Bug 1300647 - Part 1 - Don't bother reloading a zombified tab if we'r…

…e about to exit the app. r=ahunt

When pressing the back button reaches the beginning of session history for a tab opened from an external app, we both close the tab and send Firefox into background. Closing the tab leads to some other tab getting selected instead - if that other tab was zombified, this means that we'll then start restoring it.

This behaviour is
- visibly distracting, as that other tab will be visible for a split second while it starts reloading before Firefox finally disappears into the background
- wasteful of resources - while restoring a zombified tab is usually done from cache, at the very least we'll waste some CPU cycles reloading a tab even though we're in background

Therefore, in this situation the UI now alerts the session store that it needn't bother restoring that other tab if it's in a zombie state. Instead, we'll restore it the next time Firefox comes into foreground - if the tab is still selected by then.

MozReview-Commit-ID: 3FcjCZrJ0Ds

--HG--
extra : rebase_source : d071884dd1e78b7da470b042e244093e797dde61

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

@@ -2521,6 +2521,11 @@ public void run() {
 
                         if (tab.isExternal()) {
                             moveTaskToBack(true);
+                            Tab nextSelectedTab = Tabs.getInstance().getNextTab(tab);
+                            if (nextSelectedTab != null) {
+                                int nextSelectedTabId = nextSelectedTab.getId();
+                                GeckoAppShell.notifyObservers("Tab:KeepZombified", Integer.toString(nextSelectedTabId));
+                            }
                             tabs.closeTab(tab);
                             return;
                         }

mobile/android/components/SessionStore.js

@@ -86,6 +86,11 @@ SessionStore.prototype = {
   // Whether or not to send notifications for changes to the closed tabs.
   _notifyClosedTabs: false,
 
+  // If we're simultaneously closing both a tab and Firefox, we don't want
+  // to bother reloading the newly selected tab if it is zombified.
+  // The Java UI will tell us which tab to watch out for.
+  _keepAsZombieTabId: -1,
+
   init: function ss_init() {
     loggingEnabled = Services.prefs.getBoolPref("browser.sessionstore.debug_logging");
 
@@ -141,6 +146,7 @@ SessionStore.prototype = {
         observerService.addObserver(this, "quit-application", true);
         observerService.addObserver(this, "Session:Restore", true);
         observerService.addObserver(this, "Session:NotifyLocationChange", true);
+        observerService.addObserver(this, "Tab:KeepZombified", true);
         observerService.addObserver(this, "application-background", true);
         observerService.addObserver(this, "application-foreground", true);
         observerService.addObserver(this, "ClosedTabs:StartNotifications", true);
@@ -284,6 +290,13 @@ SessionStore.prototype = {
         }
         break;
       }
+      case "Tab:KeepZombified": {
+        if (aData >= 0) {
+          this._keepAsZombieTabId = aData;
+          log("Tab:KeepZombified " + aData);
+        }
+        break;
+      }
       case "application-background":
         // We receive this notification when Android's onPause callback is
         // executed. After onPause, the application may be terminated at any
@@ -301,6 +314,15 @@ SessionStore.prototype = {
         log("application-foreground");
         this._interval = Services.prefs.getIntPref("browser.sessionstore.interval");
         this._minSaveDelay = MINIMUM_SAVE_DELAY;
+
+        // If we skipped restoring a zombified tab before backgrounding,
+        // we might have to do it now instead.
+        let window = Services.wm.getMostRecentWindow("navigator:browser");
+        let tab = window.BrowserApp.selectedTab;
+
+        if (tab.browser.__SS_restore) {
+          this._restoreZombieTab(tab.browser, tab.id);
+        }
         break;
       case "ClosedTabs:StartNotifications":
         this._notifyClosedTabs = true;
@@ -667,13 +689,14 @@ SessionStore.prototype = {
 
     // Restore the resurrected browser
     if (aBrowser.__SS_restore) {
-      let data = aBrowser.__SS_data;
-      this._restoreTab(data, aBrowser);
-
-      delete aBrowser.__SS_restore;
-      aBrowser.removeAttribute("pending");
-      log("onTabSelect() restored zombie tab " + tabId);
+      if (tabId != this._keepAsZombieTabId) {
+        this._restoreZombieTab(aBrowser, tabId);
+      } else {
+        log("keeping as zombie tab " + tabId);
+      }
     }
+    // The tab id passed through Tab:KeepZombified is valid for one TabSelect only.
+    this._keepAsZombieTabId = -1;
 
     log("onTabSelect() ran for tab " + tabId);
     this.saveStateDelayed();
@@ -687,6 +710,15 @@ SessionStore.prototype = {
     }
   },
 
+  _restoreZombieTab: function ss_restoreZombieTab(aBrowser, aTabId) {
+    let data = aBrowser.__SS_data;
+    this._restoreTab(data, aBrowser);
+
+    delete aBrowser.__SS_restore;
+    aBrowser.removeAttribute("pending");
+    log("restoring zombie tab " + aTabId);
+  },
+
   onTabInput: function ss_onTabInput(aWindow, aBrowser) {
     // If this browser belongs to a zombie tab or the initial restore hasn't yet finished,
     // skip any session save activity.