Allow embedders to control Dart VM lifecycle on engine shutdown. (#10652)

This exposes the `Settings::leak_vm` flag to custom embedders. All embedder
unit-tests now shut down the VM on the shutdown of the last engine in the
process. The mechanics of VM shutdown are already tested in the Shell unit-tests
harness in the DartLifecycleUnittests set of of assertions. This just exposes
that functionality to custom embedders. Since it is part of the public stable
API, I also switched the name of the field to be something less snarky than the
field in private shell settings.

shell/platform/embedder/embedder.cc

@@ -375,6 +375,7 @@ FlutterEngineResult FlutterEngineRun(size_t version,
 
   settings.icu_data_path = icu_data_path;
   settings.assets_path = args->assets_path;
+  settings.leak_vm = !SAFE_ACCESS(args, shutdown_dart_vm_when_done, false);
 
   if (!flutter::DartVM::IsRunningPrecompiledCode()) {
     // Verify the assets path contains Dart 2 kernel assets.

shell/platform/embedder/embedder.h

@@ -695,6 +695,25 @@ typedef struct {
   // optional argument allows for the specification of task runner interfaces to
   // event loops managed by the embedder on threads it creates.
   const FlutterCustomTaskRunners* custom_task_runners;
+
+  // All `FlutterEngine` instances in the process share the same Dart VM. When
+  // the first engine is launched, it starts the Dart VM as well. It used to be
+  // the case that it was not possible to shutdown the Dart VM cleanly and start
+  // it back up in the process in a safe manner. This issue has since been
+  // patched. Unfortunately, applications already began to make use of the fact
+  // that shutting down the Flutter engine instance left a running VM in the
+  // process. Since a Flutter engine could be launched on any thread,
+  // applications would "warm up" the VM on another thread by launching
+  // an engine with no isolates and then shutting it down immediately. The main
+  // Flutter application could then be started on the main thread without having
+  // to incur the Dart VM startup costs at that time. With the new behavior,
+  // this "optimization" immediately becomes massive performance pessimization
+  // as the VM would be started up in the "warm up" phase, shut down there and
+  // then started again on the main thread. Changing this behavior was deemed to
+  // be an unacceptable breaking change. Embedders that wish to shutdown the
+  // Dart VM when the last engine is terminated in the process should opt into
+  // this behavior by setting this flag to true.
+  bool shutdown_dart_vm_when_done;
 } FlutterProjectArgs;
 
 FLUTTER_EXPORT

shell/platform/embedder/tests/embedder_config_builder.cc

@@ -12,6 +12,7 @@ EmbedderConfigBuilder::EmbedderConfigBuilder(
     InitializationPreference preference)
     : context_(context) {
   project_args_.struct_size = sizeof(project_args_);
+  project_args_.shutdown_dart_vm_when_done = true;
   project_args_.platform_message_callback =
       [](const FlutterPlatformMessage* message, void* context) {
         reinterpret_cast<EmbedderContext*>(context)->PlatformMessageCallback(

shell/platform/embedder/tests/embedder_unittests.cc

@@ -13,6 +13,7 @@
 #include "flutter/fml/message_loop.h"
 #include "flutter/fml/synchronization/waitable_event.h"
 #include "flutter/fml/thread.h"
+#include "flutter/runtime/dart_vm.h"
 #include "flutter/shell/platform/embedder/tests/embedder_config_builder.h"
 #include "flutter/shell/platform/embedder/tests/embedder_test.h"
 #include "flutter/testing/testing.h"
@@ -465,5 +466,26 @@ TEST_F(EmbedderTest, InvalidPlatformMessages) {
   ASSERT_EQ(result, kInvalidArguments);
 }
 
+//------------------------------------------------------------------------------
+/// Asserts behavior of FlutterProjectArgs::shutdown_dart_vm_when_done (which is
+/// set to true by default in these unit-tests).
+///
+TEST_F(EmbedderTest, VMShutsDownWhenNoEnginesInProcess) {
+  auto& context = GetEmbedderContext();
+  EmbedderConfigBuilder builder(context);
+
+  const auto launch_count = DartVM::GetVMLaunchCount();
+
+  {
+    auto engine = builder.LaunchEngine();
+    ASSERT_EQ(launch_count + 1u, DartVM::GetVMLaunchCount());
+  }
+
+  {
+    auto engine = builder.LaunchEngine();
+    ASSERT_EQ(launch_count + 2u, DartVM::GetVMLaunchCount());
+  }
+}
+
 }  // namespace testing
 }  // namespace flutter