Code review implementation of cacheStorage.clear()

Possibly related issue:
- https://old.reddit.com/r/firefox/comments/b3u4nj/what_is_the_estimated_time_period_for_reviewing_a/

@gwarser has been able to reproduce at will, while I have been
unable to reproduce at all. The change here is to clear the
content of the database instead of outright deleting it before
restoring backed up settings.

src/js/cachestorage.js

@@ -211,10 +211,7 @@
                     if ( ev.oldVersion === 1 ) { return; }
                     try {
                         const db = ev.target.result;
-                        const table = db.createObjectStore(
-                            STORAGE_NAME,
-                            { keyPath: 'key' }
-                        );
+                        db.createObjectStore(STORAGE_NAME, { keyPath: 'key' });
                     } catch(ex) {
                         req.onerror();
                     }
@@ -422,15 +419,18 @@
             if ( typeof callback !== 'function' ) {
                 callback = noopfn;
             }
-            disconnect();
-            try {
-                const req = indexedDB.deleteDatabase(STORAGE_NAME);
-                req.onsuccess = req.onerror = ( ) => {
+            getDb().then(db => {
+                let transaction = db.transaction(STORAGE_NAME, 'readwrite');
+                transaction.oncomplete =
+                transaction.onerror =
+                transaction.onabort = ( ) => {
                     callback();
                 };
-            } catch(ex) {
+                transaction.objectStore(STORAGE_NAME).clear();
+            }).catch(reason => {
+                console.info(`cacheStorage.clearDb() failed: ${reason}`);
                 callback();
-            }
+            });
         };
 
         return getDb().then(db => {

src/js/messaging.js

@@ -799,17 +799,21 @@ var backupUserData = function(callback) {
     µb.assets.get(µb.userFiltersPath, onUserFiltersReady);
 };
 
-var restoreUserData = function(request) {
-    var userData = request.userData;
+const restoreUserData = function(request) {
+    const userData = request.userData;
 
-    var restart = function() {
+    const restart = function() {
         vAPI.app.restart();
     };
 
-    var onAllRemoved = function() {
+    let countdown = 2;
+
+    const onAllRemoved = function() {
+        countdown -= 1;
+        if ( countdown !== 0 ) { return; }
         µBlock.saveLocalSettings();
         vAPI.storage.set(userData.userSettings);
-        var hiddenSettings = userData.hiddenSettings;
+        let hiddenSettings = userData.hiddenSettings;
         if ( hiddenSettings instanceof Object === false ) {
             hiddenSettings = µBlock.hiddenSettingsFromString(
                 userData.hiddenSettingsString || ''
@@ -840,7 +844,7 @@ var restoreUserData = function(request) {
 
     // If we are going to restore all, might as well wipe out clean local
     // storage
-    µb.cacheStorage.clear();
+    µb.cacheStorage.clear().then(( ) => { onAllRemoved(); });
     vAPI.storage.clear(onAllRemoved);
     vAPI.localStorage.removeItem('immediateHiddenSettings');
 };
@@ -856,9 +860,9 @@ const resetUserData = function() {
             vAPI.app.restart();
         }
     };
-    µb.cacheStorage.clear().then(( ) => countdown()); // 1
-    vAPI.storage.clear(countdown);             // 2
-    µb.saveLocalSettings(countdown);           // 3
+    µb.cacheStorage.clear().then(( ) => countdown());   // 1
+    vAPI.storage.clear(countdown);                      // 2
+    µb.saveLocalSettings(countdown);                    // 3
     vAPI.localStorage.removeItem('immediateHiddenSettings');
 };