xvfb: improve robustness when stale lock files exist

scripts/slave/xvfb.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Functions to setup xvfb, which is used by the linux machines.
 """
 
 import os
 import platform
 import signal
@@ skipping 42 matching lines @@
     checkstarttime = time.time()
     xdisplayproc = subprocess.Popen([xdisplaycheck_path, '--noserver'],
                                     stdout=subprocess.PIPE,
                                     stderr=subprocess.STDOUT)
     # Wait for xdisplaycheck to exit.
     logs = xdisplayproc.communicate()[0]
     if xdisplayproc.returncode == 0:
       print 'xdisplaycheck says there is a display still running, exiting...'
       raise Exception('Display already present.')
 
+    xvfb_lock_filename = '/tmp/.X%s-lock' % _XvfbDisplayIndex(slave_build_name)
+    if os.path.exists(xvfb_lock_filename):
+      print 'Removing stale xvfb lock file %r' % xvfb_lock_filename
+      try:
+        os.unlink(xvfb_lock_filename)
+      except OSError as e:
+        print 'Removing xvfb lock file failed: %s' % e
+
   # Figure out which X server to try.
   cmd = 'Xvfb'
   if server_dir and os.path.exists(server_dir):
     cmd = os.path.join(server_dir, 'Xvfb.' + platform.architecture()[0])
     if not os.path.exists(cmd):
       cmd = os.path.join(server_dir, 'Xvfb')
     if not os.path.exists(cmd):
       print 'No Xvfb found in designated server path:', server_dir
       raise Exception('No virtual server')
 
@@ skipping 56 matching lines @@
   if os.path.exists(xvfb_pid_filename):
     xvfb_pid = int(open(xvfb_pid_filename).read())
     print 'Stopping Xvfb with pid %d ...' % xvfb_pid
     # If the process doesn't exist, we raise an exception that we can ignore.
     try:
       os.kill(xvfb_pid, signal.SIGKILL)
     except OSError:
       print '... killing failed, presuming unnecessary.'
     os.remove(xvfb_pid_filename)
     print 'Xvfb pid file removed'