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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

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture