sync: use progress markers instead of timestamps during GetUpdates

chrome/browser/sync/syncable/syncable.h

Index: chrome/browser/sync/syncable/syncable.h
diff --git a/chrome/browser/sync/syncable/syncable.h b/chrome/browser/sync/syncable/syncable.h
index cccf4170e9fed440cd616badb6089abd6455138a..1c5b42ef9d75bcf7e501f801dc4364a5819afbd4 100644
--- a/chrome/browser/sync/syncable/syncable.h
+++ b/chrome/browser/sync/syncable/syncable.h
@@ -557,13 +557,6 @@ class LessEntryMetaHandles {
 };
 typedef std::set<EntryKernel, LessEntryMetaHandles> OriginalEntries;
 
-// Represents one or more model types sharing an update timestamp, as is
-// the case with a GetUpdates request.
-struct MultiTypeTimeStamp {
-  syncable::ModelTypeBitSet data_types;
-  int64 timestamp;
-};
-
 // a WriteTransaction has a writer tag describing which body of code is doing
 // the write. This is defined up here since DirectoryChangeEvent also contains
 // one.
@@ -666,8 +659,10 @@ class Directory {
     PersistedKernelInfo();
     ~PersistedKernelInfo();
 
+    void reset_download_progress(ModelType model_type);
+
     // Last sync timestamp fetched from the server.
-    int64 last_download_timestamp[MODEL_TYPE_COUNT];
+    sync_pb::DataTypeProgressMarker download_progress[MODEL_TYPE_COUNT];
     // true iff we ever reached the end of the changelog.
     ModelTypeBitSet initial_sync_ended;
     // The store birthday we were given by the server. Contents are opaque to
@@ -727,35 +722,17 @@ class Directory {
   const FilePath& file_path() const { return kernel_->db_path; }
   bool good() const { return NULL != store_; }
 
-  // The download timestamp is an index into the server's list of changes for
-  // an account.  We keep this for each datatype.  It doesn't actually map
-  // to any time scale; its name is an historical artifact.
-  int64 last_download_timestamp(ModelType type) const;
-  virtual void set_last_download_timestamp(ModelType type, int64 value);
-
-  // Find the model type or model types which have the least timestamp, and
-  // return them along with the types having that timestamp.  This is done
-  // with the intent of sequencing GetUpdates requests in an efficient way:
-  // bundling together requests that have the same timestamp, and requesting
-  // the oldest such bundles first in hopes that they might catch up to
-  // (and thus be merged with) the newer bundles.
-  MultiTypeTimeStamp GetTypesWithOldestLastDownloadTimestamp(
-      ModelTypeBitSet enabled_types) {
-    MultiTypeTimeStamp result;
-    result.timestamp = std::numeric_limits<int64>::max();
-    for (int i = 0; i < MODEL_TYPE_COUNT; ++i) {
-      if (!enabled_types[i])
-        continue;
-      int64 stamp = last_download_timestamp(ModelTypeFromInt(i));
-      if (stamp < result.timestamp) {
-        result.data_types.reset();
-        result.timestamp = stamp;
-      }
-      if (stamp == result.timestamp)
-        result.data_types.set(i);
-    }
-    return result;
-  }
+  // The download progress is an opaque token provided by the sync server
+  // to indicate the continuation state of the next GetUpdates operation.
+  void GetDownloadProgress(
+      ModelType type,
+      sync_pb::DataTypeProgressMarker* value_out) const;
+  void GetDownloadProgressAsString(
+      ModelType type,
+      std::string* value_out) const;
+  void SetDownloadProgress(
+      ModelType type,
+      const sync_pb::DataTypeProgressMarker& value);
 
   bool initial_sync_ended_for_type(ModelType type) const;
   void set_initial_sync_ended_for_type(ModelType type, bool value);
@@ -934,7 +911,6 @@ class Directory {
   // on their own; caller must guarantee exclusive access manually by holding
   // a ScopedKernelLock.
   void set_initial_sync_ended_for_type_unsafe(ModelType type, bool x);
-  void set_last_download_timestamp_unsafe(ModelType model_type, int64 x);
   void SetNotificationStateUnsafe(const std::string& notification_state);
 
   Directory& operator = (const Directory&);