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I'm seeing a problem mixing MVar and Weak: even though the MVar is still accessible from a function's scope (and the main thread is blocking on it), the Weak thinks it should be finalized.

Only seems to happen when compiled with -O2.

Platform: 64-bit Linux Reproduced with GHC versions: 6.10.4, 6.12.3, 7.0.4, 7.2.2, 7.4.1

module Main (main) where

import           Control.Concurrent
import           Control.Monad (forever, forM_)
import           Data.IORef
import           System.Mem
import           System.Mem.Weak

dispatchPendingCalls :: IORef [Weak (MVar ())] -> IO ()
dispatchPendingCalls ref = forever $ do
	threadDelay 100000
	
	pending <- atomicModifyIORef ref (\p -> ([], p))
	forM_ pending (\weak -> do
		maybeMVar <- deRefWeak weak
		case maybeMVar of
			Just mvar -> putMVar mvar ()
			Nothing -> putStrLn "dispatchReply: weak mvar is Nothing")

call :: IORef [Weak (MVar ())] -> IO ()
call ref = do
	mvar <- newEmptyMVar
	weak <- mkWeakPtr mvar (Just (putStrLn "call: finalising weak"))
	
	putStrLn "call: about to insert weak into list"
	atomicModifyIORef ref (\p -> (weak : p, ()))
	putStrLn "call: inserted weak into list"
	performGC
	takeMVar mvar
	putStrLn "call: took from mvar"

main :: IO ()
main = do
	pendingCalls <- newIORef []
	_ <- forkIO (dispatchPendingCalls pendingCalls)
	call pendingCalls

Expected output:

$ ghc --make WeakVar.hs
$ ./WeakMvar
call: about to insert weak into list
call: inserted weak into list
call: took from mvar

Actual output:

$ ghc --make -O2 WeakVar.hs
$ ./WeakMvar
call: about to insert weak into list
call: inserted weak into list
call: finalizing weak
dispatchReply: weak mvar is Nothing

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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