Started looking for the dart plugin registrant at runtime (flutter/engine#31418)

engine/src/flutter/BUILD.gn

@@ -145,6 +145,7 @@ group("flutter") {
       "//flutter/lib/spirv/test/supported_glsl_op_shaders:spirv_compile_supported_glsl_shaders",
       "//flutter/lib/spirv/test/supported_op_shaders:spirv_compile_supported_op_shaders",
       "//flutter/lib/ui:ui_unittests",
+      "//flutter/runtime:dart_plugin_registrant_unittests",
       "//flutter/runtime:no_dart_plugin_registrant_unittests",
       "//flutter/runtime:runtime_unittests",
       "//flutter/shell/common:shell_unittests",

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -962,6 +962,7 @@ FILE: ../../../flutter/runtime/dart_isolate_group_data.cc
 FILE: ../../../flutter/runtime/dart_isolate_group_data.h
 FILE: ../../../flutter/runtime/dart_isolate_unittests.cc
 FILE: ../../../flutter/runtime/dart_lifecycle_unittests.cc
+FILE: ../../../flutter/runtime/dart_plugin_registrant_unittests.cc
 FILE: ../../../flutter/runtime/dart_service_isolate.cc
 FILE: ../../../flutter/runtime/dart_service_isolate.h
 FILE: ../../../flutter/runtime/dart_service_isolate_unittests.cc
@@ -978,6 +979,7 @@ FILE: ../../../flutter/runtime/dart_vm_lifecycle.h
 FILE: ../../../flutter/runtime/dart_vm_unittests.cc
 FILE: ../../../flutter/runtime/embedder_resources.cc
 FILE: ../../../flutter/runtime/embedder_resources.h
+FILE: ../../../flutter/runtime/fixtures/dart_tool/flutter_build/dart_plugin_registrant.dart
 FILE: ../../../flutter/runtime/fixtures/no_dart_plugin_registrant_test.dart
 FILE: ../../../flutter/runtime/fixtures/runtime_test.dart
 FILE: ../../../flutter/runtime/fixtures/split_lib_test.dart

engine/src/flutter/runtime/BUILD.gn

@@ -158,4 +158,24 @@ if (enable_unittests) {
       "//flutter/testing:fixture_test",
     ]
   }
+
+  test_fixtures("plugin_registrant") {
+    dart_main = "fixtures/dart_tool/flutter_build/dart_plugin_registrant.dart"
+    use_target_as_artifact_prefix = true
+  }
+
+  executable("dart_plugin_registrant_unittests") {
+    testonly = true
+
+    sources = [ "dart_plugin_registrant_unittests.cc" ]
+
+    public_configs = [ "//flutter:export_dynamic_symbols" ]
+
+    public_deps = [
+      ":plugin_registrant",
+      "//flutter/fml",
+      "//flutter/testing",
+      "//flutter/testing:fixture_test",
+    ]
+  }
 }

engine/src/flutter/runtime/dart_isolate.cc

@@ -709,6 +709,34 @@ static void InvokeDartPluginRegistrantIfAvailable(Dart_Handle library_handle) {
   tonic::LogIfError(tonic::DartInvokeField(plugin_registrant, "register", {}));
 }
 
+namespace {
+bool EndsWith(const std::string& str, const std::string& ending) {
+  if (str.size() >= ending.size()) {
+    return (0 ==
+            str.compare(str.size() - ending.size(), ending.size(), ending));
+  } else {
+    return false;
+  }
+}
+
+Dart_Handle FindDartPluginRegistrantLibrary() {
+  // TODO(): Instead of finding this, it could be passed down from the tool.
+  Dart_Handle libraries = Dart_GetLoadedLibraries();
+  intptr_t length = 0;
+  Dart_ListLength(libraries, &length);
+  for (intptr_t i = 0; i < length; ++i) {
+    Dart_Handle library = Dart_ListGetAt(libraries, i);
+    std::string library_name =
+        tonic::DartConverter<std::string>::FromDart(Dart_ToString(library));
+    if (EndsWith(library_name,
+                 "dart_tool/flutter_build/dart_plugin_registrant.dart'")) {
+      return library;
+    }
+  }
+  return Dart_Null();
+}
+}  // namespace
+
 bool DartIsolate::RunFromLibrary(std::optional<std::string> library_name,
                                  std::optional<std::string> entrypoint,
                                  const std::vector<std::string>& args) {
@@ -727,7 +755,12 @@ bool DartIsolate::RunFromLibrary(std::optional<std::string> library_name,
                                ? tonic::ToDart(entrypoint.value().c_str())
                                : tonic::ToDart("main");
 
-  InvokeDartPluginRegistrantIfAvailable(library_handle);
+  auto dart_plugin_registrant_library = FindDartPluginRegistrantLibrary();
+  if (!Dart_IsNull(dart_plugin_registrant_library)) {
+    InvokeDartPluginRegistrantIfAvailable(dart_plugin_registrant_library);
+  } else {
+    InvokeDartPluginRegistrantIfAvailable(library_handle);
+  }
 
   auto user_entrypoint_function =
       ::Dart_GetField(library_handle, entrypoint_handle);

engine/src/flutter/runtime/dart_plugin_registrant_unittests.cc

@@ -0,0 +1,78 @@
+// Copyright 2013 The Flutter 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 "flutter/runtime/dart_isolate.h"
+
+#include "flutter/fml/paths.h"
+#include "flutter/runtime/dart_vm.h"
+#include "flutter/runtime/dart_vm_lifecycle.h"
+#include "flutter/testing/dart_isolate_runner.h"
+#include "flutter/testing/fixture_test.h"
+#include "flutter/testing/testing.h"
+
+// CREATE_NATIVE_ENTRY is leaky by design
+// NOLINTBEGIN(clang-analyzer-core.StackAddressEscape)
+
+namespace flutter {
+namespace testing {
+
+const std::string kernel_file_name = "plugin_registrant_kernel_blob.bin";
+const std::string elf_file_name = "plugin_registrant_app_elf_snapshot.so";
+
+class DartIsolateTest : public FixtureTest {
+ public:
+  DartIsolateTest() : FixtureTest(kernel_file_name, elf_file_name, "") {}
+};
+
+TEST_F(DartIsolateTest, DartPluginRegistrantIsNotPresent) {
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+
+  std::vector<std::string> messages;
+  fml::AutoResetWaitableEvent latch;
+
+  AddNativeCallback(
+      "PassMessage",
+      CREATE_NATIVE_ENTRY(([&latch, &messages](Dart_NativeArguments args) {
+        auto message = tonic::DartConverter<std::string>::FromDart(
+            Dart_GetNativeArgument(args, 0));
+        messages.push_back(message);
+        latch.Signal();
+      })));
+
+  auto settings = CreateSettingsForFixture();
+  auto did_throw_exception = false;
+  settings.unhandled_exception_callback = [&](const std::string& error,
+                                              const std::string& stack_trace) {
+    did_throw_exception = true;
+    return true;
+  };
+
+  auto vm_ref = DartVMRef::Create(settings);
+  auto thread = CreateNewThread();
+  TaskRunners task_runners(GetCurrentTestName(),  //
+                           thread,                //
+                           thread,                //
+                           thread,                //
+                           thread                 //
+  );
+
+  auto kernel_path =
+      fml::paths::JoinPaths({GetFixturesPath(), kernel_file_name});
+  auto isolate =
+      RunDartCodeInIsolate(vm_ref, settings, task_runners,
+                           "mainForPluginRegistrantTest", {}, kernel_path);
+
+  ASSERT_TRUE(isolate);
+  ASSERT_EQ(isolate->get()->GetPhase(), DartIsolate::Phase::Running);
+
+  latch.Wait();
+
+  ASSERT_EQ(messages.size(), 1u);
+  ASSERT_EQ(messages[0], "_PluginRegistrant.register() was called");
+}
+
+}  // namespace testing
+}  // namespace flutter
+
+// NOLINTEND(clang-analyzer-core.StackAddressEscape)

engine/src/flutter/runtime/fixtures/dart_tool/flutter_build/dart_plugin_registrant.dart

@@ -0,0 +1,33 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+void passMessage(String message) native 'PassMessage';
+
+bool didCallRegistrantBeforeEntrypoint = false;
+
+// Test the Dart plugin registrant.
+@pragma('vm:entry-point')
+class _PluginRegistrant {
+
+  @pragma('vm:entry-point')
+  static void register() {
+    if (didCallRegistrantBeforeEntrypoint) {
+      throw '_registerPlugins is being called twice';
+    }
+    didCallRegistrantBeforeEntrypoint = true;
+  }
+
+}
+
+
+@pragma('vm:entry-point')
+void mainForPluginRegistrantTest() {
+  if (didCallRegistrantBeforeEntrypoint) {
+    passMessage('_PluginRegistrant.register() was called');
+  } else {
+    passMessage('_PluginRegistrant.register() was not called');
+  }
+}
+
+void main() {}