[GHC] #9965: getChanContents BlockedIndefinitelyOnMVar on 7.10 RC1

7 Jan 2015

      #9965: getChanContents BlockedIndefinitelyOnMVar on 7.10 RC1
-------------------------------------+-------------------------------------
              Reporter:  jberryman   |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:              |           Version:  7.10.1-rc1
  libraries/base                     |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  Incorrect result
          Architecture:              |  at runtime
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 All of the stanzas work on 7.8.3, but the middle one works incorrectly on
 7.10:

 {{{#!hs
 module Main where

 import Control.Concurrent.Chan
 import Control.Monad

 main = do
     io <- newChan
     let inp = [0] :: [Int]
     mapM_ (writeChan io) inp
 {-
     -- WORKS:
     outp <- getChanContents io
     if 0 == head outp
         then putStrLn "OK"
         else error $ "Smoke test failed"
 -}
     -- BROKEN ON 7.10 (BlockedIndefinitelyOnMVar):
     outp <- getChanContents io
     if and (zipWith (==) inp outp)
         then putStrLn "OK"
         else error $ "Smoke test failed"
 {-
     -- WORKS:
     forM_ inp $ \xIn-> do
         xOut <- readChan io
         unless (xIn == xOut) $
             error $ "Smoke test failed"
 -}
 }}}

 I first noticed the behavior in a Chan-like library I wrote, whose
 internals are completely different from Chan, but where the
 getChanContents is a copy-paste from Chan. So I assume it has something to
 do with `unsafeInterleaveIO`.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9965
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