Impl syncing of left rooms. Factor out getting or creating filters.

lib/client.js

@@ -2117,8 +2117,30 @@ MatrixClient.prototype.search = function(opts, callback) {
  * @return {module:http-api.MatrixError} Rejects: with an error response.
  */
 MatrixClient.prototype.syncLeftRooms = function() {
+    // Guard against multiple calls whilst ongoing and multiple calls post success
+    if (this._syncedLeftRooms) {
+        console.log("Already synced left rooms");
+        return q([]); // don't call syncRooms again if it succeeded.
+    }
+    if (this._syncLeftRoomsPromise) {
+        console.log("Returning ongoing request promise");
+        return this._syncLeftRoomsPromise; // return the ongoing request
+    }
+    console.log("Making sync left rooms request");
+    var self = this;
     var syncApi = new SyncApi(this);
-    return syncApi.syncLeftRooms();
+    this._syncLeftRoomsPromise = syncApi.syncLeftRooms();
+
+    // cleanup locks
+    this._syncLeftRoomsPromise.then(function(res) {
+        console.log("Marking success of sync left room request");
+        self._syncedLeftRooms = true; // flip the bit on success
+    }).finally(function() {
+        console.log("Cleaning up request state");
+        self._syncLeftRoomsPromise = null; // cleanup ongoing request state
+    });
+
+    return this._syncLeftRoomsPromise;
 };