#4876: isEmptySampleVar returns False when threads are waiting on an empty
SampleVar
---------------------------------+------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 7.0.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect result at runtime
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The source code for Control.Concurrent.SampleVar.isEmptySampleVar:
{{{
isEmptySampleVar :: SampleVar a -> IO Bool
isEmptySampleVar (SampleVar svar) = do
(readers, _) <- readMVar svar
return (readers == 0)
}}}
should have readers <= 0, as the state readers < 0 is used to indicate the
SampleVar is empty and has threads waiting on it.
As an example:
{{{
import System.Random
import Control.Concurrent.SampleVar
import Control.Concurrent
do_something = threadDelay 100000 -- 100 ms
loop body = body >> loop body
produce, consume :: SampleVar Int -> IO ()
produce svar = do
do_something
b <- isEmptySampleVar svar
if b then randomIO >>= writeSampleVar svar else return ()
consume svar = readSampleVar svar >>= print
main = do
svar <- newEmptySampleVar
forkIO $ loop $ produce svar
forkIO $ loop $ consume svar
threadDelay 1000000 -- one second
}}}
This code deadlocks instead of printing random numbers.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4876>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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