I'm trying to create a process, and communicate with it via a handle that I provide outside the createProcess function:
stdOutH <- openFile (logDir </> "stdout.log") ReadWriteMode
hSetBuffering stdOutH LineBuffering
(_, _, _, ph) <- createProcess $
(proc "someproc" []) { std_out = UseHandle stdOutH
, std_err = UseHandle stdErrH
}
line <- hGetLine stdOutH
putStrLn $ "Got " ++ line
The "someproc" process spits a line out to the standard output, and I want to read it from the process that spawned it. However if I try to do this I get the following error:
hGetLine: illegal operation (handle is closed)
What I don't understand is why the handle is closed while the created process is running. This works if I use CreatePipe instead of UseHandle, the problem is that I only want to read the first line. But doing this requires to keep on reading from the pipe, otherwise it gets full after a certain amount of output by "someproc".
So, is there a way to use system.process to communicate two processes via stdOutH in the way described above?
This behavior of createProcess is documented:
Note that Handles provided for std_in, std_out, or std_err via the
UseHandle constructor will be closed by calling this function.
Documentation suggests to use createProcess_ function instead.
Related
I'm currently authoring an application in Haskell that relies on Yesod and its web sockets implementation.
I was wondering what is the correct way to acquire and release resources for a WebSocketT handler.
For example, in the following naive case...
chatApp :: WebSocketsT Handler ()
chatApp = do
let outgoingFlow = forever $ deliverOutgoingMessages
let incomingFlow = forever $ deliverIncomingMessages
bracket_ acquireResource
releaseResource
(race_ outgoingFlow incomingFlow)
... releaseResource does not seem to be called when a client disconnects abruptly or purposefully.
This is what I ended up doing over the weekend. This is essentially a replication of how web socket background ping process works, apart for the fact that I'm not swallowing the ping send exception when the other end is no longer reachable, but rather using it to detect the disconnection.
echoApp' :: WebSocketsT Handler ()
echoApp' = do
conn <- ask
let acquire = putStrLn "Acquiring..."
release = putStrLn "Releasing"
hardWork = (threadDelay 600000000)
ping i = do
threadDelay (30 * 1000 * 1000)
WS.sendPing conn (T.pack $ show i)
ping (i + 1)
liftIO $ bracket_ acquire release $ race_ hardWork (ping 1)
The downside of this approach is that there's still an up to 30 seconds window when the web socket process is lingering, but at least the resource gets eventually released in a more or less controllable way.
Consider something like this:
...
handleShutdown :: ThreadId -> IO ()
handleShutdown tid = doSomethingFunny >> throwTo tid ExitSuccess
main = do
...
installHandler sigTERM (Catch $ myThreadId >>= handleShutdown) Nothing
forever $ do
stuff
...
If sigINT (Ctrl+C) is handled in this manner, the process finishes nicely. However, it seems like sigTERM is being used by Haskell internally and the code above doesn't exit from the main process at all. Is there a way to exit the process from a sigTERM handler without using an MVar and a custom loop? I couldn't find any information on the sigTERM handling anywhere (didn't read ghc sources, that's just too much for me to handle).
Update:
The following works:
main = do
...
tid <- myThreadId -- This moved out of the Catch handler below.
installHandler sigTERM (Catch $ handleShutdown tid) Nothing
forever $ do
stuff
...
Sorry for short answer, but on mobile.
You want to run myThreadId from outside of the handler itself to get the main thread's ID. You're currently getting the ID of the signal handler itself.
I am currently attempting to play audio files in Haskell using OpenAl. In order to do so, I am trying to get the example code at the ALUT git repository (https://github.com/haskell-openal/ALUT/blob/master/examples/Basic/PlayFile.hs) to work. However, it refuses to produce any sound. What am I missing here?
{-
PlayFile.hs (adapted from playfile.c in freealut)
Copyright (c) Sven Panne 2005-2016
This file is part of the ALUT package & distributed under a BSD-style license.
See the file LICENSE.
-}
import Control.Monad ( when, unless )
import Data.List ( intersperse )
import Sound.ALUT
import System.Exit ( exitFailure )
import System.IO ( hPutStrLn, stderr )
-- This program loads and plays a variety of files.
playFile :: FilePath -> IO ()
playFile fileName = do
-- Create an AL buffer from the given sound file.
buf <- createBuffer (File fileName)
-- Generate a single source, attach the buffer to it and start playing.
source <- genObjectName
buffer source $= Just buf
play [source]
-- Normally nothing should go wrong above, but one never knows...
errs <- get alErrors
unless (null errs) $ do
hPutStrLn stderr (concat (intersperse "," [ d | ALError _ d <- errs ]))
exitFailure
-- Check every 0.1 seconds if the sound is still playing.
let waitWhilePlaying = do
sleep 0.1
state <- get (sourceState source)
when (state == Playing) $
waitWhilePlaying
waitWhilePlaying
main :: IO ()
main = do
-- Initialise ALUT and eat any ALUT-specific commandline flags.
withProgNameAndArgs runALUT $ \progName args -> do
-- Check for correct usage.
unless (length args == 1) $ do
hPutStrLn stderr ("usage: " ++ progName ++ " <fileName>")
exitFailure
-- If everything is OK, play the sound file and exit when finished.
playFile (head args)
Unfortunately, while I don't get any errors, I also can\t hear any sound. Pavucontrol also does not seem to detect anything (no extra streams appear under the Playback tab).
Their HelloWorld example on the same git repository also gave neither errors nor sound.
I also tried the OpenALInfo function on the same git repository (https://github.com/haskell-openal/ALUT/blob/master/examples/Basic/OpenALInfo.hs), which further proves that I'm actually connecting to OpenAL, and gives some information about the versions which may or may not be useful:
ALC version: 1.1
ALC extensions:
ALC_ENUMERATE_ALL_EXT, ALC_ENUMERATION_EXT, ALC_EXT_CAPTURE,
ALC_EXT_DEDICATED, ALC_EXT_disconnect, ALC_EXT_EFX,
ALC_EXT_thread_local_context, ALC_SOFTX_device_clock,
ALC_SOFT_HRTF, ALC_SOFT_loopback, ALC_SOFT_pause_device
AL version: 1.1 ALSOFT 1.17.2
AL renderer: OpenAL Soft
AL vendor: OpenAL Community
AL extensions:
AL_EXT_ALAW, AL_EXT_BFORMAT, AL_EXT_DOUBLE,
AL_EXT_EXPONENT_DISTANCE, AL_EXT_FLOAT32, AL_EXT_IMA4,
AL_EXT_LINEAR_DISTANCE, AL_EXT_MCFORMATS, AL_EXT_MULAW,
AL_EXT_MULAW_BFORMAT, AL_EXT_MULAW_MCFORMATS, AL_EXT_OFFSET,
AL_EXT_source_distance_model, AL_LOKI_quadriphonic,
AL_SOFT_block_alignment, AL_SOFT_buffer_samples,
AL_SOFT_buffer_sub_data, AL_SOFT_deferred_updates,
AL_SOFT_direct_channels, AL_SOFT_loop_points, AL_SOFT_MSADPCM,
AL_SOFT_source_latency, AL_SOFT_source_length
Well, it turns out I posted here a bit too quickly. There was no problem with my code, but rather with my OpenAl settings. By adding
drivers=pulse,alsa
to /etc/openal/alsoft.conf OpenAl works. This is described in https://wiki.archlinux.org/index.php/PulseAudio#OpenAL.
I'm using the GHC API to parse a module. If the module contains syntax errors the GHC API writes them to stdout. This interferes with my program, which has another way to report errors. Example session:
$ prog ../stack/src/Stack/Package.hs
../stack/src/Stack/Package.hs:669:0:
error: missing binary operator before token "("
#if MIN_VERSION_Cabal(1, 22, 0)
^
../stack/src/Stack/Package.hs:783:0:
error: missing binary operator before token "("
#if MIN_VERSION_Cabal(1, 22, 0)
^
../stack/src/Stack/Package.hs
error: 1:1 argon: phase `C pre-processor' failed (exitcode = 1)
Only the last one should be outputted. How can I make sure the GHC API does not output anything? I'd like to avoid libraries like silently which solve the problem by redirecting stdout to a temporary file.
I already tried to use GHC.defaultErrorHandler, but while I can catch the exception, GHC API still writes to stdout. Relevant code:
-- | Parse a module with specific instructions for the C pre-processor.
parseModuleWithCpp :: CppOptions
-> FilePath
-> IO (Either (Span, String) LModule)
parseModuleWithCpp cppOptions file =
GHC.defaultErrorHandler GHC.defaultFatalMessager (GHC.FlushOut $ return ()) $
GHC.runGhc (Just libdir) $ do
dflags <- initDynFlags file
let useCpp = GHC.xopt GHC.Opt_Cpp dflags
fileContents <-
if useCpp
then getPreprocessedSrcDirect cppOptions file
else GHC.liftIO $ readFile file
return $
case parseFile dflags file fileContents of
GHC.PFailed ss m -> Left (srcSpanToSpan ss, GHC.showSDoc dflags m)
GHC.POk _ pmod -> Right pmod
Moreover, with this approach I cannot catch the error message (I just get ExitFailure). Removing the line with GHC.defaultErrorHandler gives me the output shown above.
Many thanks to #adamse for pointing me in the right direction! I have found the answer in Hint's code.
It suffices to override logging in the dynamic flags:
initDynFlags :: GHC.GhcMonad m => FilePath -> m GHC.DynFlags
initDynFlags file = do
dflags0 <- GHC.getSessionDynFlags
src_opts <- GHC.liftIO $ GHC.getOptionsFromFile dflags0 file
(dflags1, _, _) <- GHC.parseDynamicFilePragma dflags0 src_opts
let dflags2 = dflags1 { GHC.log_action = customLogAction }
void $ GHC.setSessionDynFlags dflags2
return dflags2
customLogAction :: GHC.LogAction
customLogAction dflags severity _ _ msg =
case severity of
GHC.SevFatal -> fail $ GHC.showSDoc dflags msg
_ -> return () -- do nothing in the other cases (debug, info, etc.)
The default implementation of GHC.log_action can be found here:
http://haddock.stackage.org/lts-3.10/ghc-7.10.2/src/DynFlags.html#defaultLogAction
The code for parsing remains the same in my question, after having removed the line about GHC.defaultErrorHandler, which is no longer needed, assuming one catches exceptions himself.
I have seen this question before and then the answer was to temporarily redirect stdout and stderr.
To redirect stdout to a file as an example:
import GHC.IO.Handle
import System.IO
main = do file <- openFile "stdout" WriteMode
stdout' <- hDuplicate stdout -- you might want to keep track
-- of the original stdout
hDuplicateTo file stdout -- makes the second Handle a
-- duplicate of the first
putStrLn "hi"
hClose file
I'm using runCommand from System.Process but I use
"cd " ++ path ++ " & " ++ args
And it's not good, even will not work if path is on different local drive in windows.
How can I handle current directory change for runCommand ?
setCurrentDirectory from System.Directory changes the working directory of the main program.
If you do that before using runCommand, the command should use that directory too.
By looking at the source code of runCommand you can realize is just a thin wrapper for createProcess which is the one doing the real work. Here is an example taken from the createProcess documentation, which has been edited for convenience to address this question.
(_, Just hout, _, _) <- createProcess (proc "/path/to/my/executable" [])
{ cwd = Just "/path/to/working-directory"
, std_out = CreatePipe }