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

[GHC]

#8804: BlockedIndefinitelyOnMVar thrown for an MVar which is still weakly
accessible from another thread
------------------------------------------------+--------------------------
        Reporter:  bholst                       |            Owner:
            Type:  bug                          |  simonmar
        Priority:  normal                       |           Status:  closed
       Component:  Runtime System               |        Milestone:
      Resolution:  invalid                      |          Version:  7.6.3
Operating System:  Unknown/Multiple             |         Keywords:
 Type of failure:  Incorrect result at runtime  |     Architecture:
       Test Case:                               |  Unknown/Multiple
        Blocking:                               |       Difficulty:
                                                |  Unknown
                                                |       Blocked By:
                                                |  Related Tickets:
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Comment (by bholst):

 You mean the waiting thread is garbage collected in this example and
 that's why the MVar will never be filled?

 Then keeping the thread reachable through another MVar should do the
 trick?
 {{{
 import Control.Concurrent
 import System.Mem.Weak
 import Data.Maybe
 import Control.Monad

 main = do
   my <- myThreadId
   m <- newEmptyMVar
   w <- mkWeakMVar m (return ())
   o <- newEmptyMVar
   _ <- forkIO $ do
     threadDelay 1000000
     n <- deRefWeak w
     when (isJust n) $ putMVar (fromJust n) ()
     putMVar o ()
   takeMVar m
   takeMVar o
 }}}
 This program has exactly the same output.

 I understand that the ''MVar m'' is not reachable from the other thread,
 but nevertheless the main thread is not blocked indefinately on this MVar.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8804#comment:2>
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