Build new sandbox after request is done

This is basic PR to build a new sandbox per visit after current request is complete. Specifically making sure the creation of sandbox is not tied to the request timeline itself but is done as an after effect. This is so that we do not build sandbox at the start of the request which is slow.

Not specifically adding tests as this is just an internal implementation change. When we add queue management, I'll add more test cases of requests more than queue size or less than to cover those usecases.

Author: kratiahuja

src/ember-app.js

@@ -188,6 +188,16 @@ class EmberApp {
     }
   }
 
+  /**
+   * Builds a new application instance sandbox as a micro-task.
+   */
+  buildNewApplicationInstance() {
+    return Promise.resolve().then(() => {
+      let app = this.buildApp();
+      return app;
+    });
+  }
+
   /**
    * @private
    *
@@ -227,6 +237,29 @@ class EmberApp {
     return app;
   }
 
+  /**
+   * @private
+   *
+   * Get the new sandbox off if it is being created, otherwise create a new one on demand.
+   * The later is needed when the current request hasn't finished or wasn't build with sandbox
+   * per request turned on and a new request comes in.
+   *
+   */
+  async _getNewApplicationInstance() {
+    let app;
+
+    if (this._pendingNewApplicationInstance) {
+      let pendingAppInstancePromise = this._pendingNewApplicationInstance;
+      this._pendingNewApplicationInstance = undefined;
+      app = await pendingAppInstancePromise;
+    } else {
+      // if there is no current pending application instance, create a new one on-demand.
+      app = await this.buildApp();
+    }
+
+    return app;
+  }
+
   /**
    * @private
    *
@@ -252,7 +285,7 @@ class EmberApp {
   async _visit(path, fastbootInfo, bootOptions, result, buildSandboxPerVisit) {
     let shouldBuildApp = buildSandboxPerVisit || this._applicationInstance === undefined;
 
-    let app = shouldBuildApp ? await this.buildApp() : this._applicationInstance;
+    let app = shouldBuildApp ? await this._getNewApplicationInstance() : this._applicationInstance;
 
     if (buildSandboxPerVisit) {
       // entangle the specific application instance to the result, so it can be
@@ -305,6 +338,7 @@ class EmberApp {
     let html = options.html || this.html;
     let disableShoebox = options.disableShoebox || false;
     let destroyAppInstanceInMs = parseInt(options.destroyAppInstanceInMs, 10);
+    let buildSandboxPerVisit = options.buildSandboxPerVisit || false;
 
     let shouldRender = options.shouldRender !== undefined ? options.shouldRender : true;
     let bootOptions = buildBootOptions(shouldRender);
@@ -331,13 +365,7 @@ class EmberApp {
     }
 
     try {
-      await this._visit(
-        path,
-        fastbootInfo,
-        bootOptions,
-        result,
-        options.buildSandboxPerVisit === true
-      );
+      await this._visit(path, fastbootInfo, bootOptions, result, buildSandboxPerVisit);
 
       if (!disableShoebox) {
         // if shoebox is not disabled, then create the shoebox and send API data
@@ -355,6 +383,12 @@ class EmberApp {
       result._destroy();
 
       clearTimeout(destroyAppInstanceTimer);
+
+      if (buildSandboxPerVisit) {
+        // if sandbox was built for this visit, then build a new sandbox for the next incoming request
+        // which is invoked using buildSandboxPerVisit
+        this._pendingNewApplicationInstance = this.buildNewApplicationInstance();
+      }
     }
 
     return result;