Bug 1651844 - Ensure that readyPromise is resolved when startup() exi…

…ts early r=rpl

This is necessary because otherwise callers of policy.readyPromise can
get stuck when an extension fails to start up.

Differential Revision: https://phabricator.services.mozilla.com/D83143

dom/chrome-webidl/WebExtensionPolicy.webidl

@@ -230,9 +230,11 @@ interface WebExtensionPolicy {
    * This may be used to delay operations, such as loading extension pages,
    * which depend on extensions being fully initialized.
    *
-   * Note: This will always be either a Promise<WebExtensionPolicy> or null,
+   * Note: This will always be either a Promise<WebExtensionPolicy?> or null,
    * but the WebIDL grammar does not allow us to specify a nullable Promise
    * type.
+   *
+   * Note: This could resolve to null when the startup was interrupted.
    */
   readonly attribute object? readyPromise;
 
@@ -270,5 +272,5 @@ dictionary WebExtensionInit {
   sequence<DOMString>? backgroundScripts = null;
   DOMString? backgroundWorkerScript = null;
 
-  Promise<WebExtensionPolicy> readyPromise;
+  Promise<WebExtensionPolicy?> readyPromise;
 };

toolkit/components/extensions/Extension.jsm

@@ -2396,6 +2396,8 @@ class Extension extends ExtensionData {
   async startup() {
     this.state = "Startup";
 
+    // readyPromise is resolved with the policy upon success,
+    // and with null if startup was interrupted.
     let resolveReadyPromise;
     let readyPromise = new Promise(resolve => {
       resolveReadyPromise = resolve;
@@ -2560,6 +2562,9 @@ class Extension extends ExtensionData {
       throw e;
     } finally {
       ExtensionTelemetry.extensionStartup.stopwatchFinish(this);
+      // Mark readyPromise as resolved in case it has not happened before,
+      // e.g. due to an early return or an error.
+      resolveReadyPromise(null);
     }
   }
 

toolkit/components/extensions/test/xpcshell/test_ext_extension_startup_failure.js

@@ -21,6 +21,11 @@ add_task(async function extension_startup_early_error() {
 
   let startupPromise = extension.startup();
 
+  let policy = WebExtensionPolicy.getByID(EXTENSION_ID);
+  ok(policy, "WebExtensionPolicy instantiated at startup");
+  let readyPromise = policy.readyPromise;
+  ok(readyPromise, "WebExtensionPolicy.readyPromise is set");
+
   await Assert.rejects(
     startupPromise,
     /dummy error/,
@@ -32,4 +37,10 @@ add_task(async function extension_startup_early_error() {
     null,
     "WebExtensionPolicy should be unregistered"
   );
+
+  Assert.equal(
+    await readyPromise,
+    null,
+    "policy.readyPromise should be resolved with null"
+  );
 });