xvfb: improve robustness when stale lock files exist

scripts/slave/xvfb.py

Index: scripts/slave/xvfb.py
diff --git a/scripts/slave/xvfb.py b/scripts/slave/xvfb.py
index 62dff858bdfd3dc3be1d334a9d233f6b09d74ccf..835f4662ff0cf46d1cb2001eaf98bb54d81e88be 100644
--- a/scripts/slave/xvfb.py
+++ b/scripts/slave/xvfb.py
@@ -60,6 +60,14 @@ def StartVirtualX(slave_build_name, build_dir, with_wm=True, server_dir=None):
       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):