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Subject: Re: [boost] [gsoc] Boost.Process done
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-09-15 09:18:53

Jeremy Maitin-Shepard wrote:
> On 09/14/2010 10:58 AM, Stewart, Robert wrote:
> > Jeremy Maitin-Shepard
> >> On 09/14/2010 04:04 AM, Stewart, Robert wrote:
> >>
> >>> Indeed, calling waitpid() for each child, in turn, is not a good
> >>> idea from a scalability perspective. Putting all of
> >>> Boost.Process' children into a unique process group and using
> >>> waitpid(), with WNOHANG, to wait for children in that process
> >>> group could work though.
> >>
> >> The process group approach does have some nice properties, but
> >> I'm not sure it is reasonable for the library to impose such a
> >> restriction. (Maybe it is, I am simply not sufficiently
> >> familiar with process groups and their intended
> >> purpose/practical use to know.) One thing of note is that a
> >> process can change its own process group id.
> >
> > From Wikipedia: "Process groups are used to control the
> > distribution of signals."
> >
> > The idea is that you can send a signal to all members of a
> > process group, but using that to monitor a signal from any member
> > of the group via waitpid() seems quit within scope, particularly
> > since that is part of the waitpid() interface.
> >
> > I'm also not familiar with the behaviors and uses of process
> > groups so I don't know if this would just shift the problem in
> > some way, albeit to presumably less common application types.
> In addition to the fact that you could still run into the same
> problem if you allow users to change process groups (you would
> have to loop through arbitrarily many process groups), I feel
> that imposing non-standard restrictions, even if seemingly
> minor, is not really appropriate for a Boost Process library.

I've done some more reading on the subject. According to [1],
the purpose of process groups is, as suggested by, but not stated
in, Wikipedia [2], for shells to manage processes associated with
a tty. However, [3] notes the reason for creating a process
group, in the context of the tutorial at least, is so "one may
kill all the processes in the process group without having to
keep track of how many processes have been forked and all of
their process id's." Of course, "kill" can mean send any signal,
not just SIGTERM or SIGKILL, for example.

Note that Boost.Process would need to set the process group for
each child and would likely use the first child's PID as the
process group ID. Since the parent would not be part of the new
process group and the children would inherit the parent's
session, the process group won't be orphaned unless the parent
process dies. In that case, the children will get SIGHUP
followed by SIGCONT to alert them to the loss of the controlling
process; that should be documented so library clients can
consider handling that case (likely by ignoring SIGHUP). See [5]
for more on this.

Aside from wanting to signal all child processes, I consider it
to be unusual for an application to set its process group and
much more so for a client of Boost.Process to do so [4]. A
Boost.Process client would only want to set the process group of
its children to permit the use of kill() to signal them all at
once. If there's a mechanism for doing that on Windows, the
library would need to provide a portable means to do so, and that
means clients wouldn't need to rely on kill() and a process
group to support it. Consequently, Boost.Process could preclude
the manipulation of process groups and using kill() on child
processes via a process group.

> I took a look at the process spawning facilities in GLib and Qt
> (documentation only). There might be other libraries that
> would also be relevant. See:
> <>
> <>
> Note: The Main Loop documentation describes the "child watch"
> facilities, which are particularly relevant to our discussion.

I found little of use there except that they disallow calling
waitpid(-1); did I miss something important? Using a process
group means there's no need to do that anyway.

> It seems a key goal is for Boost Process to be interoperable
> with the process creation in GLib and Qt. GLib may also
> provide some useful design inspiration.

I agree on both counts. Avoiding waitpid(-1) seems necessary for
this and reasons you've stated before. Using waitpid(-pgid),
however, would fit nicely.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

[1] <>
[2] <>
[3] <>, see
section entitled, "Kill all processes in a process group"
[4] A client of Boost.Process, being somewhat hampered by the
restraints of portability, would necessarily not have as complex
process management needs as a shell, unless I'm much mistaken.
[5] <>

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