Add a git cache for gclient sync operations.

gclient.py

Index: gclient.py
diff --git a/gclient.py b/gclient.py
index 1f4801b9464af43a0bf9e252cde59053de0e23bf..69d8047775c42ce82ba98187c2e79ba1733d45fd 100755
--- a/gclient.py
+++ b/gclient.py
@@ -66,6 +66,7 @@ Specifying a target OS
 
 __version__ = "0.6.4"
 
+import collections
 import copy
 import logging
 import optparse
@@ -75,6 +76,7 @@ import posixpath
 import pprint
 import re
 import sys
+import threading
 import urllib
 import urlparse
 
@@ -1543,9 +1545,20 @@ def CMDsync(parser, args):
                          'actual HEAD revision from the repository')
   parser.add_option('--upstream', action='store_true',
                     help='Make repo state match upstream branch.')
+  parser.add_option('--cache-dir',
+                    help='For git, cache all repos into this dir and do shared '
+                         'clones from the cache, instead of cloning directly '
+                         'from the remote.')
   (options, args) = parser.parse_args(args)
   client = GClient.LoadCurrentConfig(options)
 
+  if options.cache_dir:
+    # If a given cache is used in a solution more than once, prevent multiple
+    # threads from updating it simultaneously. |cache_lock| is used to prevent
+    # races on the defaultdict's construction of Locks.
+    options.cache_lock = threading.Lock()

[szager1, 2013/07/01 23:38:29]

options.cache_lock shouldn't be necessary; vivifying a new entry in
options.cache_locks is thread-safe and atomic, because threading.Lock is a
built-in type. 

[iannucci, 2013/07/01 23:55:52]

defaultdict guarantees that item addition is atomic?

In that case, I'm happy to ditch the uberlock.

[szager1, 2013/07/02 00:08:05]

As long as the __init__ method for the subject class is atomic (which is true
for all built-in types, including threading.Lock), defaultdict is thread-safe.

+    options.cache_locks = collections.defaultdict(threading.Lock)
+
   if not client:
     raise gclient_utils.Error('client not configured; see \'gclient config\'')