[GHC] #7719: System.Timeout.timeout may leak <<timeout>> exceptions

[GHC]

#7719: System.Timeout.timeout may leak <<timeout>> exceptions
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Reporter:  int-e         |          Owner:                
    Type:  bug           |         Status:  new           
Priority:  normal        |      Component:  libraries/base
 Version:  7.7           |       Keywords:                
      Os:  Linux         |   Architecture:  x86_64 (amd64)
 Failure:  None/Unknown  |      Blockedby:                
Blocking:                |        Related:                
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Comment(by int-e):

 A possible scenario for two nested threads on a single capability is the
 following, based on the fact that {{{threadDelay}}} masks exceptions while
 using {{{takeMVar}}} to wait for a timeout.
 {{{
 A  = timeout n (timeout m (return ())
 M  = TimerManager
 Ti = timer threads

 A: outer timeout ... set up T1
 A: inner timeout ... set up T2
 A: threadDelay (insert into M's queue, block on MVar)
         T1: threadDelay (insert into M's queue, block on MVar)
                 T2: threadDelay (insert into M's queue, block on MVar)
                         M: process queue, performing the takeMVar for A,
 T1, T2
 A: killThread T2 ... blocks, because T2 is now ready to run and has
 exceptions blocked!
         T1: throws TimeOut to A, which succeeds.
                 T2: throws TimeOut to A, which is enqueued.
 A: catches T1 in the inner timeout, re-raises T1 to the outer timeout,
 completes the call.
 A: Now catches the pending TimeOut from T2.
 }}}

 (Examples using multiple capabilities running in parallel are easier to
 construct.)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7719#comment:1>
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