kcrash, fork, and stdout/stderr

[David Faure]

On lundi 4 décembre 2017 05:43:14 CET Thiago Macieira wrote:
> > Yes, no kdeinit involved. Here's the setup:
> > 
> > $ bin/kcrashtest testAutoRestartDirectly
> > 
> >       starts test_crasher (with QProcess, using waitForFinished())
> >       
> >           which crashes :)
> >           KCrash catches that, and since the Autorestart flag is set,
> >           
> >              sets the env var KCRASH_AUTO_RESTARTED,
> >              and starts test_crasher again (it checks that env var, so no
> > 
> > infinite loop)
> > 
> > The first test_crasher then exits (after waitpid(), but that's just
> > waiting
> > for the child to start, right?).
> 
> Sorry, more details here. Can you provide the strace for clone, pipe, pipe2,
> dup, dup2, dup3, execve, waitpid, waitid, wait4?
> 
> QProcess starts a process.
> 
> Does that process start something else? Or is QProcess's direct child that
> crashes?

It's exactly as I described. It's QProcess's direct child which crashes
(and which has a segv handler enabled, KCrash).

> The crash handler is in-process, correct? Then you execve in place?

fork+execve

https://lxr.kde.org/source/frameworks/kcrash/src/kcrash.cpp#0733

> Or do you fork a child at that point? fork from inside a signal handler is
> an incredibly bad idea, don't do it.

Well, I guess that's why it's the fallback from the main strategy which is 
"ask kdeinit to restart that application" (even if it doesn't provide a 
kdeinit module). But kdeinit might not be running, outside plasma workspace.

This has always been like that, it goes back to 2006 (Luboš), it was discussed 
with you https://marc.info/?l=kde-core-devel&m=113699766611213&w=2
("There's still a fallback to use fork() in case using kdeinit fails for 
any reason.")

> That makes QProcess not create a pipe and instead pass its own stderr to the
> child. Then it's SEP.

Yep, so from my POV the problem is fixed.

Thanks for the additional input!

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE Frameworks 5