[Android Refactor] Reland: Merge WebappDataStorage#LAST_USED_UNSET and LAST_USED_INVALID

chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chrome.browser.webapps;
 
 import android.content.Context;
 import android.content.SharedPreferences;
 import android.content.pm.PackageManager;
 import android.os.AsyncTask;
@@ skipping 94 matching lines @@
      * @param webappId The id of the web app to register.
      * @param callback The callback to run with the WebappDataStorage argument.
      * @return The storage object for the web app.
      */
     public void register(final String webappId, final FetchWebappDataStorageCallback callback) {
         new AsyncTask<Void, Void, WebappDataStorage>() {
             @Override
             protected final WebappDataStorage doInBackground(Void... nothing) {
                 // Create the WebappDataStorage on the background thread, as this must create and
                 // open a new SharedPreferences.
-                return WebappDataStorage.open(webappId);
+                WebappDataStorage storage = WebappDataStorage.open(webappId);
+                // The first read from {@link WebappDataStorage} must be done in the background.

[dominickn, 2017/06/13 01:36:57]

This is an interesting restriction! Is it new?

[pkotwicz, 2017/06/13 03:14:29]

Thank you for your question! It forced me to do more investigation. The strict
mode exception is thrown by SharedPreferencesImpl#awaitLoadLocked() if the
SharedPreferences have not finished loading.
http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/...

We would get a strict mode exception if we tried to access SharedPreferences
immediately after the strict mode block in WebappActivity#preInflationStartup()
(I checked that this is the case)

[dominickn, 2017/06/13 23:13:21]

Perhaps this bit of code should be moved to warmUpSharedPrefs then? The whole
purpose of warming shared prefs is to avoid this issue in one go....

+                // Otherwise a strict mode exception is thrown. Do it now.
+                storage.getLastUsedTime();
+                return storage;
             }
 
             @Override
             protected final void onPostExecute(WebappDataStorage storage) {
-                // Guarantee that last used time != WebappDataStorage.LAST_USED_INVALID. Must be
-                // run on the main thread as SharedPreferences.Editor.apply() is called.
+                // Update the last used time in order to prevent
+                // {@link WebappRegistry@unregisterOldWebapps()} from deleting the
+                // WebappDataStorage. Must be run on the main thread as
+                // SharedPreferences.Editor.apply() is called.
                 mStorages.put(webappId, storage);
                 mPreferences.edit().putStringSet(KEY_WEBAPP_SET, mStorages.keySet()).apply();
                 storage.updateLastUsedTime();
                 if (callback != null) callback.onWebappDataStorageRetrieved(storage);
             }
         }.execute();
     }
 
     /**
      * Returns the WebappDataStorage object for webappId, or null if one cannot be found.
@@ skipping 147 matching lines @@
                 }
             }
         } else {
             if (webapps.contains(idToInitialize)
                     && (replaceExisting || !mStorages.containsKey(idToInitialize))) {
                 mStorages.put(idToInitialize, WebappDataStorage.open(idToInitialize));
             }
         }
     }
 }