Pickle parsed git config files

We now cache the output of `git config --list` for each of our
GitConfig instances in a Python pickle file.  These can be read
back in using only the Python interpreter at a much faster rate
than we can fork+exec the git config process.

If the corresponding git config file has a newer modification
timestamp than the pickle file, we delete the pickle file and
regenerate it.  This ensures that any edits made by the user
will be taken into account the next time we consult the file.

This reduces the time for a no-op repo sync from 0.847s to 0.269s.

Signed-off-by: Shawn O. Pearce <sop@google.com>

git_config.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import cPickle
 import os
 import re
 import sys
@@ -57,6 +58,9 @@ class GitConfig(object):
     self._cache_dict = None
     self._remotes = {}
     self._branches = {}
+    self._pickle = os.path.join(
+      os.path.dirname(self.file),
+      '.repopickle_' + os.path.basename(self.file))
 
   def Has(self, name, include_defaults = True):
     """Return true if this configuration file has the key.
@@ -170,6 +174,40 @@ class GitConfig(object):
     return self._cache_dict
 
   def _Read(self):
+    d = self._ReadPickle()
+    if d is None:
+      d = self._ReadGit()
+      self._SavePickle(d)
+    return d
+
+  def _ReadPickle(self):
+    try:
+      if os.path.getmtime(self._pickle) \
+      <= os.path.getmtime(self.file):
+        os.remove(self._pickle)
+        return None
+    except OSError:
+      return None
+    try:
+      return cPickle.load(open(self._pickle, 'r'))
+    except IOError:
+      os.remove(self._pickle)
+      return None
+    except cPickle.PickleError:
+      os.remove(self._pickle)
+      return None
+
+  def _SavePickle(self, cache):
+    try:
+      cPickle.dump(cache,
+                   open(self._pickle, 'w'),
+                   cPickle.HIGHEST_PROTOCOL)
+    except IOError:
+      os.remove(self._pickle)
+    except cPickle.PickleError:
+      os.remove(self._pickle)
+
+  def _ReadGit(self):
     d = self._do('--null', '--list')
     c = {}
     while d: