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[GHC] #4245: ghci panic: thread blocked indefinitely in an MVar operation

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[GHC] #4245: ghci panic: thread blocked indefinitely in an MVar operation

#4245: ghci panic: thread blocked indefinitely in an MVar operation
--------------------------+-------------------------------------------------
Reporter: pturnbull | Owner:
Type: bug | Status: new
Priority: normal | Component: GHCi
Version: 6.12.3 | Keywords: MVar
Testcase: | Blockedby:
Os: MacOS X | Blocking:
Architecture: x86 | Failure: GHCi crash
--------------------------+-------------------------------------------------
I stumbled across this error today:
{{{
phil ~
$ ghci
GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude>
[1]+ Stopped ghci
phil ~
$ fg
ghci
^C^C

Prelude>
ghc: panic! (the 'impossible' happened)
(GHC version 6.12.3 for i386-apple-darwin):
thread blocked indefinitely in an MVar operation

Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

phil ~
$
}}}

Steps to reproduce:
* start ghci
* ctrl-z
* fg
* ctrl-c
* ctrl-c
* enter
* wait ~2 seconds

I'm not sure if it is related but while waiting for the impossible to
happen, any characters typed are not echoed to the screen.

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4245>
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The Glasgow Haskell Compiler
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#4245: ghci panic: thread blocked indefinitely in an MVar operation
---------------------------------+------------------------------------------
Reporter: pturnbull | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.0.2
Component: GHCi | Version: 7.0.1
Keywords: MVar | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: x86 | Failure: GHCi crash
---------------------------------+------------------------------------------
Changes (by Jafet):

* version: 6.12.3 => 7.0.1
* os: MacOS X => Unknown/Multiple

Comment:

I've encountered the same bug, but in the Windows console with GHC 7.0.1.

To reproduce:
1. Start {{{ghci}}} from cmd.exe (or Cygwin bash) in a console.
2. Press {{{^C}}}.
3. Hold down the return key.

{{{
C:\>ghci
GHCi, version 7.0.1: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude> ^C
C:\>
Prelude>
C:\>
Prelude>
C:\>

... after a moment ...

Prelude>
C:\>
Prelude>
ghc.exe: panic! (the 'impossible' happened)
(GHC version 7.0.1 for i386-unknown-mingw32):
thread blocked indefinitely in an MVar operation

Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

C:\>
C:\>
}}}

After {{{^C}}}, GHC backgrounds itself from cmd.exe and subsequent input
finds its way to the shell:

{{{
Prelude> ^C
C:\>
Prelude> input
'input' is not recognized as an internal or external command,
operable program or batch file.
C:\>input
'nput' is not recognized as an internal or external command,
operable program or batch file.
}}}

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4245#comment:5>
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#4245: ghci panic: thread blocked indefinitely in an MVar operation
-------------------------------+--------------------------------------------
Reporter: pturnbull | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.0.2
Component: GHCi | Version: 6.12.3
Keywords: MVar | Testcase:
Blockedby: | Difficulty:
Os: MacOS X | Blocking:
Architecture: x86_64 (amd64) | Failure: GHCi crash
-------------------------------+--------------------------------------------
Changes (by paterno):

* version: 7.0.1 => 6.12.3
* os: Unknown/Multiple => MacOS X
* architecture: x86 => x86_64 (amd64)

Comment:

I can verify this problem appears in the Mac OS X release as well.
My output is below.
{{{
bash-$ uname -a
Darwin thomas 10.6.0 Darwin Kernel Version 10.6.0: Wed Nov 10 18:11:58 PST
2010; root:xnu-1504.9.26~3/RELEASE_X86_64 x86_64

bash-$ ghci
GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude>
[1]+ Stopped ghci
bash-$ fg
ghci
^C^C

Prelude>
ghc: panic! (the 'impossible' happened)
(GHC version 6.12.3 for i386-apple-darwin):
thread blocked indefinitely in an MVar operation

Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4245#comment:8>
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#4245: ghci panic: thread blocked indefinitely in an MVar operation
-------------------------------+--------------------------------------------
Reporter: pturnbull | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.2.1
Component: GHCi | Version: 6.12.3
Keywords: MVar | Testcase:
Blockedby: | Difficulty:
Os: MacOS X | Blocking:
Architecture: x86_64 (amd64) | Failure: GHCi crash
-------------------------------+--------------------------------------------

Comment(by Artyom.Kazak):

I'm using Windows 7, GHC 7.0.2. I don't know is it the same bug…

I was playing with my module called Helpers.Numeric located in file called
Numeric.hs. As you can see, I compiled it using GHC and by mistake ran
Numeric.hs instead of Numeric.exe, so I had GHCi in GHCi. Then I wanted to
enter ':q', but entered just 'q', and the inner GHCi crashed.

{{{
*Helpers.Numeric Math.Root.Finder> :! ghc --make -O2 -main-is
Halpers.Numeric Numeric.hs
[2 of 2] Compiling Helpers.Numeric ( Numeric.hs, Numeric.o )
*Helpers.Numeric Math.Root.Finder> :! Numeric.hs
GHCi, version 7.0.2: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Ok, modules loaded: Helpers.Numeric, Ratio'.
Prelude Helpers.Numeric>
Prelude Helpers.Numeric> q

<interactive>:1:1: Not in scope: `q'
Prelude Helpers.Numeric>
ghc.exe: panic! (the 'impossible' happened)
(GHC version 7.0.2 for i386-unknown-mingw32):
thread blocked indefinitely in an MVar operation

Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4245#comment:11>
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#4245: ghci panic: thread blocked indefinitely in an MVar operation
-------------------------------+--------------------------------------------
Reporter: pturnbull | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.2.1
Component: GHCi | Version: 6.12.3
Keywords: MVar | Testcase:
Blockedby: | Difficulty:
Os: MacOS X | Blocking:
Architecture: x86_64 (amd64) | Failure: GHCi crash
-------------------------------+--------------------------------------------
Changes (by fryguybob):

* cc: fryguybob@… (added)

Comment:

I've experienced the behavior from the last reporter on Windows with
7.0.2. For a while I've tried to get something repeatable to no avail.
When it is happening it seems to happen fairly frequently. Some days I
never run into it. I have never had it happen with HEAD which makes me
suspect that #4531 fixed it. Some things that have triggered it for me:

* Typing `q` instead of `:q` (other things not in scope too).
* Hitting tab then `y` to display all.
* Pasting a long line. When it fails in this case I have been pasting a
`let` expression and initially just an `l` shows. I hit backspace and
paste again then it fails with the full line.

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4245#comment:12>
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#4245: ghci panic: thread blocked indefinitely in an MVar operation
-------------------------------+--------------------------------------------
Reporter: pturnbull | Owner: igloo
Type: bug | Status: new
Priority: high | Milestone: 7.4.1
Component: GHCi | Version: 6.12.3
Keywords: MVar | Testcase:
Blockedby: | Difficulty:
Os: MacOS X | Blocking:
Architecture: x86_64 (amd64) | Failure: GHCi crash
-------------------------------+--------------------------------------------

Comment(by igloo):

With `z.hs`:
{{{
import Control.Concurrent
import Control.Exception
import Prelude hiding (catch)
import System.IO

main :: IO ()
main = do mv <- newEmptyMVar
tid <- forkIO $ otherThread mv
takeMVar mv
`catch` \e ->
do putStrLn ("Got exception: " ++ show (e :: SomeException))
putStrLn "Killing thread"
killThread tid
putStrLn "Killed thread"

otherThread :: MVar () -> IO ()
otherThread mv = do putStrLn "Waiting for input"
_ <- hWaitForInput stdin (-1)
putStrLn "There's some input"
putMVar mv ()
}}}
if we compile with HEAD on OS X (x86 or x86_64) with the threaded RTS:
{{{
ghc --make z -threaded
}}}
then run it, `^Z` it, `fg` it, then `^C` it, then the `killThread` blocks:
{{{
$ ./z
Waiting for input
^Z
[1]+ Stopped ./z
212:examples ian$ fg
./z
^CGot exception: user interrupt
Killing thread
}}}
Another `^C` silently terminates the program.

It doesn't happen if:
* We don't do the `^Z`/`fg`
* We don't use the threaded RTS
* It is run on Linux

So now the question is, why does it happen and how do we fix it?

(I believe Haskeline doing something like the above, mixed in with using
an MVar, is the underlying case of the GHCi bug)

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4245#comment:19>
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