[Haskell-beginners] forkProcess behaviour

[PICCA Frederic-Emmanuel]

> Hi Frederic,

Hello Michael

Thanks a lot for your informative answer.

> Let me give a high level recommendation: don't use forkProcess. It's a very dangerous function, which can result in confusing behavior and impossible-to-debug problems. There are cases where it can be made to work, but (1) it's complicated, (2) I'm not sure anyone has ever figured out all of those caveats, and (3) it's certainly not documented properly.

> forkProcess is little more than a call to the fork() system call, creating a brand new child process which will run the IO action provided. The runtimes of the two processes will not be > connected to each other at all. It would be impossible to, say, throw an exception from a thread in the parent process to a thread in the child process.

> I could say a lot more about this, but I think I'll just reiterate my original recommendation: don't use forkProcess :)

> Instead, for this kind of use case of changing user/group IDs, I'd recommend using a normal external process call via the process package[1]. I'm not sure of your use case exactly, > but I see three ways of making this work:

> * Generate two Haskell executables
> * Put the code for both the parent and child into a single executable, and use command line arguments or env vars to control which behavior is run
> * If you don't have any real logic in the Haskell code, and instead are just using some other program: you can call that directly

I am quite close to this model, since I have only once executable and the command line parameters allows to execute the different job like this

autoprocessing-exe [exec|submit|server] <jobname> parameters

I start my service via the server command without parameters.
Then from another computer, I can submit jobs to that server.
And locally I can execute jobs with the exec command.

What you proposed is just to execute the autoprocessing-command line with s/submit/exec in order to execute a child process.

I think that I can do this.

Now sometimes this process hang and I want to add a timeout, can I just use timeout for this purpose.

2) I do not want to have communication between my server process and the child one, so in that case is it worth changing the code in order to use process instead of forkPRocess.

With process I will need to had lot's of code in oder to convert my job type into command line.

I have for now the right Parser command line -> jobtype, but not the other way around.
This is why I used forkProcess et first.

Once again thanks for your valuable comments.

Frédéric