this fixes #252

js/async.js

@@ -197,28 +197,10 @@ function onMessageHandler(request, sender, callback) {
             HTTPSB.reloadAllLocalAssets();
             break;
 
-        case 'contentScriptHasLocalStorage':
-            response = contentScriptLocalStorageHandler(request.url);
-            break;
-
-        case 'contentScriptSummary':
-            contentScriptSummaryHandler(request, sender);
-            break;
-
         case 'forceReloadTab':
             HTTPSB.forceReload(request.pageURL);
             break;
 
-        case 'checkScriptBlacklisted':
-            response = {
-                scriptBlacklisted: HTTPSB.blacklisted(
-                    request.url,
-                    'script',
-                    HTTPSB.URI.hostnameFromURI(request.url)
-                    )
-                };
-            break;
-
         case 'gotoExtensionURL':
             gotoExtensionURL(request.url);
             break;

js/contentscript-uaspoof.js

@@ -0,0 +1,87 @@
+/*******************************************************************************
+
+    httpswitchboard - a Chromium browser extension to black/white list requests.
+    Copyright (C) 2013  Raymond Hill
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see {http://www.gnu.org/licenses/}.
+
+    Home: https://github.com/gorhill/httpswitchboard
+*/
+
+/******************************************************************************/
+
+// https://github.com/gorhill/httpswitchboard/issues/252
+
+(function() {
+
+/******************************************************************************/
+
+var navigatorSpoofer = ' \
+(function() { \
+    var spoofedUserAgent = "{{ua}}"; \
+    if ( spoofedUserAgent === navigator.userAgent ) { \
+        return; \
+    } \
+    var realNavigator = navigator; \
+    var SpoofedNavigator = function(ua) { \
+        this.navigator = navigator; \
+    }; \
+    var spoofedNavigator = new SpoofedNavigator(spoofedUserAgent); \
+    var makeFunction = function(n, k) { \
+        n[k] = function() { \
+            return this.navigator[k].apply(this.navigator, arguments); }; \
+    }; \
+    for ( var k in realNavigator ) { \
+        if ( typeof realNavigator[k] === "function" ) { \
+            makeFunction(spoofedNavigator, k); \
+        } else { \
+            spoofedNavigator[k] = realNavigator[k]; \
+        } \
+    } \
+    spoofedNavigator.appVersion = spoofedUserAgent; \
+    spoofedNavigator.userAgent = spoofedUserAgent; \
+    Object.freeze(spoofedNavigator); \
+    /*Object.setPrototypeOf(spoofedNavigator, Object.getPrototypeOf(realNavigator));*/ \
+    navigator = window.navigator = spoofedNavigator; \
+})();\
+';
+
+/******************************************************************************/
+
+// Because window.userAgent is read-only, we need to create a fake Navigator
+// object to contain out fake user-agent string.
+// Because objects created by a content script are local to the content script
+// and not visible to the web page itself (and vice versa), we need the context
+// of the web page to create the fake Navigator object directly, and the only
+// way to do this is to inject appropriate javascript code into the web page.
+
+var injectNavigatorSpoofer = function(spoofedUserAgent) {
+    if ( typeof spoofedUserAgent !== 'string' ) {
+        return;
+    }
+    if ( spoofedUserAgent === navigator.userAgent ) {
+        return;
+    }
+    var script = document.createElement('script');
+    script.type = 'text/javascript';
+    var js = document.createTextNode(navigatorSpoofer.replace('{{ua}}', spoofedUserAgent));
+    script.appendChild(js);
+    document.documentElement.appendChild(script, document.documentElement.firstChild);
+};
+
+chrome.runtime.sendMessage({ what: 'getUserAgentReplaceStr' }, injectNavigatorSpoofer);
+
+/******************************************************************************/
+
+})();

js/contentscripthandlers.js

@@ -21,7 +21,11 @@
 
 /******************************************************************************/
 
-function contentScriptSummaryHandler(details, sender) {
+(function() {
+
+/******************************************************************************/
+
+var contentScriptSummaryHandler = function(details, sender) {
     // TODO: Investigate "Error in response to tabs.executeScript: TypeError:
     // Cannot read property 'locationURL' of null" (2013-11-12). When can this
     // happens? 
@@ -66,11 +70,11 @@ function contentScriptSummaryHandler(details, sender) {
 
     // https://github.com/gorhill/httpswitchboard/issues/181
     httpsb.onPageLoadCompleted(pageURL);
-}
+};
 
 /******************************************************************************/
 
-function contentScriptLocalStorageHandler(pageURL) {
+var contentScriptLocalStorageHandler = function(pageURL) {
     var httpsb = HTTPSB;
     var httpsburi = httpsb.URI.set(pageURL);
     var response = httpsb.blacklisted(pageURL, 'cookie', httpsburi.hostname);
@@ -85,5 +89,55 @@ function contentScriptLocalStorageHandler(pageURL) {
         httpsb.localStorageRemovedCounter++;
     }
     return response;
-}
+};
+
+/******************************************************************************/
+
+// Handling stuff asynchronously simplifies code
+
+var onMessageHandler = function(request, sender, callback) {
+    if ( !request || !request.what ) {
+        return;
+    }
+
+    var response;
+
+    switch ( request.what ) {
+
+    case 'contentScriptHasLocalStorage':
+        response = contentScriptLocalStorageHandler(request.url);
+        break;
+
+    case 'contentScriptSummary':
+        contentScriptSummaryHandler(request, sender);
+        break;
+
+    case 'checkScriptBlacklisted':
+        response = {
+            scriptBlacklisted: HTTPSB.blacklisted(
+                request.url,
+                'script',
+                HTTPSB.URI.hostnameFromURI(request.url)
+                )
+            };
+        break;
+
+    case 'getUserAgentReplaceStr':
+        response = HTTPSB.userSettings.spoofUserAgent ? HTTPSB.userAgentReplaceStr : undefined;
+        break;
+
+    default:
+         // console.error('HTTP Switchboard > onMessage > unknown request: %o', request);
+        break;
+    }
+
+    if ( response !== undefined && callback ) {
+        callback(response);
+    }
+};
+
+chrome.runtime.onMessage.addListener(onMessageHandler);
+
+/******************************************************************************/
 
+})();

js/traffic.js

@@ -400,6 +400,13 @@ var onBeforeSendHeadersHandler = function(details) {
 
     if ( httpsb.userSettings.spoofUserAgent ) {
         changed = foilUserAgent(httpsb, details) || changed;
+        // https://github.com/gorhill/httpswitchboard/issues/252
+        // To avoid potential mismatch between the user agent from HTTP headers
+        // and the user agent from subrequests and the window.navigator object,
+        // I could always store here the effective user agent, but I am really
+        // not convinced it is worth the added overhead given the low
+        // probability and the benign consequence if it ever happen. Can always
+        // be revised if ever I become aware a mismatch is a terrible thing
     }
 
     if ( changed ) {

manifest.json

@@ -19,12 +19,20 @@
     "background": {
         "page": "background.html"
     },
-    "content_scripts": [{
+    "content_scripts": [
+    {
         "matches": ["http://*/*", "https://*/*"],
         "js": ["js/contentscript.js"],
         "run_at": "document_end",
         "all_frames": true
-    }],
+        },
+    {
+        "matches": ["http://*/*", "https://*/*"],
+        "js": ["js/contentscript-uaspoof.js"],
+        "run_at": "document_start",
+        "all_frames": true
+        }
+    ],
     "homepage_url": "https://github.com/gorhill/httpswitchboard/wiki",
     "minimum_chrome_version": "22.0",
     "default_locale": "en",