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[Libguestfs] setpgid() before exec'ing qemu


I am writing a python test suite that uses oz (https://github.com/clalancette/oz)
which uses libquestfs. I am simulating a small cloud and have some apps that need
to talk to the guests.

All works really well until I needed to start another process.
So I start a process then "oz" starts up a VM (it uses libquestfs)
then when "oz" cleans up it calls libquestfs.kill_subprocess().
At this point my test locks up.

A bacetrace of the python code is:
#0  0x00000036fe80ef1e in waitpid () from /lib64/libpthread.so.0
#1  0x0000003629c0df7f in guestfs_close () from /usr/lib64/libguestfs.so.0
then a mess of python stuff

If I attach to it with strace it shows:

It looks like libguestfs's children are already dead
pstree 13107

so it's seems like it's waiting on my process to die.

So I changed the oz code to call close (in python "del obj") and all is well,
however I was looking in libquestfs code and saw:
#if 0
    /* Set up a new process group, so we can signal this process
     * and all subprocesses (eg. if qemu is really a shell script).
   setpgid (0, 0);

Any reason this is "if def'ed" out?
this was done in commit 88f69eb03160a62d38 
I was thinking this might prevent my problem from occuring in the first place?

Any special reason you are using kill_subprocess()?
    def kill_subprocess (self):
         u"""This kills the qemu subprocess. You should never need to
          call this.
         return libguestfsmod.kill_subprocess (self._o)

Angus Salkeld

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