Merge pull request #495 from ipfs-shipyard/feat/hide-window.ipfs

This PR enables Firefox users to disable creation of `window.ipfs` attribute via _Preferences_ screen, solving fingerprinting issue raised in #451.

It requires webpack, so should be merged after #498 

### Background

Existing `tabs.executeScript` API with `runAt: 'document_start'` flag was executing too late and page scripts were  unable to detect `window.ipfs` correctly.  More info on the underlying issue: #368 #362 (comment)

As a workaround that worked in both Chrome and Firefox, we were forced to always load content script via manifest definition. This meant no control over when it is loaded and no easy off switch. If `window.ipfs` was disabled via _Preferences_, it was throwing an Error on access, but remained in the scope.

Mozilla added [`contentScripts`](https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/contentScripts/register) API in Firefox 59. The key difference between `tabs.executeScript` and `contentScripts` API is that the latter provides guarantee to execute before anything else on the page, passing [our synchronous smoke test](https://ipfs-shipyard.github.io/ipfs-companion/docs/examples/window.ipfs-fallback.html). 

### Known Issues

Chrome does not provide `contentScripts` API so it will continue to statically load `content_script` via  manifest.  Hopefully with time, other browser vendors will adopt the new API.

add-on/manifest.firefox.json

@@ -18,6 +18,7 @@
     "default_title": "__MSG_pageAction_titleNonIpfs__",
     "default_popup": "dist/popup/page-action/index.html"
   },
+  "content_scripts": [ ],
   "protocol_handlers": [
     {
       "protocol": "web+dweb",

add-on/src/contentScripts/ipfs-proxy/content.js

@@ -7,14 +7,12 @@ const injectScript = require('./inject-script')
 
 function init () {
   const port = browser.runtime.connect({ name: 'ipfs-proxy' })
-
   // Forward on messages from background to the page and vice versa
   port.onMessage.addListener((data) => {
     if (data && data.sender && data.sender.startsWith('postmsg-rpc/')) {
       window.postMessage(data, '*')
     }
   })
-
   window.addEventListener('message', (msg) => {
     if (msg.data && msg.data.sender && msg.data.sender.startsWith('postmsg-rpc/')) {
       port.postMessage(msg.data)
@@ -24,8 +22,30 @@ function init () {
   injectScript(rawCode)
 }
 
-// Restricting window.ipfs to Secure Context
-// See: https://github.com/ipfs-shipyard/ipfs-companion/issues/476
-if (window.isSecureContext) {
+function injectIpfsProxy () {
+  // Skip if proxy is already present
+  if (window.ipfs) {
+    return false
+  }
+  // Restricting window.ipfs to Secure Context
+  // See: https://github.com/ipfs-shipyard/ipfs-companion/issues/476
+  if (!window.isSecureContext) {
+    return false
+  }
+  // Skip if not in HTML context
+  // Check 1/2
+  const doctype = window.document.doctype
+  if (doctype && doctype.name !== 'html') {
+    return false
+  }
+  // Check 2/2
+  if (document.documentElement.nodeName !== 'HTML') {
+    return false
+  }
+  // Should be ok by now
+  return true
+}
+
+if (injectIpfsProxy()) {
   init()
 }

add-on/src/lib/ipfs-companion.js

@@ -30,6 +30,7 @@ module.exports = async function init () {
   var contextMenus
   var apiStatusUpdateInterval
   var ipfsProxy
+  var ipfsProxyContentScript
   const idleInSecs = 5 * 60
   const browserActionPortName = 'browser-action-port'
 
@@ -60,6 +61,7 @@ module.exports = async function init () {
     })
     modifyRequest = createRequestModifier(getState, dnsLink, ipfsPathValidator, runtime)
     ipfsProxy = createIpfsProxy(() => ipfs, getState)
+    ipfsProxyContentScript = await registerIpfsProxyContentScript()
     registerListeners()
     await setApiStatusUpdateInterval(options.ipfsApiPollMs)
     await storeMissingOptions(
@@ -95,6 +97,33 @@ module.exports = async function init () {
     }
   }
 
+  // Register Content Script responsible for loading window.ipfs (ipfsProxy)
+  //
+  // The key difference between tabs.executeScript and contentScripts API
+  // is the latter provides guarantee to execute before anything else.
+  // https://github.com/ipfs-shipyard/ipfs-companion/issues/451#issuecomment-382669093
+  async function registerIpfsProxyContentScript (previousHandle) {
+    previousHandle = previousHandle || ipfsProxyContentScript
+    if (previousHandle) {
+      previousHandle.unregister()
+    }
+    if (!state.ipfsProxy || !browser.contentScripts) {
+      // no-op if window.ipfs is disabled in Preferences
+      // or if runtime has no contentScript API
+      // (Chrome loads content script via manifest)
+      return
+    }
+    const newHandle = await browser.contentScripts.register({
+      matches: ['<all_urls>'],
+      js: [
+        {file: '/dist/bundles/ipfsProxyContentScript.bundle.js'}
+      ],
+      allFrames: true,
+      runAt: 'document_start'
+    })
+    return newHandle
+  }
+
   // HTTP Request Hooks
   // ===================================================================
 
@@ -544,13 +573,16 @@ module.exports = async function init () {
         case 'useCustomGateway':
           state.redirect = change.newValue
           break
+        case 'ipfsProxy':
+          state[key] = change.newValue
+          ipfsProxyContentScript = await registerIpfsProxyContentScript()
+          break
         case 'linkify':
         case 'catchUnhandledProtocols':
         case 'displayNotifications':
         case 'automaticMode':
         case 'dnslink':
         case 'preloadAtPublicGateway':
-        case 'ipfsProxy':
           state[key] = change.newValue
           break
       }
@@ -616,6 +648,10 @@ module.exports = async function init () {
       contextMenus = null
       ipfsProxy.destroy()
       ipfsProxy = null
+      if (ipfsProxyContentScript) {
+        ipfsProxyContentScript.unregister()
+        ipfsProxyContentScript = null
+      }
       await destroyIpfsClient()
     }
   }