        |          Blocking:
     Difficulty:          |
  Unknown                 |
     Blocked By:          |
Related Tickets:          |
--------------------------+------------------------------------------------
 GHC's runtime system throws a BlockedIndefinatelyOnMVar exception
 when the thread blocking an the MVar is the only one having a direct
 access to it. It assumes that there will be nothing written into the MVar
 in the future.
 This would be the correct behaviour if there were no weak references.
 The runtime system even throws the Exception when another thread still has
 a weak reference to the MVar. Consider the following example:
 {{{
 import Control.Concurrent
 import System.Mem.Weak
 import Data.Maybe
 import Control.Monad

 main = do
   m <- newEmptyMVar
   w <- mkWeakMVar m (return ())
   forkIO $ do
     threadDelay 1000000
     n <- deRefWeak w
     when (isJust n) $ putMVar (fromJust n) ()
   takeMVar m
 }}}

 At the time of ''takeMVar'' the forked thread has a weak reference to
 the MVar and it will put a value in it. However, the runtime system throws
 the Exception:

 {{{
 % ghc BlockingOnMVar.hs -threaded
 Linking BlockingOnMVar ...
 % ./BlockingOnMVar +RTS -N2
 BlockingOnMVar: thread blocked indefinitely in an MVar operation
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8804
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