Bug 33335 - does anyone understand dbus-spawn.c?
Summary: does anyone understand dbus-spawn.c?
Alias: None
Product: dbus
Classification: Unclassified
Component: core (show other bugs)
Version: unspecified
Hardware: Other All
: medium enhancement
Assignee: Havoc Pennington
QA Contact: John (J5) Palmieri
Keywords: patch
Depends on:
Reported: 2011-01-21 07:09 UTC by Simon McVittie
Modified: 2014-01-14 10:55 UTC (History)
4 users (show)

See Also:
i915 platform:
i915 features:


Description Simon McVittie 2011-01-21 07:09:03 UTC
On Bug #32992, Colin wrote:
> Stepping back a bit, does anyone have any idea what the heck is going on in
> dbus-spawn.c?  Why is there an "intermediate child between the main process and
> the grandchild"??  Why aren't we just executing the child directly?
> The commit message doesn't say...

Meanwhile, the source code has this comment, dating back to Havoc's initial introduction of DBusBabysitter:

> /*
>  * I'm pretty sure this whole spawn file could be made simpler,
>  * if you thought about it a bit.
>  */

One thing that's particularly confusing is that there are three processes:

- the main process (typically dbus-daemon)
  - the "babysitter process"
    - the grandchild, which calls exec()

and in the main process, there's a DBusBabysitter object. Most of the comments, variable names, etc., say "the babysitter", so it's unclear whether they're talking about the DBusBabysitter object or the babysitter process.
Comment 1 Simon McVittie 2011-03-03 03:18:47 UTC
Back on Bug #32992, Colin wrote:
> Apparently this is a trick to avoid having the parent deal with SIGCHLD if it
> wasn't expecting it, see:
> http://compgroups.net/comp.unix.programmer/the-need-for-double-forking
Comment 2 Simon McVittie 2013-10-08 12:05:00 UTC
See also Attachment #74834 [details], Attachment #74836 [details], Attachment #74837 [details], Attachment #74839 [details] on Bug #60859 - in particular, the second of those documents why dbus-spawn looks like it does.
Comment 3 Simon McVittie 2014-01-14 10:55:42 UTC
Patches documenting this were merged.

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