[dancer-users] How can a fork+system() Dancer app not create zombie processes?

[Shlomi Fish]

Hi Andy,

On Mon, 3 Apr 2017 12:21:36 +0100
Andrew Beverley <andy at andybev.com> wrote:

> On Mon, 3 Apr 2017 13:22:16 +0300 Shlomi Fish wrote:
> > > Forking a web app is not as simple as it first seems.   
> > 
> > Do you mean forking any web app or just Dancer apps?   
> 
> Yes, any web application, and probably any application that runs
> continuously. The parent process needs to reap the child processes.
> 

sorry it took me so long to try that. I've now tried your suggestion of
double-forking and I'm still getting zombies. See:

* https://bitbucket.org/shlomif/app-notifier/branch/zombies-processes-bug

How can it be fixed?

Regards,

	Shlomi Fish

> > > You need to take
> > > an approach like the following (hopefully the comments will explain
> > > without me regurgitating here, but feel free to ask for
> > > clarification):
> > > 
> > > https://github.com/ctrlo/GADS/blob/cb8c9478dee8eac1f40a44486634173bed8d666d/lib/GADS/Record.pm#L1068
> > >   
> > 
> > I don't understand the code there in part because it uses too many
> > custom APIs  
> 
> As Racke says, you just need to use that concept. I.e. the parent
> process forks a child; the child process terminates immediately after
> it has spawned a grandchild, which allows the parent to reap the child.
> The grandchild is then left hanging and becomes init's responsibility.
> 
> > like "guard"s  
> 
> You should consider a guard, to ensure that nothing in the grandchild
> causes it to hang around. For example, if there is an exception which
> is then caught before the _exit(), you can end up with the grandchild
> hanging around serving web requests to nobody ;-)
> 
> > and "try".  
> 
> You can ignore that.
> 
> > Furthermore, the licence is the AGPLv3 which cannot be used inside my
> > MIT Expat code.  
> 
> IANAL, but I wouldn't say that small snippet of code is copywritable.
> Nonetheless, I can speak with authority for Ctrl O, and you are welcome
> to freely copy without condition that whole forking section if you want.
> 
> Andy



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Shlomi Fish       http://www.shlomifish.org/
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