Do threading.Lock the Right Way.

R=cmp@chromium.org, dnj@chromium.org, iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/499003002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291573 0039d316-1c4b-4281-b951-d872f2087c98

git_cache.py

@@ -200,13 +200,12 @@ class Mirror(object):
 
   @classmethod
   def SetCachePath(cls, cachepath):
-    cls.cachepath_lock.acquire()
+    with cls.cachepath_lock:
       setattr(cls, 'cachepath', cachepath)
-    cls.cachepath_lock.release()
 
   @classmethod
   def GetCachePath(cls):
-    cls.cachepath_lock.acquire()
+    with cls.cachepath_lock:
       if not hasattr(cls, 'cachepath'):
         try:
           cachepath = subprocess.check_output(
@@ -214,10 +213,9 @@ class Mirror(object):
         except subprocess.CalledProcessError:
           cachepath = None
         if not cachepath:
-        cls.cachepath_lock.release()
-        raise RuntimeError('No global cache.cachepath git configuration found.')
+          raise RuntimeError(
+              'No global cache.cachepath git configuration found.')
         setattr(cls, 'cachepath', cachepath)
-    cls.cachepath_lock.release()
       return getattr(cls, 'cachepath')
 
   def RunGit(self, cmd, **kwargs):