D-Bus testing: ignore failures to kill processes which have quit

ShutdownProcess() has a race condition between calling popen.poll()
and os.kill(): the process might quit in the meantime. The check
also was missing for the initial kill().

Better check first and continue when kill() fails because the
process was not found.

test/test-dbus.py

@@ -226,13 +226,23 @@ class TimeoutTest:
         self.failIf(end - start < 5)
         self.failIf(end - start >= 6)
 
+def TryKill(pid, signal):
+    try:
+        os.kill(pid, signal)
+    except OSError, ex:
+        # might have quit in the meantime, deal with the race
+        # condition
+        if ex.errno != 3:
+            raise ex
+
 def ShutdownSubprocess(popen, timeout):
     start = time.time()
-    os.kill(popen.pid, signal.SIGTERM)
+    if popen.poll() == None:
+        TryKill(popen.pid, signal.SIGTERM)
     while popen.poll() == None and start + timeout >= time.time():
         time.sleep(0.01)
     if popen.poll() == None:
-        os.kill(popen.pid, signal.SIGKILL)
+        TryKill(popen.pid, signal.SIGKILL)
         while popen.poll() == None and start + timeout + 1 >= time.time():
             time.sleep(0.01)
         return False