Issue 3815 - Fix TabAllocator. Now it returns tab with initialized outerWindowID

lib/crawler.js

 /*
  * This Source Code is subject to the terms of the Mozilla Public License
  * version 2.0 (the "License"). You can obtain a copy of the License at
  * http://mozilla.org/MPL/2.0/.
  */
 
 /**
  * @module crawler
  */
 
@@ skipping 23 matching lines @@
  * @constructor
  */
 function TabAllocator(browser, maxtabs)
 {
   this._browser = browser;
   this._tabs = 0;
   this._maxtabs = maxtabs;
   // The queue containing resolve functions of promises waiting for a tab.
   this._resolvers = [];
   // Keep at least one tab alive to prevent browser from closing itself.
-  let tabToRemove = this._browser.tabs[0];
-  this._browser.removeAllTabsBut(tabToRemove);
-  // this._tab is a keep alive tab
-  this._tab = this._createTab().then(tab =>
-  {
-    // Starting from Firefox 48 (nightly) the sequence of calls addTab and
-    // removeTab can cause a closing of the browser because a new tab is still
-    // not here. Because of that we need to remove the previous tab only after
-    // the new tab is ready.
-    this._browser.removeTab(tabToRemove);
-    return tab;
-  });

[Wladimir Palant, 2016/09/14 15:00:13]

There is little point pre-allocating a tab, you can just as well have
this._tabKeepingWindowAlive and remove that tab in _createTab before resolving.
You can also use the same code to get rid of the race condition on release -
just set this._tabKeepingWindowAlive instead of removing the last tab. You won't
need to mess with the bookkeeping then, tab count is updated either way -
currently verifying that the tab count is correct under all circumstances is
really complicated.

[sergei, 2016/09/15 15:33:27]

Done.

+  this._tabKeepingWindowAlive = this._browser.tabs[0];
+  this._browser.removeAllTabsBut(this._tabKeepingWindowAlive);
 }
 TabAllocator.prototype = {
+  _removeTabKeepingWindowAlive: function()
+  {
+    if (!this._tabKeepingWindowAlive)
+      return;
+    this._browser.removeTab(this._tabKeepingWindowAlive);
+    delete this._tabKeepingWindowAlive;
+  },
+
   /**
    * Creates a blank tab in this._browser.
    *
    * @return {Promise.<tab>} promise which resolves once the tab is fully initialized.
    */
   _createTab: function()
   {
     this._tabs++;
     let tab = this._browser.addTab("about:blank");
     if (tab.linkedBrowser.outerWindowID)
+    {
+      this._removeTabKeepingWindowAlive();
       return Promise.resolve(tab);
+    }
     return new Promise((resolve, reject) =>
     {
       let onBrowserInit = (msg) =>
       {
-        // https://bugzilla.mozilla.org/show_bug.cgi?id=1256602#c1

[Wladimir Palant, 2016/09/14 15:00:13]

Please don't use URL-only comments, it should be obvious what this is about. For
example:

// "Browser:Init" message is sent once the browser is ready, see
https://bugzil.la/1256602#c1

Also, this comment is misplaced, should be above the addMessageListener call.

[sergei, 2016/09/15 15:33:27]

Done.

         tab.linkedBrowser.messageManager.removeMessageListener("Browser:Init", onBrowserInit);
+        this._removeTabKeepingWindowAlive();
         resolve(tab);
       };
+      // "Browser:Init" message is sent once the browser is ready, see
+      // https://bugzil.la/1256602#c1
       tab.linkedBrowser.messageManager.addMessageListener("Browser:Init", onBrowserInit);
     });
   },
 
   /**
    * Returns a promise that will resolve into a tab once a tab is allocated.
    * The tab cannot be used by other tasks until releaseTab() is called.
    *
    * @result {Promise.<tab>}
    */
   getTab: function()
   {
-    if (this._tab)
-    {
-      let tab = this._tab;
-      delete this._tab;
-      return tab;
-    }
     if (this._tabs < this._maxtabs)
       return this._createTab();
     return new Promise((resolve, reject) => this._resolvers.push(resolve));
   },
 
   /**
    * Adds a tab back to the pool so that it can be used by other tasks.
    *
    * @param {tab} tab
    */
   releaseTab: function(tab)
   {
-    // If we are about to close last tab don't close it immediately rather
-    // allocate a new blank tab and close the current one afterwards.
-    if (this._tabs == 1)
-    {
-      this._tab = this._createTab().then((resultTab) =>
-      {
-        this.releaseTab(tab);
-        return resultTab;
-      });
-      return;
-    }
-
-    this._browser.removeTab(tab);
+    // If we are about to close last tab don't close it immediately to keep
+    // the window alive. It will be closed when a new tab is created.
+    if (this._tabs > 1)
+      this._browser.removeTab(tab);
+    else
+    {
+      // navigate away from early opened URL
+      tab.linkedBrowser.loadURI('about:blank', null, null);
+      this._tabKeepingWindowAlive = tab;
+    }
+
     this._tabs--;
-    if (this._resolvers.length)
-    {
-      if (this._tab)
-      {
-        this._resolvers.shift()(this._tab);
-        delete this._tab;
-      }
-      else if (this._tabs < this._maxtabs)
-      {
-        this._resolvers.shift()(this._createTab());
-      }
+    if (this._resolvers.length && this._tabs < this._maxtabs)
+    {
+      this._resolvers.shift()(this._createTab());
     }
   },
 };
 
 /**
  * Observes page loads in a particular tabbed browser.
  *
  * @param {tabbrowser} browser
  *    The tabbed browser to be observed
  * @param {int} timeout
@@ skipping 265 matching lines @@
 
 function reportException(e)
 {
   let stack = "";
   if (e && typeof e == "object" && "stack" in e)
     stack = e.stack + "\n";
 
   Cu.reportError(e);
   dump(e + "\n" + stack + "\n");
 }