[GHC] #8684: hWaitForInput cannot be interrupted by async exceptions on unix

[GHC]

#8684: hWaitForInput cannot be interrupted by async exceptions on unix
-------------------------------------+-------------------------------------
        Reporter:  nh2               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  7.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:  13497, 13525      |             Blocking:
 Related Tickets:  #12912, #13525    |  Differential Rev(s):  Phab:D42
       Wiki Page:                    |
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Comment (by nh2):

 OK new info with the power of `-debug` and `+RTS -Ds`:

 In non`-threaded`, it does raise the exception, but _after_ the entire
 `hWaitForInput` is over.

 I can see it in `cap 0: raising exception in thread 2.`, and also the
 reason:

 `timeout`'s `throwTo ` also only happens **after** `hWaitForInput` is over
 (it prints `cap 0: throwTo: from thread 1 to thread 2` only at the very
 end).

 So that means it's not the exception handling that's not working, it's the
 throwing.

 My C code returns back into Haskell land, but there is no exception there
 at the time so it continues.


 ----


 On my debug commit
 https://github.com/nh2/ghc/blob/49a5e9ce7062da7594bfd86ca7af92796b84b52a/libraries/base/cbits/inputReady.c#L53-L68
 that looks like this (relevant output without `-debug` in
 [https://gist.github.com/nh2/f543030a9b68b1530b19087d0c655115 this gist]):

 {{{
 cap 0: running thread 1 (ThreadRunGHC)
 cap 0: thread 1 stopped (suspended while making a foreign call)
 fdReady called with msecs = 20
 fdReady res = -1
 cap 0: running thread 1 (ThreadRunGHC)
 cap 0: thread 1 stopped (suspended while making a foreign call)
 fdReady called with msecs = 10
 fdReady res = -1
 cap 0: running thread 1 (ThreadRunGHC)
 cap 0: thread 1 stopped (suspended while making a foreign call)
 fdReady called with msecs = 0
 fdReady res = 0
 cap 0: running thread 1 (ThreadRunGHC)
 cap 0: throwTo: from thread 1 to thread 2
         thread    2 @ 0x42001059d0 is blocked until 558048911098409
 (TSO_DIRTY)
 cap 0: raising exception in thread 2.
 cap 0: thread 1 stopped (finished)
 bound thread (1) finished
 task exiting
 }}}

 Observe here how `cap 0: throwTo: from thread 1 to thread 2` is after the
 `fdReady called with msecs = 0` (`hWaitForInput` ran its entire 5
 seconds).

 ----

 So now we just need to find out why `timeout` doesn't result in a prompt
 `cap 0: throwTo: from thread 1 to thread 2` after 1 second.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8684#comment:16>
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