Issue 6504, 6458, 6446 - Update selectors based on DOM mutations

lib/content/elemHideEmulation.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 464 matching lines @@
 }
 
 function CSSSelectorManager(root)
 {
   this._root = root;
 
   this.selectors = [];
   this.filters = [];
 
   this._lookup = new Set();
+
+  this._extended = false;
 }
 
 CSSSelectorManager.prototype = {
   add(selectors, filters)
   {
     for (let i = 0; i < selectors.length; i++)
     {
       this.selectors.push(selectors[i]);
       this.filters.push(filters[i]);
 
       this._lookup.add(selectors[i]);
+
+      if (!this._extended && selectors[i].startsWith(":root > "))
+        this._extended = true;
     }
   },
 
   has(selector)
   {
     return this._lookup.has(selector);
   },
 
   setAt(index, selector)
   {
     this._lookup.delete(this.selectors[index]);
     this._lookup.add(selector);
 
     this.selectors[index] = selector;
   },
 
   deleteAt(index)
   {
     this._lookup.delete(this.selectors[index]);
 
     this.selectors.splice(index, 1);
     this.filters.splice(index, 1);
   },
 
   update(mutations)
   {
+    // If there are no extended CSS selectors (i.e. :-abp-*), there's nothing
+    // to update.
+    if (!this._extended || this.selectors.length == 0)

[Manish Jethani, 2018/03/21 15:35:26]

This is useful. In EasyList the number of sites with emulation filters is 45:

  grep '#?#' easylist.txt | cut -f1 -d'#' | sort -u | tr ',' "\n" | wc -l

While the number of sites with extended CSS selector syntax is only 4:

  grep '#?#.*-abp' easylist.txt | cut -f1 -d'#' | sort -u | tr ',' "\n" | wc -l

If a site doesn't use :-abp-* there won't be any path-based selectors (those
starting with ":root > ") and there's no need to update anything.

Also if there are no selectors, which is true for facebook.com when the page is
loading (but not true afterwards), we can skip all the unnecessary code.

+      return false;
+
     let changed = false;
 
     for (let mutation of mutations)
     {
       if (this._updateForMutation(mutation))
         changed = true;
     }
 
     return changed;
   },
@@ skipping 401 matching lines @@
   onLoad(event)
   {
     let stylesheet = event.target.sheet;
     if (stylesheet)
       this.queueFiltering([stylesheet]);
   },
 
   observe(mutations)
   {
     if (this.css.update(mutations))
       this.addSelectorsFunc(this.css.selectors, this.css.filters);

[hub, 2018/03/23 15:54:25]

My concern here is that the intent of queueFiltering was to have 
addSelectorsFunc() run asynchronously. This here throw it off a bit.

[Manish Jethani, 2018/03/23 16:55:33]

The thing is that I'm trying to avoid running full filtering on every DOM
mutation, but at the same time we have to keep the selectors up to date
(otherwise they hide the wrong elements, i.e. false positives and false
negatives). I thought about making this asynchronous, but I wonder if it's
necessary. MutationObserver events are already asynchronous (multiple mutations
are batched together). But I'll see if I can throw in a bit of asynchronicity
here.

 
     this.queueFiltering(null, mutations);
   },
 
   apply(patterns)
   {
     this.patterns = [];
     for (let pattern of patterns)
     {
       let selectors = this.parseSelector(pattern.selector);
@@ skipping 12 matching lines @@
           characterData: shouldObserveCharacterData(this.patterns),
           subtree: true
         }
       );
       this.document.addEventListener("load", this.onLoad.bind(this), true);
     }
   }
 };
 
 exports.ElemHideEmulation = ElemHideEmulation;