Thread blocked indefinitely in an MVar operation

Question

I have been attempting to debug a problem when using multiple MVars, however to no luck.

My code uses two MVars: one to store the servers current state, and another to pass network events to and from the client threads. However after connecting and disconnecting several times, the server stops sending data upon new clients connecting (presumably because the network events MVar is emptied for whatever reason) and eventually trips up with the error: *** Exception: thread blocked indefinitely in an MVar operation

I have concluded the following in trying to debug this issue over the last few days:

The functions used to modify the MVar(s) wont throw exceptions
The problem does not occur until a client either connects, or connects then disconnects
The issue seems to occur randomly (sometimes several clients can connect then disconnect, other times it happens right away)

I have isolated the problem to three files:

https://github.com/Mattiemus/IMC-Server/blob/master/IMC.hs (the exception gets thrown in sense)
https://github.com/Mattiemus/IMC-Server/blob/master/IMC/Networking/Server.hs (Modifined within application handleClient, and cleanupClient)
https://github.com/Mattiemus/IMC-Server/blob/master/IMC/Utilities/Concurrency.hs (functions which push and pop to a list stored in an MVar)

I am totally out of ideas, as I only use modifyMVar and withMVar (so surely it should never be totally left empty) - my only assumption is that maybe an Exception is being thrown while modifying the MVar, however I think this is highly unlikely.

Any help is appreciated, this problem has been bugging me for some time now.

I don't think most people will want to search through so much code to find this bug. Perhaps writing some tests will help you find it. — user2407038
I have modified the question to further pin down the problem to 4 main functions. What sort of tests would I need to write to isolate this problem, the problem seems to eventually occur regardless of weather i remove all modifiers over the either the server or events MVar, leading me to believe I am somehow using them incorrectly. There are multiple producers over the events MVar (one thread for each client) however only a single consumer - maybe this is the issue? — Mattiemus
In sense you are using an MVar very much like a channel. Why not use a Chan instead? — Chris Kuklewicz
modifyMVar and withMVar are safe. I do not see swapMVar used, which is unsafe, is it used elsewhere? — Chris Kuklewicz
@ChrisKuklewicz swapMVar was removed in favor of using modifyMVar_ due to this problem, however the core issue still persists. I dont use Chan because I dont want to block while the channel is empty, I want to use it as if there is simply an empty list of events. See popAllMVar in Concurrency.hs — Mattiemus

Mattiemus Mattiemus · Accepted Answer · 2015-06-18T02:41:46

Three days later and its solved: Was actually unrelated to either the networking or concurrency code, and infact caused by my incorrect re-implementation of Yampas dpSwitch in Netwire. Corrected code posted below for anyone wishing to implement this function:

dpSwitch :: (Monoid e, Applicative m, Monad m, T.Traversable col) => (forall wire. a -> col wire -> col (b, wire))
     -> col (Wire s e m b c)
     -> Wire s e m (a, col c) (Event d)
     -> (col (Wire s e m b c) -> d -> Wire s e m a (col c))
     -> Wire s e m a (col c)
dpSwitch route wireCol switchEvtGen continuation = WGen $ gen wireCol switchEvtGen
where
    gen wires switchEvtGenWire _ (Left x) = return (Left mempty, WGen $ gen wires switchEvtGenWire)
    gen wires switchEvtGenWire ws (Right x) = do            
        let routings = route x wires
        wireSteps <- T.sequenceA (fmap (\(wireInput, wire) -> stepWire wire ws (Right wireInput)) routings)
        let wireOutputs = T.sequenceA (fmap fst wireSteps)
            steppedWires = fmap snd wireSteps
        case wireOutputs of
            Left wireInhibitedOutput -> return (Left wireInhibitedOutput, WGen $ gen steppedWires switchEvtGenWire)
            Right wireResultOutput -> do
                (event, steppedSwitchEvtGenWire) <- stepWire switchEvtGenWire ws (Right (x, wireResultOutput))
                case event of
                    Left eventInhibited -> return (Left eventInhibited, WGen $ gen steppedWires steppedSwitchEvtGenWire)
                    Right NoEvent -> return (wireOutputs, WGen $ gen steppedWires steppedSwitchEvtGenWire)
                    Right (Event e) -> return (wireOutputs, continuation steppedWires e)

Thread blocked indefinitely in an MVar operation

3 Answers