Ability to list registered keys in multistore_file.

Reviewed in https://codereview.appspot.com/8547048/.

Fixes issue #263.
diff --git a/oauth2client/multistore_file.py b/oauth2client/multistore_file.py
index e1b39f7..4d2c091 100644
--- a/oauth2client/multistore_file.py
+++ b/oauth2client/multistore_file.py
@@ -125,6 +125,43 @@
     An object derived from client.Storage for getting/setting the
     credential.
   """
+  multistore = _get_multistore(filename, warn_on_readonly=warn_on_readonly)
+  key = util.dict_to_tuple_key(key_dict)
+  return multistore._get_storage(key)
+
+
+@util.positional(1)
+def get_all_credential_keys(filename, warn_on_readonly=True):
+  """Gets all the registered credential keys in the given Multistore.
+
+  Args:
+    filename: The JSON file storing a set of credentials
+    warn_on_readonly: if True, log a warning if the store is readonly
+
+  Returns:
+    A list of the credential keys present in the file.  They are returned as
+    dictionaries that can be passed into get_credential_storage_custom_key to
+    get the actual credentials.
+  """
+  multistore = _get_multistore(filename, warn_on_readonly=warn_on_readonly)
+  multistore._lock()
+  try:
+    return multistore._get_all_credential_keys()
+  finally:
+    multistore._unlock()
+
+
+@util.positional(1)
+def _get_multistore(filename, warn_on_readonly=True):
+  """A helper method to initialize the multistore with proper locking.
+
+  Args:
+    filename: The JSON file storing a set of credentials
+    warn_on_readonly: if True, log a warning if the store is readonly
+
+  Returns:
+    A multistore object
+  """
   filename = os.path.expanduser(filename)
   _multistores_lock.acquire()
   try:
@@ -132,8 +169,7 @@
         filename, _MultiStore(filename, warn_on_readonly=warn_on_readonly))
   finally:
     _multistores_lock.release()
-  key = util.dict_to_tuple_key(key_dict)
-  return multistore._get_storage(key)
+  return multistore
 
 
 class _MultiStore(object):
@@ -356,6 +392,14 @@
       raw_creds.append({'key': raw_key, 'credential': raw_cred})
     self._locked_json_write(raw_data)
 
+  def _get_all_credential_keys(self):
+    """Gets all the registered credential keys in the multistore.
+
+    Returns:
+      A list of dictionaries corresponding to all the keys currently registered
+    """
+    return [dict(key) for key in self._data.keys()]
+
   def _get_credential(self, key):
     """Get a credential from the multistore.