Bug 33335

Summary:	does anyone understand dbus-spawn.c?
Product:	dbus	Reporter:	Simon McVittie <smcv>
Component:	core	Assignee:	Havoc Pennington <hp>
Status:	RESOLVED FIXED	QA Contact:	John (J5) Palmieri <johnp>
Severity:	enhancement
Priority:	medium	CC:	chengwei.yang.cn, cosimo.alfarano, smcv, walters
Version:	unspecified	Keywords:	patch
Hardware:	Other
OS:	All
Whiteboard:
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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