[GHC] #9030: An async exception handler that blocks throwTo until handler finishes running

23 Apr 2014

      #9030: An async exception handler that blocks throwTo until handler finishes
running
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       Reporter:  jberryman         |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 I'd like to be able to handle an async exception in thread `A` such that
 if another thread `B` calls `throwTo`, that call blocks until my handler
 in `A` has a chance to ''finish''. Currently `throwTo` only blocks until
 the exception is "received" (which I don't understand precisely).

 My application is a concurrent FIFO queue library, in which a blocked
 reader must perform a cleanup action on async exceptions, else the next
 write will be lost:

 {{{
 testBlockedRecovery = do
     c <- newChan
     started <- newEmptyMVar
     rid <- forkIO (putMVar started () >> readChan c)
     takeMVar started >> threadDelay 1000
     throwTo rid ThreadKilled
     -- we race the exception-handler in `readChan` here...
     writeChan c ()
     -- If this write won, then the write was lost and we block
 indefinitely below:
     readChan c
     putStrLn "OK"
 }}}

 If in my library I could catch the exception in such a way that `throwTo`
 blocked until I could handle it, then the test above would pass and the
 behavior of my Chan would match the standard library.

 This might seem like a lame example, but I think this functionality would
 be very generally useful. It would let you recover to a state using an
 exception handler such that you can reason in terms of linearizability
 about the pre- and post- exception states; I assume that's sort of the
 thinking behind making `throwTo` synchronous in the first place.

 Thanks and please let me know if there's actually a way to do what I'm
 asking currently, or if any of that wasn't clear.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9030
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