#4934: threadWaitRead works incorrectly on nonthreaded RTS
---------------------------------+------------------------------------------
Reporter: slyfox | Owner:
Type: bug | Status: new
Priority: normal | Component: Runtime System
Version: 7.0.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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I found out it in xmobar on ghc-7.0.1.
When I ran it I got the following message:
{{{
xmobar: file descriptor 8954336 out of range for select (0--1024).
Recompile with -threaded to work around this.
}}}
'''-threaded''' option works, but the message is absolutely misleading.
I've decided to track where so large descriptor could come,
but there was no such place.
All xmobar does is:
{{{
do x11connection_fd <- connectionNumber d
threadWaitRead (Fd fd) -- here we die
}}}
'''x11connection_fd''' has value '''3''' at the time of call.
This value comes from '''libX11''' (via FFI, so I suspect it's a problem).
Another example (might be unrelated):
{{{
-- a.hs
import Control.Concurrent
-- 12 is picked at random as guaranteed to be invalid FD
main = threadWaitRead 12 >> print "yet input!"
}}}
Building:
{{{
$ ghc --make -fforce-recomp a.hs -o a
[1 of 1] Compiling Main ( a.hs, a.o )
Linking a ...
$ ghc --make -fforce-recomp a.hs -threaded -o a.threaded
[1 of 1] Compiling Main ( a.hs, a.o )
Linking a.threaded ...
}}}
Running:
{{{
$ ./a.threaded
a.threaded: epollControl: invalid argument (Bad file descriptor)
}}}
Looks good.
And nonthreaded:
{{{
$ ./a
"yet input!"
}}}
According to strace there was an error but it was not reported.
{{{
sf tmp # strace -etrace='select,write' ./a >/dev/null
select(13, [12], [], NULL, {134, 217727}) = -1 EBADF (Bad file descriptor)
# this one
select(2, [], [1], NULL, {0, 0}) = 1 (out [1], left {0, 0})
write(1, "\"yet input!\"\n", 13) = 13
}}}
Side note: Haskeline seems to workaround this problem on it's own way.
'''libraries/haskeline/System/Console/Haskeline/Backend/Posix.hsc'''
{{{
-- Different versions of ghc work better using different functions.
blockUntilInput :: Handle -> IO ()
#if __GLASGOW_HASKELL__ >= 611
-- threadWaitRead doesn't work with the new ghc IO library,
-- because it keeps a buffer even when NoBuffering is set.
blockUntilInput h = hWaitForInput h (-1) >> return ()
#else
-- hWaitForInput doesn't work with -threaded on ghc < 6.10
-- (#2363 in ghc's trac)
blockUntilInput h = unsafeHandleToFd h >>= threadWaitRead
#endif
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4934>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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