[GHC] #15312: getChanContents exception behavior seems a bit odd

26 Jun 2018

      #15312: getChanContents exception behavior seems a bit odd
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Core           |           Version:  8.4.3
  Libraries                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I've been playing around with `Control.Concurrent.Chan` today. Something
 seems a bit off:

 {{{#!hs
 -- Bug.hs
 import Control.Concurrent (forkIO, yield)
 import Control.Concurrent.Chan
 import Data.List (elem)
 import Control.Exception
 import Control.Concurrent.MVar

 data Ex = Ex deriving Show
 instance Exception Ex

 main = do
   ch <- newChan
   sync1 <- newEmptyMVar
   sync2 <- newEmptyMVar
   forkIO $ do {writeList2Chan ch [1..3*10^6 :: Int]; putMVar sync1 ()}
   yield
   writeChan ch (-12)
   cont <- getChanContents ch

   tid <- forkIO $ do
            evaluate (last cont)
            putMVar sync2 ()

   yield
   throwTo tid Ex

   print (elem (3*10^6) cont)
   takeMVar sync1
   tryTakeMVar sync2
 }}}

 When I run this single-threaded (`+RTS -N1`), it prints

 {{{
 Bug: Ex
 Bug: Ex
 }}}

 One of the thunks in the lazy list gets overwritten by the (asynchronous)
 exception. This seems a bit surprising; is it the way it should be? Does
 `hGetContents` do this too?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15312
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