processes: Fix kill of processes run under sudo

sudo drops forwarding of signals sent by processes of the same
process group, which means by default will drop signals from
parent and children processes. By moving it to another group, we
will later be able to kill it.
Note: sudo documentation is wrong, since it states it only drops
signals from children.

See following link for more information:
https://stackoverflow.com/questions/34337840/cant-terminate-a-sudo-process-created-with-python-in-ubuntu-15-10

Change-Id: I25990234aaa496c501ff45ad7f7fd549d6f188da

src/osmo_gsm_tester/process.py

@@ -157,16 +157,27 @@ class Process(log.Origin):
         return f
 
     def launch(self):
+        preexec_fn = None
         log.dbg('cd %r; %s %s' % (
                 os.path.abspath(str(self.run_dir)),
                 ' '.join(['%s=%r'%(k,v) for k,v in self.popen_kwargs.get('env', {}).items()]),
                 ' '.join(self.popen_args)))
 
+        if self.popen_args[0] == "sudo":
+            # sudo drops forwarding of signals sent by processes of the same
+            # process group, which means by default will drop signals from
+            # parent and children processes. By moving it to another group, we
+            # will later be able to kill it.
+            # Note: sudo documentation is wrong, since it states it only drops
+            # signals from children.
+            preexec_fn = os.setpgrp
+
         self.process_obj = subprocess.Popen(
             self.popen_args,
             stdout=self.make_output_log('stdout'),
             stderr=self.make_output_log('stderr'),
             stdin=subprocess.PIPE,
+            preexec_fn=preexec_fn,
             shell=False,
             cwd=self.run_dir.path,
             **self.popen_kwargs)