#3291: "Thread blocked indefinitely" kills the program despite being caught
---------------------------------+------------------------------------------
Reporter: batterseapower | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Runtime System | Version: 6.10.3
Severity: normal | Resolution: invalid
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Changes (by simonmar):
* status: new => closed
* difficulty: => Unknown
* resolution: => invalid
Old description:
> Perhaps this is just me misunderstanding the semantics of exceptions, but
> the following program:
>
> """
> import Test.HUnit.Lang
> import Control.Concurrent.STM
>
> import Control.Exception
>
> import Control.Concurrent
> import Control.Concurrent.MVar
>
> main = do
> mv <- newEmptyMVar
> forkIO $ do
> r <- performTestCase (atomically retry)
> print "Yeaahh!"
> print r
> evaluate r
> putMVar mv r
> print "Cool, let's see what we get"
> r <- takeMVar mv
> print r
> print "Am I printed?"
> """
>
> Should in my opinion, when run, print "Am I printed". This is because the
> "Thread blocked indefinitely" exception is caught by HUnit. However, this
> happens instead:
>
> """
> mbolingbr...@mb566 ~/Junk
> $ ./Repro
> "Cool, let's see what we get"
> "Yeaahh!"
> Just (False,"thread blocked indefinitely")
> Repro: thread blocked indefinitely
> """
>
> NB: I get the expected behaviour if I run it within GHCi, but not if I
> compile the program.
>
> This is causing problems for test-framework (see
> http://bsp.lighthouseapp.com/projects/15661-hs-test-framework/tickets/1
> -exits-immediately-on-thread-blocked-indefinitely-exception#ticket-1-2)
New description:
Perhaps this is just me misunderstanding the semantics of exceptions, but
the following program:
{{{
import Test.HUnit.Lang
import Control.Concurrent.STM
import Control.Exception
import Control.Concurrent
import Control.Concurrent.MVar
main = do
mv <- newEmptyMVar
forkIO $ do
r <- performTestCase (atomically retry)
print "Yeaahh!"
print r
evaluate r
putMVar mv r
print "Cool, let's see what we get"
r <- takeMVar mv
print r
print "Am I printed?"
}}}
Should in my opinion, when run, print "Am I printed". This is because the
"Thread blocked indefinitely" exception is caught by HUnit. However, this
happens instead:
{{{
mbolingbr...@mb566 ~/Junk
$ ./Repro
"Cool, let's see what we get"
"Yeaahh!"
Just (False,"thread blocked indefinitely")
Repro: thread blocked indefinitely
}}}
NB: I get the expected behaviour if I run it within GHCi, but not if I
compile the program.
This is causing problems for test-framework (see
http://bsp.lighthouseapp.com/projects/15661-hs-test-framework/tickets/1
-exits-immediately-on-thread-blocked-indefinitely-exception#ticket-1-2)
Comment:
The GC determines which threads are blocked indefinitely: those that are
unreachable from any root can never be woken up. So what's happening here
is that when the forkIO'd thread becomes unreachable, the main thread is
also unreachable. Both threads therefore get BlockedIndefinitely
exceptions.
If you want the main thread to stay alive, you can add a
{{{
myThreadId >>= newStablePtr
}}}
to `main`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3291#comment:2>
GHC <http://www.haskell.org/ghc/>
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