Worker: Throw an ErrorEvent on a failure of module loading

This behavior is defined in the HTML spec:

  Step 13: "If the algorithm asynchronously completes with null, queue a task to
  fire an event named error at worker, and return."
  https://html.spec.whatwg.org/multipage/workers.html#worker-processing-model

An ErrorEvent is created on a worker thread, and passed to the main thread to
dispatch it on a worker object in a document context.

Bug: 843031
Change-Id: I1fd82cba1e02b1658bb2b556742f7a206ee754a7
Reviewed-on: https://chromium-review.googlesource.com/1058998
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558903}

third_party/WebKit/LayoutTests/external/wpt/workers/modules/dedicated-worker-import.html

@@ -52,4 +52,25 @@ promise_test(() => {
       .then(e => assert_equals(e.data, 'TypeError'));
 }, 'importScripts() on module worker should throw an exception.');
 
+promise_test(() => {
+  const scriptURL = 'resources/non-existent-worker.js';
+  const worker = new Worker(scriptURL, { type: 'module' });
+  return new Promise(resolve => worker.onerror = resolve);
+}, 'Worker construction for non-existent script should throw an exception.');
+
+promise_test(() => {
+  const scriptURL = 'resources/static-import-non-existent-script-worker.js';
+  const worker = new Worker(scriptURL, { type: 'module' });
+  return new Promise(resolve => worker.onerror = resolve);
+}, 'Static import for non-existent script should throw an exception.');
+
+promise_test(() => {
+  const script_url = './non-existent-worker.js';
+  const worker = new Worker('resources/dynamic-import-given-url-worker.js',
+                            { type: 'module' });
+  worker.postMessage(script_url);
+  return new Promise(resolve => worker.onmessage = resolve)
+      .then(msg_event => assert_equals(msg_event.data, 'ERROR'));
+}, 'Dynamic import for non-existent script should throw an exception.');
+
 </script>

third_party/WebKit/LayoutTests/external/wpt/workers/modules/resources/dynamic-import-given-url-worker.js

 // Dynamically import the script URL sent by postMessage().
 self.addEventListener('message', e => {
-  import(e.data);
+  import(e.data).catch(error_event => postMessage('ERROR'));
 });

third_party/WebKit/LayoutTests/external/wpt/workers/modules/resources/static-import-non-existent-script-worker.js

+import './non-existent-script.js';

third_party/blink/renderer/core/workers/worker_module_tree_client.cc

@@ -5,6 +5,7 @@
 #include "third_party/blink/renderer/core/workers/worker_module_tree_client.h"
 
 #include "third_party/blink/renderer/bindings/core/v8/script_value.h"
+#include "third_party/blink/renderer/core/events/error_event.h"
 #include "third_party/blink/renderer/core/execution_context/execution_context.h"
 #include "third_party/blink/renderer/core/script/module_script.h"
 #include "third_party/blink/renderer/core/workers/worker_global_scope.h"
@@ -20,24 +21,29 @@ WorkerModuleTreeClient::WorkerModuleTreeClient(Modulator* modulator)
 // https://html.spec.whatwg.org/multipage/workers.html#worker-processing-model
 void WorkerModuleTreeClient::NotifyModuleTreeLoadFinished(
     ModuleScript* module_script) {
+  auto* execution_context =
+      ExecutionContext::From(modulator_->GetScriptState());
+
   if (!module_script) {
-    // Step 11: ... "If the algorithm asynchronously completes with null, queue
-    // a task to fire an event named error at worker, and abort these steps."
-    // ...
-    // TODO(nhiroki): Throw an ErrorEvent at the Worker object on the owner
-    // Document.
+    // Step 13: "If the algorithm asynchronously completes with null, queue
+    // a task to fire an event named error at worker, and return."
+    // This ErrorEvent object is just used for passing error information to a
+    // worker object on the parent context thread and not dispatched directly.
+    execution_context->ExceptionThrown(
+        ErrorEvent::Create("Failed to load a module script.",
+                           SourceLocation::Capture(), nullptr /* world */));
     return;
   }
 
-  // Step 11: ... "Otherwise, continue the rest of these steps after the
-  // algorithm's asynchronous completion, with script being the asynchronous
-  // completion value." ...
+  // Step 13: "Otherwise, continue the rest of these steps after the algorithm's
+  // asynchronous completion, with script being the asynchronous completion
+  // value."
 
   ScriptValue error = modulator_->ExecuteModule(
       module_script, Modulator::CaptureEvalErrorFlag::kReport);
-  WorkerGlobalScope* global_scope =
-      ToWorkerGlobalScope(ExecutionContext::From(modulator_->GetScriptState()));
-  global_scope->ReportingProxy().DidEvaluateModuleScript(error.IsEmpty());
+  ToWorkerGlobalScope(execution_context)
+      ->ReportingProxy()
+      .DidEvaluateModuleScript(error.IsEmpty());
 }
 
 void WorkerModuleTreeClient::Trace(blink::Visitor* visitor) {