Abstract domain provider from its stream transport (#6670)

* Abstract domain provider from its stream transport

Creating new provider-consuming extensions, like [a new
platform](https://github.com/MetaMask/metamask-extension/blob/develop/docs/porting_to_new_environment.md)
can be frustrating for new contributors because our provider
construction has been tangled with our streaming interface.

Here I've broken up our streaming domain connection from the provider
construction, so developers can more easily construct local and
domain-restricted providers without dealing with streams.

* Abstract public API from stream interface

* clean up noop

* Document non-streaming interface

* getSiteMetadata must be async

* Clean up filters on stream end

* Document cleaning up filters

* Allow named filterMiddleware to be cleaned up

* Linted

* Require site metadata

* Destroy any destroyable middleware during cleanup

* Lint

2 files changed, 65 insertions, 15 deletions

diff --git a/app/scripts/metamask-controller.js b/app/scripts/metamask-controller.js
index 393154c60..17d38d43c 100644
--- a/app/scripts/metamask-controller.js
+++ b/app/scripts/metamask-controller.js
@@ -1391,6 +1391,32 @@ module.exports = class MetamaskController extends EventEmitter {
    * @param {string} origin - The URI of the requesting resource.
    */
   setupProviderConnection (outStream, origin, publicApi) {
+    const getSiteMetadata = publicApi && publicApi.getSiteMetadata
+    const engine = this.setupProviderEngine(origin, getSiteMetadata)
+
+    // setup connection
+    const providerStream = createEngineStream({ engine })
+
+    pump(
+      outStream,
+      providerStream,
+      outStream,
+      (err) => {
+        // cleanup filter polyfill middleware
+        engine._middleware.forEach((mid) => {
+          if (mid.destroy && typeof mid.destroy === 'function') {
+            mid.destroy()
+          }
+        })
+        if (err) log.error(err)
+      }
+    )
+  }
+
+  /**
+   * A method for creating a provider that is safely restricted for the requesting domain.
+   **/
+  setupProviderEngine (origin, getSiteMetadata) {
     // setup json rpc engine stack
     const engine = new RpcEngine()
     const provider = this.provider
@@ -1398,6 +1424,7 @@ module.exports = class MetamaskController extends EventEmitter {
 
     // create filter polyfill middleware
     const filterMiddleware = createFilterMiddleware({ provider, blockTracker })
+
     // create subscription polyfill middleware
     const subscriptionManager = createSubscriptionManager({ provider, blockTracker })
     subscriptionManager.events.on('notification', (message) => engine.emit('notification', message))
@@ -1413,24 +1440,11 @@ module.exports = class MetamaskController extends EventEmitter {
     // requestAccounts
     engine.push(this.providerApprovalController.createMiddleware({
       origin,
-      getSiteMetadata: publicApi && publicApi.getSiteMetadata,
+      getSiteMetadata,
     }))
     // forward to metamask primary provider
     engine.push(providerAsMiddleware(provider))
-
-    // setup connection
-    const providerStream = createEngineStream({ engine })
-
-    pump(
-      outStream,
-      providerStream,
-      outStream,
-      (err) => {
-        // cleanup filter polyfill middleware
-        filterMiddleware.destroy()
-        if (err) log.error(err)
-      }
-    )
+    return engine
   }
 
   /**
@@ -1797,3 +1811,4 @@ module.exports = class MetamaskController extends EventEmitter {
     return this.keyringController.setLocked()
   }
 }
+
diff --git a/docs/porting_to_new_environment.md b/docs/porting_to_new_environment.md
index 9c4021781..070ab69fa 100644
--- a/docs/porting_to_new_environment.md
+++ b/docs/porting_to_new_environment.md
@@ -10,6 +10,41 @@ The `metamask-background` describes the file at `app/scripts/background.js`, whi
 
 When a new site is visited, the WebExtension creates a new `ContentScript` in that page's context, which can be seen at `app/scripts/contentscript.js`. This script represents a per-page setup process, which creates the per-page `web3` api, connects it to the background script via the Port API (wrapped in a [stream abstraction](https://github.com/substack/stream-handbook)), and injected into the DOM before anything loads.
 
+You can choose to use this streaming interface to connect to the MetaMask controller over any transport you can wrap with a stream, by connecting it to the [metamask-inpage-provider](https://github.com/MetaMask/metamask-inpage-provider), but you can also construct a provider per domain like this:
+
+```javascript
+const providerFromEngine = require('eth-json-rpc-middleware/providerFromEngine')
+
+/**
+* returns a provider restricted to the requesting domain
+**/
+function incomingConnection (domain, getSiteMetadata) {
+  const engine = metamaskController.setupProviderEngine(domain, getSiteMetadata)
+  const provider = providerFromEngine(engine)
+  return provider
+}
+```
+
+Please note if you take this approach, you are responsible for cleaning up the filters when the page is closed:
+
+```
+const filterMiddleware = engine._middleware.filter(mid => mid.name === 'filterMiddleware')[0]
+filterMiddleware.destroy()
+```
+
+### getSiteMetadata()
+
+This method is used to enhance our confirmation screens with images and text representing the requesting domain.
+
+It should return a promise that resolves with an object with the following properties:
+
+- `name`: The requesting site's name.
+- `icon`: A URI representing the site's logo.
+
+### Using the Streams Interface
+
+Only use this if you intend to construct the [metamask-inpage-provider](https://github.com/MetaMask/metamask-inpage-provider) over a stream!
+
 The most confusing part about porting MetaMask to a new platform is the way we provide the Web3 API over a series of streams between contexts. Once you understand how we create the [MetamaskInpageProvider](https://github.com/MetaMask/metamask-inpage-provider/blob/master/index.js) in the [inpage.js script](../app/scripts/inpage.js), you will be able to understand how the [port-stream](../app/scripts/lib/port-stream.js) is just a thin wrapper around the [postMessage API](https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage), and a similar stream API can be wrapped around any communication channel to communicate with the `MetaMaskController` via its `setupUntrustedCommunication(stream, domain)` method.
 
 ### The MetaMask Controller