id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
767,withMVar family have a bug,simonmar,,"{{{withMVar}}} is defined like this:

{{{
withMVar :: MVar a -> (a -> IO b) -> IO b
withMVar m io = 
  block $ do
    a <- takeMVar m
    b <- catch (unblock (io a))
      	    (\e -> do putMVar m a; throw e)
    putMVar m a
    return b
}}}

unfortunately this has a (very rare) bug: {{{catch}}} can raise a stack overflow exception, which would leave the {{{MVar}}} empty.

This is a tricky one.  Perhaps in the event of a stack overflow, catch should ensure that the exception is passed directly to its handler, and we always add some extra stack space for the handler.",bug,closed,high,6.8.1,Compiler,6.4.1,fixed,,,Unknown/Multiple,Unknown/Multiple,,Unknown,N/A,,,