Docserver: Add commit history and _reset_commit servlet

chrome/common/extensions/docs/server2/commit_tracker.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import collections
+import datetime
+
 from object_store_creator import ObjectStoreCreator
 from future import Future
 
 
+# The maximum number of commit IDs to retain in a named commit's history deque.
+_MAX_COMMIT_HISTORY_LENGTH = 50
+
+
+class CachedCommit(object):
+  '''Object type which is stored for each entry in a named commit's history.
+  |datetime| is used as a timestamp for when the commit cache was completed,

[not at google - send to devlin, 2015/02/03 18:17:20]

TBH I'd just construct this here rather than passing it in, since it's only for
debugging. Up to you.

[Ken Rockot(use gerrit already), 2015/02/03 20:12:10]

We want it to be this way so we can store the datetime at which a given commit
was made live; that chronological ordering makes the debugging tool more useful.

+  and is only meant to provide a loose ordering of commits for administrative
+  servlets to display.'''
+  def __init__(self, commit_id, datetime):
+    self.commit_id = commit_id
+    self.datetime = datetime
+
+
 class CommitTracker(object):
   '''Utility class for managing and querying the storage of various named commit
   IDs.'''
   def __init__(self, object_store_creator):
-    # The object store should never be created empty since the sole purpose of
+    # The object stores should never be created empty since the sole purpose of
     # this tracker is to persist named commit data across requests.
     self._store = object_store_creator.Create(CommitTracker, start_empty=False)
+    self._history_store = object_store_creator.Create(CommitTracker,
+        category='history', start_empty=False)
 
   def Get(self, key):
     return self._store.Get(key)
 
   def Set(self, key, commit):
-    return self._store.Set(key, commit)
+    return (self._store.Set(key, commit)
+      .Then(lambda _: self._UpdateHistory(key, commit)))
+
+  def GetHistory(self, key):
+    '''Fetches the commit ID history for a named commit. If the commit has no
+    history, this will return an empty collection.'''
+    return (self._history_store.Get(key)
+        .Then(lambda history: () if history is None else history))
+
+  def _UpdateHistory(self, key, commit):
+    '''Appends a commit ID to a named commit's tracked history.'''
+    def create_or_amend_history(history):
+      if history is None:
+        history = collections.deque([], maxlen=50)
+      history.append(CachedCommit(commit, datetime.datetime.now()))
+      return self._history_store.Set(key, history)
+    return self._history_store.Get(key).Then(create_or_amend_history)