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Re: MinGW open-process, take N

From: Eli Zaretskii
Subject: Re: MinGW open-process, take N
Date: Thu, 14 Jul 2016 21:41:18 +0300

> From: Andy Wingo <address@hidden>
> Cc: address@hidden,  address@hidden,  address@hidden
> Date: Thu, 14 Jul 2016 20:11:28 +0200
> On Thu 14 Jul 2016 17:34, Eli Zaretskii <address@hidden> writes:
> >> > The process ID is indeed an int, but my code hides a process handle
> >> > inside it.
> >> 
> >> If you don't mind my asking: why? :)
> >
> > Because that's the only way on Windows to make sure the process object
> > is kept around by the kernel.  Integer PIDs are reused very quickly on
> > Windows, so by the time you get to scm_waitpid or some other function
> > that wants to query the process, that PID might not exist, or even
> > name a different process.  Having a handle open on the process
> > prevents the process object from being recycled.  Also, _cwait, used
> > by the Windows emulation of waitpid, needs a handle.
> Are you saying that Windows doesn't wait until you waitpid() on a PID to
> reap it?

If there's no handle open on a process, then when it exits, its object
is recycled and its PID can easily be reused (on a busy system), yes.

> >> The caller effectively just returns the PID to Scheme, at which
> >> point it has a not-quite-PID floating around.  Sounds like trouble
> >> to me.
> >
> > Not sure why you think it's trouble.  Can you explain?
> Just that spawn_child isn't the only way to get a PID.  getpid would be
> the most obvious one, but reading a PID value over a socket or whatever
> is also possible.  But then your code is now introducing not-quite-PIDs
> as well.  What if an interface expects one but gets the other?

I see your point, and I think I can fix this, together with the pid_t
width issue in 64-bit build, if I introduce an array private to
posix-w32.c that will hold the PIDs of all processes known to Guile
for which waitpid did not yet return an exit code, and their
corresponding process handles.  Then we can return an int to Scheme,
and the w32 functions will get hold of the handle by looking up the
PID in that array.

How's that sound?  If you agree, I can work on this tomorrow.

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