IO manager can leak MVars if threads are killed

If a thread is killed in threadWaitRead and no data ever comes in on the file descriptor it was waiting on, the IO manager will not remove the FdData structure from its queue of waited-for events. This causes the queue to slowly grow in size.

Test case:

-- Compile with: ghc -threaded --make

import Control.Monad
import Control.Concurrent

main = do
  forM_ [0..10000] $ \i -> do
    when (i`mod`100==0) $ print i
    t <- forkIO $ threadWaitRead 0
    threadDelay 5000
    killThread t

This is not hugely likely to affect real programs, but could lead to a (perhaps accidental) denial of service against a long-running application.

Trac metadata

Trac field	Value
Version	7.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	tibbe
Operating system
Architecture