[GHC] #7493: STM and TVar report incorrect results

[GHC]

#7493: STM and TVar report incorrect results
----------------------------------------+-----------------------------------
Reporter:  parcs                        |          Owner:                  
    Type:  bug                          |         Status:  new             
Priority:  normal                       |      Component:  Runtime System  
 Version:  7.6.1                        |       Keywords:                  
      Os:  Unknown/Multiple             |   Architecture:  Unknown/Multiple
 Failure:  Incorrect result at runtime  |      Blockedby:                  
Blocking:                               |        Related:                  
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 On Haskell Cafe, I posted:

 I'm getting strange behavior when using the 'many' combinator to read zero
 or more items off of a TQueue with readTQueue. The script that exhibits
 this behavior is as follows:

 {{{
 import Control.Concurrent.STM
 import Control.Concurrent
 import Control.Monad
 import Control.Applicative

 main = do
     q <- newTQueueIO
     atomically $ writeTQueue q True
     atomically $ writeTQueue q False
     forever $ do
         xs <- atomically $ many $ readTQueue q
         print xs
         threadDelay 500000
 }}}

 I'd expect the output of the script to be:
 {{{
 [True,False]
 []
 []
 ...
 }}}

 However, that is not the case: the actual output of the script is:
 {{{
 [True,False]
 [True,False]
 [True,False]
 ...
 }}}

 If 1 element (say, True) is written into the TQueue instead of 2, then the
 output of the script is:
 {{{
 [True]
 []
 []
 ...
 }}}

 Which is expected behavior, but inconsistent with the behavior when the
 TQueue has 2 or more elements in it.

 Is this considered a bug, or undocumented behavior of TQueue?

 ----

 Bas vas Dijk noted that this may be a bug in STM, and provided a condensed
 test case which reproduces the behavior of my original script:

 {{{
 $ cat stmTest.hs
 import Control.Concurrent.STM
 main = do
   x <- atomically $ do
          t <- newTVar 1
          writeTVar t 2
          ((readTVar t >> retry) `orElse` return ()) `orElse` return ()
          readTVar t
   print x

 $ ghc --make stmTest.hs -fforce-recomp -threaded -o stmTest && ./stmTest
 [1 of 1] Compiling Main             ( stmTest.hs, stmTest.o )
 Linking stmTest ...
 1
 }}}

 The program prints 1 when it should print 2.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7493>
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