Added TextMetrics protection

Fixes #448

lib/modifiedTextMetricsAPI.js

@@ -0,0 +1,96 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+(function(){
+	"use strict";
+	
+	let scope;
+	if ((typeof exports) !== "undefined"){
+		scope = exports;
+	}
+	else {
+		scope = require.register("./modifiedTextMetricsAPI", {});
+	}
+	
+	const {checkerWrapper, setProperties, getStatusByFlag} = require("./modifiedAPIFunctions");
+	const {byteArrayToString: hash} = require("./hash");
+	const {cache} = require("./modifiedDOMRectAPI");
+	const valueCache = cache.valueCache;
+	
+	function getValueHash(value){
+		return hash(new Float32Array([value]));
+	}
+	
+	let randomSupply = null;
+	scope.setRandomSupply = function(supply){
+		randomSupply = supply;
+	};
+	
+	function getFakeValue(window, value, i, prefs){
+		const valueHash = getValueHash(value);
+		const cache = valueCache[i];
+		let cachedValue = cache[valueHash];
+		if (typeof cachedValue === "number"){
+			return cachedValue;
+		}
+		if ((value * prefs("domRectIntegerFactor", window.location)) % 1 === 0){
+			cache[valueHash] = value;
+			return value;
+		}
+		else {
+			const rng = randomSupply.getRng(5, window);
+			const fakedValue = value + 0.01 * (rng(i) / 0xffffffff - 0.5);
+			const fakedHash = getValueHash(fakedValue);
+			cache[valueHash] = fakedValue;
+			cache[fakedHash] = fakedValue;
+			return fakedValue;
+		}
+	}
+	
+	function generateChangedTextMetricsPropertyGetter(property, cacheIndex){
+		const changedGetter = {
+			objectGetters: [
+				function(window){return window.TextMetrics && window.TextMetrics.prototype;}
+			],
+			name: property,
+			getterGenerator: function(checker){
+				const temp = {
+					get [property](){
+						return checkerWrapper(checker, this, arguments, function(args, check){
+							const {prefs, notify, window, original} = check;
+							const originalValue = original.call(this, ...args);
+							const returnValue = getFakeValue(window, originalValue, cacheIndex, prefs);
+							if (originalValue !== returnValue){
+								notify("fakedTextMetricsReadout");
+							}
+							return returnValue;
+						});
+					}
+				};
+				return Object.getOwnPropertyDescriptor(temp, property).get;
+			}
+		};
+		return changedGetter;
+	}
+	
+	scope.changedGetters = [
+		generateChangedTextMetricsPropertyGetter("width", cache.WIDTH),
+		generateChangedTextMetricsPropertyGetter("actualBoundingBoxAscent", cache.OTHER),
+		generateChangedTextMetricsPropertyGetter("actualBoundingBoxDescent", cache.OTHER),
+		generateChangedTextMetricsPropertyGetter("actualBoundingBoxLeft", cache.OTHER),
+		generateChangedTextMetricsPropertyGetter("actualBoundingBoxRight", cache.OTHER),
+		generateChangedTextMetricsPropertyGetter("alphabeticBaseline", cache.OTHER),
+		generateChangedTextMetricsPropertyGetter("emHeightAscent", cache.OTHER),
+		generateChangedTextMetricsPropertyGetter("emHeightDescent", cache.OTHER),
+		generateChangedTextMetricsPropertyGetter("fontBoundingBoxAscent", cache.OTHER),
+		generateChangedTextMetricsPropertyGetter("fontBoundingBoxDescent", cache.OTHER),
+		generateChangedTextMetricsPropertyGetter("hangingBaseline", cache.OTHER),
+		generateChangedTextMetricsPropertyGetter("ideographicBaseline", cache.OTHER),
+	];
+	
+	setProperties({}, scope.changedGetters, {
+		type: "readout",
+		getStatus: getStatusByFlag("protectTextMetrics"),
+		api: "textMetrics"
+	});
+}());