[Libguestfs] guestfs_launch() fails when C application is started as a systemd service

[Richard W.M. Jones]

On Thu, Nov 08, 2018 at 04:51:31PM +0200, Peter Dimitrov wrote:
> Here are strace outputs per process.
> 
> strace_output.22076 is the plugin's pid. (A little before forking)

Ah ha.

Close reading of the waitpid(2) man page says:

       ECHILD (for  waitpid() or waitid()) The process specified by pid (wait‐
              pid()) or idtype and id (waitid()) does not exist or  is  not  a
              child  of  the  calling process.  (This can happen for one's own
              child if the action for SIGCHLD is set to SIG_IGN.  See also the
              Linux Notes section about threads.)

I'm going to guess that collectd is leaking the signal handler setting
into the child process instead of resetting it.  This is a bug in
collectd.

It's surprisingly hard to correctly fork a process in Unix.  Here's
what libvirt does, which is the most comprehensive code that I know
of.  It involves resetting multiple things before running the child:

  https://libvirt.org/git/?p=libvirt.git;a=blob;f=src/util/vircommand.c;h=de937f6f9aa91abb518eac98bfac9dcf37e1f5df;hb=HEAD#l280

While you're getting the collectd bug fixed, the easiest workaround is
probably to add:

  signal (SIGCHLD, SIG_DFL);

in your code.

Rich.

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