Increase log information when a RootHelperProcess fails

If a RootHelperProcess does not start, add more information to the
exception raised: the command return code, the stdout and the stderr.

Change-Id: I229e926341c5e6c8b06f59950e3ae09864d0f1f6
Closes-Bug: #1861221

neutron/tests/common/net_helpers.py

@@ -327,11 +327,25 @@ class RootHelperProcess(subprocess.Popen):
             if utils.pid_invoked_with_cmdline(child_pid, self.cmd):
                 return True
 
-        common_utils.wait_until_true(
-            child_is_running,
-            timeout,
-            exception=RuntimeError("Process %s hasn't been spawned "
-                                   "in %d seconds" % (self.cmd, timeout)))
+        try:
+            common_utils.wait_until_true(child_is_running, timeout)
+        except common_utils.WaitTimeout:
+            # If there is an error, the stderr and stdout pipes usually have
+            # information returned by the command executed. If not, timeout
+            # the pipe communication quickly.
+            stdout = stderr = ''
+            try:
+                stdout, stderr = self.communicate(timeout=0.5)
+            except subprocess.TimeoutExpired:
+                pass
+            msg = ("Process %(cmd)s hasn't been spawned in %(seconds)d "
+                   "seconds. Return code: %(ret_code)s, stdout: %(stdout)s, "
+                   "sdterr: %(stderr)s" %
+                   {'cmd': self.cmd, 'seconds': timeout,
+                    'ret_code': self.returncode, 'stdout': stdout,
+                    'stderr': stderr})
+            raise RuntimeError(msg)
+
         self.child_pid = utils.get_root_helper_child_pid(
             self.pid, self.cmd, run_as_root=True)