[ES6 modules] ModuleTreeLinker::FetchDescendants should not assume urls isn't empty.

third_party/WebKit/Source/core/loader/modulescript/ModuleTreeLinkerTest.cpp

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/loader/modulescript/ModuleTreeLinker.h"
 
 #include "bindings/core/v8/ScriptModule.h"
 #include "bindings/core/v8/ScriptState.h"
 #include "bindings/core/v8/V8BindingForCore.h"
 #include "bindings/core/v8/V8BindingForTesting.h"
@@ skipping 485 matching lines @@
 
   GetModulator()->ResolveDependentTreeFetch(
       url_dep2, ModuleTreeLinkerTestModulator::ResolveResult::kSuccess);
 
   EXPECT_TRUE(client->WasNotifyFinished());
   ASSERT_TRUE(client->GetModuleScript());
   EXPECT_EQ(client->GetModuleScript()->InstantiationState(),
             ModuleInstantiationState::kInstantiated);
 }
 
+TEST_F(ModuleTreeLinkerTest, FetchDependencyOfCyclicGraph) {
+  ModuleTreeLinkerRegistry* registry = ModuleTreeLinkerRegistry::Create();
+
+  KURL url(kParsedURLString, "http://example.com/a.js");
+  ModuleScriptFetchRequest module_request(
+      url, String(), kParserInserted, WebURLRequest::kFetchCredentialsModeOmit);
+  TestModuleTreeClient* client = new TestModuleTreeClient;
+  registry->Fetch(
+      module_request,
+      AncestorList{KURL(kParsedURLString, "http://example.com/a.js")},
+      ModuleGraphLevel::kDependentModuleFetch, GetModulator(), client);
+
+  EXPECT_FALSE(client->WasNotifyFinished())
+      << "ModuleTreeLinker should always finish asynchronously.";
+  EXPECT_FALSE(client->GetModuleScript());
+
+  GetModulator()->ResolveSingleModuleScriptFetch(
+      url, {"./a.js"}, ModuleInstantiationState::kUninstantiated);
+
+  auto ancestor_list = GetModulator()->GetAncestorListForTreeFetch(url);
+  EXPECT_EQ(0u, ancestor_list.size());
+
+  EXPECT_TRUE(client->WasNotifyFinished());
+  ASSERT_TRUE(client->GetModuleScript());
+  EXPECT_EQ(client->GetModuleScript()->InstantiationState(),
+            ModuleInstantiationState::kInstantiated);
+}
+
 }  // namespace blink