Disable future calls to register() when extension has finished loading

src/extension-support/tw-unsandboxed-extension-runner.js

@@ -3,7 +3,12 @@ const BlockType = require('../extension-support/block-type');
 const TargetType = require('../extension-support/target-type');
 const AsyncLimiter = require('../util/async-limiter');
 
-const loadUnsandboxedExtension = (extensionURL, vm) => new Promise((resolve, reject) => {
+/**
+ * Sets up the global.Scratch API for an unsandboxed extension.
+ * @param {VirtualMachine} vm
+ * @returns {Promise<object[]>} Resolves with a list of extension objects when Scratch.extensions.register is called.
+ */
+const createUnsandboxedExtensionAPI = vm => new Promise(resolve => {
     global.Scratch = global.Scratch || {};
     global.Scratch.vm = vm;
     global.Scratch.renderer = vm.runtime.renderer;
@@ -12,9 +17,6 @@ const loadUnsandboxedExtension = (extensionURL, vm) => new Promise((resolve, rej
     global.Scratch.TargetType = TargetType;
     global.Scratch.ArgumentType = ArgumentType;
 
-    // When we load the script, wait for it to eventually call Scratch.extensions.register
-    // before resolving. Note that extension scripts may register multiple extensions.
-
     const extensionObjects = [];
     const register = extensionObject => {
         extensionObjects.push(extensionObject);
@@ -24,11 +26,35 @@ const loadUnsandboxedExtension = (extensionURL, vm) => new Promise((resolve, rej
         unsandboxed: true,
         register
     };
+});
+
+/**
+ * Disable the existing global.Scratch unsandboxed extension APIs.
+ * This helps debug poorly designed extensions.
+ */
+const teardownUnsandboxedExtensionAPI = () => {
+    // We can assume global.Scratch already exists.
+    global.Scratch.extensions.register = () => {
+        throw new Error('Too late to register new extensions.');
+    };
+};
+
+/**
+ * Load an unsandboxed extension from an arbitrary URL. This is dangerous.
+ * @param {string} extensionURL
+ * @param {Virtualmachine} vm
+ * @returns {Promise<object[]>} Resolves with a list of extension objects if the extension was loaded successfully.
+ */
+const loadUnsandboxedExtension = (extensionURL, vm) => new Promise((resolve, reject) => {
+    createUnsandboxedExtensionAPI(vm).then(resolve);
 
     const script = document.createElement('script');
     script.onerror = () => reject(new Error(`Error in unsandboxed script ${extensionURL}`));
     script.src = extensionURL;
     document.body.appendChild(script);
+}).then(objects => {
+    teardownUnsandboxedExtensionAPI();
+    return objects;
 });
 
 // Because loading unsandboxed extensions requires messing with global state (global.Scratch),
@@ -37,5 +63,6 @@ const limiter = new AsyncLimiter(loadUnsandboxedExtension, 1);
 const load = (extensionURL, vm) => limiter.do(extensionURL, vm);
 
 module.exports = {
+    createUnsandboxedExtensionAPI,
     load
 };