Issue 7096 - Make it possible to lazy initialize a filter's subscriptions

lib/filterStorage.js

Index: lib/filterStorage.js
===================================================================
--- a/lib/filterStorage.js
+++ b/lib/filterStorage.js
@@ -123,16 +123,43 @@
    * @type {number}
    */
   get subscriptionCount()
   {
     return this.knownSubscriptions.size;
   }
 
   /**
+   * Yields subscriptions in the storage to which the given filter belongs.
+   * @param {Filter} filter
+   * @yields {Subscription}
+   */
+  *subscriptionsForFilter(filter)
+  {
+    if (!filter.subscriptionsAdded)
+      addSubscriptionsToFilter(filter);
+
+    yield* filter.subscriptions();
+  }
+
+  /**
+   * Returns the number of subscriptions in the storage to which the given
+   * filter belongs.
+   * @param {Filter} filter
+   * @return {number}
+   */
+  getSubscriptionCountForFilter(filter)
+  {
+    if (!filter.subscriptionsAdded)
+      addSubscriptionsToFilter(filter);
+
+    return filter.subscriptionCount;
+  }
+
+  /**
    * Finds the filter group that a filter should be added to by default. Will
    * return <code>null</code> if this group doesn't exist yet.
    * @param {Filter} filter
    * @returns {?SpecialSubscription}
    */
   getGroupForFilter(filter)
   {
     let generalSubscription = null;
@@ -646,16 +673,32 @@
  * Reads the user's filters from disk, manages them in memory, and writes them
  * back to disk.
  */
 let filterStorage = new FilterStorage();
 
 exports.filterStorage = filterStorage;
 
 /**
+ * Adds to the filter any subscriptions in the storage to which a filter
+ * belongs.
+ * @param {Filter} filter
+ */
+function addSubscriptionsToFilter(filter)
+{
+  for (let subscription of filterStorage.subscriptions())
+  {
+    if (subscription.hasFilter(filter))
+      filter.addSubscription(subscription);
+  }
+
+  filter.subscriptionsAdded = true;
+}
+
+/**
  * Connects a subscription to its filters without any notifications.
  * @param {Subscription} subscription The subscription that should be
  *   connected to its filters.
  * @param {?Array.<Filter>} [filters] A list of filters to which the
  *   subscription should be connected. If this is not given, the subscription
  *   is connected to its own filters.
  */
 function connectSubscriptionFilters(subscription, filters)