Bug 1120441 - Don't try to show tab-history panel if app has been shutdown r=sebastian

Showing the tab history panel involves a gecko call to retrieve tab history. This
can be slow, meaning we have no idea what state the app will be in when the tab
history data is returned. Thus we need to protect the code that shows the tab
history fragment against a number of scenarios to avoid crashes in those cases
where the app might be shutting down:

1. If onSaveInstanceState() has been called (which might happen before or after onPause(),
   and might be linked to app shutdown - but the docs don't appear to give any guarantees),
   fragment transactions cannot be performed. We protect against this by accepting loss
   of state in fragment transactions.
2. If the Activity has been completely destroyed, trying to perform a fragment transaction
   will likewise fail. We protect against this by not even trying to perform the transaction
   if we definitively know that the Activity is being shut down (ie isFinishing()).

In both of these cases, we simply must accept that we're potentially losing UI state: i.e.
a user could request the tab history panel via long-back-press, followed by exiting the app;
we now end up never ever showing the panel. This scenario doesn't seem like a major loss - and
fixing this issue properly would require significant investment (i.e. we would need to either
cache tab history on frontend side, or cache the tab-history panel request - and it's not clear
users will still care about seeing the panel the next time they open firefox).

MozReview-Commit-ID: JsAK1By8yqn

--HG--
extra : rebase_source : 9a3b64d2829a969e4de2775c2714874aa8708031

mobile/android/base/java/org/mozilla/gecko/BrowserApp.java

@@ -648,9 +648,20 @@ public class BrowserApp extends GeckoApp
                 runOnUiThread(new Runnable() {
                     @Override
                     public void run() {
+                        if (BrowserApp.this.isFinishing()) {
+                            // TabHistoryController is rather slow - and involves calling into Gecko
+                            // to retrieve tab history. That means there can be a significant
+                            // delay between the back-button long-press, and onShowHistory()
+                            // being called. Hence we need to guard against the Activity being
+                            // shut down (in which case trying to perform UI changes, such as showing
+                            // fragments below, will crash).
+                            return;
+                        }
+
                         final TabHistoryFragment fragment = TabHistoryFragment.newInstance(historyPageList, toIndex);
                         final FragmentManager fragmentManager = getSupportFragmentManager();
                         GeckoAppShell.vibrateOnHapticFeedbackEnabled(getResources().getIntArray(R.array.long_press_vibrate_msec));
+                        if (BrowserApp.this.isForegrounded())
                         fragment.show(R.id.tab_history_panel, fragmentManager.beginTransaction(), TAB_HISTORY_FRAGMENT_TAG);
                     }
                 });
@@ -1061,6 +1072,7 @@ public class BrowserApp extends GeckoApp
     @Override
     public void onResume() {
         super.onResume();
+
         if (mIsAbortingAppLaunch) {
             return;
         }

mobile/android/base/java/org/mozilla/gecko/tabs/TabHistoryFragment.java

@@ -124,7 +124,10 @@ public class TabHistoryFragment extends Fragment implements OnItemClickListener,
         dismissed = false;
         transaction.add(containerViewId, this, tag);
         transaction.addToBackStack(tag);
-        backStackId = transaction.commit();
+        // Populating the tab history requires a gecko call (which can be slow) - therefore the app
+        // state by the time we try to show this fragment is unknown, and we could be in the
+        // middle of shutting down:
+        backStackId = transaction.commitAllowingStateLoss();
     }
 
     // Pop the fragment from backstack if it exists.