Expose option to allow a new sandbox per visit

This PR does a few things:

Firstly, it allows the end consumer to decide
if they want a sandbox per visit or to share a single sandbox over all
visits.

Second, it changes the default of `buildSandboxPerVisit` back to
`false` therefore sharing a single sandbox for many `visit` invocations.

src/ember-app.js

@@ -185,6 +185,9 @@ class EmberApp {
   destroy() {
     // TODO: expose as public api (through the top level) so that we can
     // cleanup pre-warmed visits
+    if (this._applicationInstance) {
+      this._applicationInstance.destroy();
+    }
   }
 
   /**
@@ -242,12 +245,25 @@ class EmberApp {
    * @param {Object} bootOptions An object containing the boot options that are used by
    *                             by ember to decide whether it needs to do rendering or not.
    * @param {Object} result
+   * @param {Boolean} buildSandboxPerVisit if true, a new sandbox will
+   *                                       **always** be created, otherwise one
+   *                                       is created for the first request
+   *                                       only
    * @return {Promise<instance>} instance
    */
-  async visitRoute(path, fastbootInfo, bootOptions, result) {
-    let app = await this.buildApp();
-    result.applicationInstance = app;
-
+  async _visit(path, fastbootInfo, bootOptions, result, buildSandboxPerVisit) {
+    let shouldBuildApp = buildSandboxPerVisit || this._applicationInstance === undefined;
+
+    let app = shouldBuildApp ? await this.buildApp() : this._applicationInstance;
+
+    // entangle the specific application instance to the result, so it can be
+    // destroyed when result._destroy() is called (after the visit is
+    // completed)
+    if (buildSandboxPerVisit) {
+      result.applicationInstance = app;
+    } else {
+      this._applicationInstance = app;
+    }
     await app.boot();
 
     let instance = await app.buildInstance();
@@ -278,6 +294,7 @@ class EmberApp {
    * @param {Boolean} [options.shouldRender] whether the app should do rendering or not. If set to false, it puts the app in routing-only.
    * @param {Boolean} [options.disableShoebox] whether we should send the API data in the shoebox. If set to false, it will not send the API data used for rendering the app on server side in the index.html.
    * @param {Integer} [options.destroyAppInstanceInMs] whether to destroy the instance in the given number of ms. This is a failure mechanism to not wedge the Node process (See: https://github.com/ember-fastboot/fastboot/issues/90)
+   * @param {Boolean} [options.buildSandboxPerVisit] whether to create a new sandbox context per-visit, or reuse the existing sandbox
    * @param {ClientRequest}
    * @param {ClientResponse}
    * @returns {Promise<Result>} result
@@ -314,7 +331,13 @@ class EmberApp {
     }
 
     try {
-      await this.visitRoute(path, fastbootInfo, bootOptions, result);
+      await this._visit(
+        path,
+        fastbootInfo,
+        bootOptions,
+        result,
+        options.buildSandboxPerVisit === true
+      );
 
       if (!disableShoebox) {
         // if shoebox is not disabled, then create the shoebox and send API data

src/index.js

@@ -75,6 +75,7 @@ class FastBoot {
    * @param {Boolean} [options.shouldRender] whether the app should do rendering or not. If set to false, it puts the app in routing-only.
    * @param {Boolean} [options.disableShoebox] whether we should send the API data in the shoebox. If set to false, it will not send the API data used for rendering the app on server side in the index.html.
    * @param {Integer} [options.destroyAppInstanceInMs] whether to destroy the instance in the given number of ms. This is a failure mechanism to not wedge the Node process (See: https://github.com/ember-fastboot/fastboot/issues/90)
+   * @param {Boolean} [options.buildSandboxPerVisit] whether to create a new sandbox context per-visit (slows down each visit, but guarantees no prototype leakages can occur), or reuse the existing sandbox (faster per-request, but each request shares the same set of prototypes)
    * @returns {Promise<Result>} result
    */
   async visit(path, options = {}) {

test/fastboot-test.js

@@ -401,45 +401,48 @@ describe('FastBoot', function() {
       });
   });
 
-  it('in app prototype mutations do not leak across visits', async function() {
+  it('in app prototype mutations do not leak across visits with buildSandboxPerVisit=true', async function() {
     this.timeout(3000);
 
     var fastboot = new FastBoot({
       distPath: fixture('app-with-prototype-mutations'),
     });
 
-    let result = await fastboot.visit('/');
+    let result = await fastboot.visit('/', { buildSandboxPerVisit: true });
     let html = await result.html();
 
     expect(html).to.match(/Items: 1/);
 
-    result = await fastboot.visit('/');
+    result = await fastboot.visit('/', { buildSandboxPerVisit: true });
     html = await result.html();
 
     expect(html).to.match(/Items: 1/);
 
-    result = await fastboot.visit('/');
+    result = await fastboot.visit('/', { buildSandboxPerVisit: true });
     html = await result.html();
 
     expect(html).to.match(/Items: 1/);
   });
 
-  it('errors can be properly attributed', async function() {
+  it('errors can be properly attributed with buildSandboxPerVisit=true', async function() {
     this.timeout(3000);
 
     var fastboot = new FastBoot({
       distPath: fixture('onerror-per-visit'),
     });
 
     let first = fastboot.visit('/slow/100/reject', {
+      buildSandboxPerVisit: true,
       request: { url: '/slow/100/reject', headers: {} },
     });
 
     let second = fastboot.visit('/slow/50/resolve', {
+      buildSandboxPerVisit: true,
       request: { url: '/slow/50/resolve', headers: {} },
     });
 
     let third = fastboot.visit('/slow/25/resolve', {
+      buildSandboxPerVisit: true,
       request: { url: '/slow/25/resolve', headers: {} },
     });