modifyMVar does not restore value if callback returns error value
modifyMVar is currently implemented as follows:
modifyMVar :: MVar a -> (a -> IO (a,b)) -> IO b
modifyMVar m io =
mask $ \restore -> do
a <- takeMVar m
(a',b) <- restore (io a) `onException` putMVar m a
putMVar m a'
return bThe problem is that it forces the (a',b) outside of the exception handler. If forcing this throws an exception, putMVar will not be called, and a subsequent withMVar or similar will hang. Example:
> import Control.Concurrent.MVar
> mv <- newMVar 'x'
> modifyMVar mv $ \_ -> return undefined
*** Exception: Prelude.undefined
> withMVar mv print
-- hang --Perhaps we can fix it like this:
- (a',b) <- restore (io a) `onException` putMVar m a
+ (a',b) <- restore (io a >>= evaluate) `onException` putMVar m aTrac metadata
| Trac field | Value |
|---|---|
| Version | 7.7 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | libraries/base |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |