sync: Loop committing items without downloading updates

Since its inception sync has required all commits to be preceded by a
GetUpdates request. This was done to try to ensure we detect and resolve
conflicts on the client, rather than having two conflicting commits
collide at the server. It could never work perfectly, but it was likely
to work in most cases and the server would bounce the commit if it
didn't.

Now we have a new server that doesn't always give us the latest version
of a node when we ask for it, especially when the request was not
solicited by the server (ie. not triggered by notification). The update
before commit is now much less likely to detect conflicts. Even worse,
the useless update requests can be surprisingly expensive in certain
scenarios.

This change allows us to avoid fetching updates between 'batches' of
commits.  This should improve performance in the case where we have lots
of items to commit (ie. first time sync) and reduce load on the server.

This CL has some far-reaching effects. This is in part because I decided
to refactor the commit code, but major changes would have been required
with or without the refactor. Highlights include:
- The commit-related SyncerCommands are now paramaterized with local
  variables, allowing us to remove many members from the SyncSession
  classes.
- Several syncer states have been collapsed into one COMMIT state
  which redirects to the new BuildAndPostCommits() function.
- The PostCommitMessageCommand had been reduced to one line, which
  has now been inlined into the BuildAndPostCommits() function.
- The unsynced_handles counter has been removed for now. Following this
  change, it would have always been zero unless an error was encountered
  during the commit. The functions that reference it expect different
  behaviour and would have been broken by this change.
- The code to put extensions activity data back into the
  ExtensionsActivityMonitor in case of failure has been moved around to
  avoid double-counting if we have to post many commit messages.
- A CONFLICT response from the server is now treated as a transient
  error.  If we had continued to treat it as a success we might risk
  going into an infinite loop. See comment in code for details.
- The "purposeless anachronism" conflicting_new_folder_ids_ has been
  removed.

Review URL: https://chromiumcodereview.appspot.com/10210009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@139159 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 11 insertions, 5 deletions

diff --git a/sync/sessions/ordered_commit_set.h b/sync/sessions/ordered_commit_set.h
index 8551c07..f8e6938 100644
--- a/sync/sessions/ordered_commit_set.h
+++ b/sync/sessions/ordered_commit_set.h
@@ -64,14 +64,17 @@ class OrderedCommitSet {
   // belonging to |group|.  This is useful when you need to process a commit
   // response one ModelSafeGroup at a time. See GetCommitIdAt for how the
   // indices contained in the returned Projection can be used.
-  const Projection& GetCommitIdProjection(browser_sync::ModelSafeGroup group) {
-    return projections_[group];
-  }
+  const Projection& GetCommitIdProjection(
+      browser_sync::ModelSafeGroup group) const;
 
-  int Size() const {
+  size_t Size() const {
     return commit_ids_.size();
   }
 
+  bool Empty() const {
+    return Size() == 0;
+  }
+
   // Returns true iff any of the commit ids added to this set have model type
   // BOOKMARKS.
   bool HasBookmarkCommitId() const;
@@ -80,6 +83,9 @@ class OrderedCommitSet {
   void AppendReverse(const OrderedCommitSet& other);
   void Truncate(size_t max_size);
 
+  // Removes all entries from this set.
+  void Clear();
+
   void operator=(const OrderedCommitSet& other);
  private:
   // A set of CommitIdProjections associated with particular ModelSafeGroups.
@@ -92,7 +98,7 @@ class OrderedCommitSet {
     syncable::ModelType group;
   };
 
-  CommitItem GetCommitItemAt(const int position) const;
+  CommitItem GetCommitItemAt(const size_t position) const;
 
   // These lists are different views of the same items; e.g they are
   // isomorphic.