[Haskell-cafe] GHC.Conc.threadStatus - documentation of ThreadDied :: ThreadStatus

[Olaf Klinke]

On Mon, 2022-03-07 at 19:59 +0000, coot at coot.me wrote:
> Hi Olaf,
> 
> `forkIO` is rather a low level.  It's more common to use async package (https://hackage.haskell.org/package/async).  Async has `waitCatch` which allows you to wait for a thread to finish and get access to either an exception which killed the thread or the result of the thread handler.
> 
> Best regards,
> Marcin Szamotulski
> 
> Sent with ProtonMail secure email.

Thanks for pointing this out, Marcin. 
Async seems to offer much better abstractions than what GHC.Conc
provides for  ThreadId. 
I have the impression, though, that Async was written for threads that
are supposed to do their work and eventually terminate. 
In my application, a webserver forks several perpetually running
threads and offers supervision to the user. Therefore withAsync is not
perfectly suited, as we do not know upfront when and what we're going
to do with the Async handle. I resorted to the following pattern. 

import Control.Concurrent.Async
import Control.Concurrent
import Control.Exception (SomeException)

type MyThread = (IO (),MVar (Async ()))

startThread :: MyThread -> IO ()
startThread (action,var) = withAsync action (putMVar var)

pauseThread :: MyThread -> IO ()
pauseThread (_,var) = do
    a <- takeMVar var
    cancel a
   
data MyThreadStatus = Paused | Running | Died SomeException
threadStatus :: MyThread -> IO MyThreadStatus
threadStatus (_,var) = do
    running <- tryReadMVar var
    case running of
        Nothing -> return Paused
        Just a -> do
            finished <- poll a
            case finished of
                Nothing         -> return Running
                Just (Right _)  -> return Paused
                Just (Left why) -> return (Died why)

-- Olaf

> 
> ------- Original Message -------
> 
> On Monday, March 7th, 2022 at 10:56, Olaf Klinke <olf at aatal-apotheke.de> wrote:
> 
> > Dear Cafe,
> > 
> > I had expected to see ThreadDied in the small example below.
> > 
> > But when I compile with
> > 
> > ghc --make -threaded -with-rtsopts=-N2
> > 
> > The output is:
> > 
> > threadStatus: user error (child thread is crashing!)
> > 
> > The status of my child is:
> > 
> > ThreadFinished
> > 
> > The output is not really a lie. But how do I determine whether a child
> > 
> > thread has exited normally or not? Wouldn't you say a call to fail (or
> > 
> > any other throwIO) should count as ThreadDied?
> > 
> > The documentation of GHC.Conc.forkIO says:
> > 
> > "... passes all other exceptions to the uncaught exception handler."
> > 
> > and the documentation for GHC.Conc.ThreadStatus says:
> > 
> > ThreadDied -- the thread received an uncaught exception
> > 
> > One can provoke ThreadDied by using throwTo from the parent thread. So
> > 
> > the emphasis in the documentation of ThreadDied should be on the word
> > 
> > "received".
> > 
> > This is a case of misleading documentation, in my humble opinion.
> > 
> > The constructor should not be named ThreadDied because that suggests
> > 
> > inclusion of internal reasons.
> > 
> > Olaf
> > 
> > -- begin threadStatus.hs
> > 
> > import Control.Concurrent
> > 
> > import GHC.Conc
> > 
> > main = mainThread
> > 
> > childThread :: IO ()
> > 
> > childThread = fail "child thread is crashing!"
> > 
> > mainThread :: IO ()
> > 
> > mainThread = do
> > 
> > child <- forkIO childThread
> > 
> > threadDelay 5000
> > 
> > status <- threadStatus child
> > 
> > putStr "The status of my child is: "
> > 
> > print status
> > 
> > -- end threadStatus.hs
> > 
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