[GHC] #7970: Thread GC frees roots before thread actually finishes

7 Jun 2013

      #7970: Thread GC frees roots before thread actually finishes
-----------------------------+----------------------------------------------
Reporter:  joeyadams         |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Runtime System  
 Version:  7.6.3             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  Runtime crash     |      Blockedby:                  
Blocking:                    |        Related:  7170            
-----------------------------+----------------------------------------------
 In the following program, an IORef is garbage collected after a
 `NonTermination` exception, but is subsequently accessed:

 {{{
 import Control.Exception as E
 import Data.IORef
 import System.Mem.Weak

 main :: IO ()
 main = do
     ref <- newIORef 'x'
     weak <- mkWeakIORef ref $ putStrLn "IORef finalized"
     let check = deRefWeak weak >>= \m -> case m of
             Nothing -> putStrLn "IORef was GCed"
             Just ref' -> do
                 x <- readIORef ref'
                 putStrLn $ "IORef still alive, and contains " ++ show x
     let loop = loop
     check
     loop `catch` \ex -> do
         putStrLn $ "caught exception: " ++ show (ex :: SomeException)
         check
     readIORef ref >>= print
 }}}

 Output:

 {{{
 IORef still alive, and contains 'x'
 IORef finalized
 caught exception: <<loop>>
 IORef was GCed
 'x'
 }}}

 The same happens with other thread deadlocks, such as:

  * newEmptyMVar >>= takeMVar

  * atomically retry

 It does not happen when a `StackOverflow` or `UserInterrupt` exception is
 caught.

 This also affects `ForeignPtr`; see the attached "database" example.  This
 is what really triggered #7170.  I marked this "Runtime crash" because it
 can lead to a `ForeignPtr` being accessed after the garbage collector
 finalized it.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7970
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