Avoid copying the contents of large platform message responses (flutter/engine#4947)

Assets are loaded via platform messages, and currently asset payloads are
being copied into Dart typed data buffers.  This change uses external
typed data objects that wrap the existing buffer if copying would be
expensive.

See https://github.com/flutter/flutter/issues/16291

engine/src/flutter/lib/ui/window/platform_message_response_dart.cc

@@ -7,12 +7,41 @@
 #include <utility>
 
 #include "flutter/common/threads.h"
+#include "flutter/lib/ui/window/window.h"
 #include "lib/fxl/functional/make_copyable.h"
 #include "lib/tonic/dart_state.h"
 #include "lib/tonic/logging/dart_invoke.h"
 
 namespace blink {
 
+namespace {
+
+// Avoid copying the contents of messages beyond a certain size.
+const int kMessageCopyThreshold = 1000;
+
+void MessageDataFinalizer(void* isolate_callback_data,
+                          Dart_WeakPersistentHandle handle,
+                          void* peer) {
+  std::vector<uint8_t>* data = reinterpret_cast<std::vector<uint8_t>*>(peer);
+  delete data;
+}
+
+Dart_Handle WrapByteData(std::vector<uint8_t> data) {
+  if (data.size() < kMessageCopyThreshold) {
+    return ToByteData(data);
+  } else {
+    std::vector<uint8_t>* heap_data = new std::vector<uint8_t>(std::move(data));
+    Dart_Handle data_handle = Dart_NewExternalTypedData(
+        Dart_TypedData_kByteData, heap_data->data(), heap_data->size());
+    DART_CHECK_VALID(data_handle);
+    Dart_NewWeakPersistentHandle(data_handle, heap_data, heap_data->size(),
+                                 MessageDataFinalizer);
+    return data_handle;
+  }
+}
+
+}  // anonymous namespace
+
 PlatformMessageResponseDart::PlatformMessageResponseDart(
     tonic::DartPersistentValue callback)
     : callback_(std::move(callback)) {}
@@ -38,19 +67,7 @@ void PlatformMessageResponseDart::Complete(std::vector<uint8_t> data) {
           return;
         tonic::DartState::Scope scope(dart_state);
 
-        Dart_Handle byte_buffer =
-            Dart_NewTypedData(Dart_TypedData_kByteData, data.size());
-        DART_CHECK_VALID(byte_buffer);
-
-        void* buffer;
-        intptr_t length;
-        Dart_TypedData_Type type;
-        DART_CHECK_VALID(
-            Dart_TypedDataAcquireData(byte_buffer, &type, &buffer, &length));
-        FXL_CHECK(type == Dart_TypedData_kByteData);
-        FXL_CHECK(static_cast<size_t>(length) == data.size());
-        memcpy(buffer, data.data(), length);
-        Dart_TypedDataReleaseData(byte_buffer);
+        Dart_Handle byte_buffer = WrapByteData(std::move(data));
         tonic::DartInvoke(callback.Release(), {byte_buffer});
       }));
 }

engine/src/flutter/lib/ui/window/window.cc

@@ -22,23 +22,6 @@ using tonic::ToDart;
 namespace blink {
 namespace {
 
-Dart_Handle ToByteData(const std::vector<uint8_t>& buffer) {
-  Dart_Handle data_handle =
-      Dart_NewTypedData(Dart_TypedData_kByteData, buffer.size());
-  if (Dart_IsError(data_handle))
-    return data_handle;
-
-  Dart_TypedData_Type type;
-  void* data = nullptr;
-  intptr_t num_bytes = 0;
-  FXL_CHECK(!Dart_IsError(
-      Dart_TypedDataAcquireData(data_handle, &type, &data, &num_bytes)));
-
-  memcpy(data, buffer.data(), num_bytes);
-  Dart_TypedDataReleaseData(data_handle);
-  return data_handle;
-}
-
 void DefaultRouteName(Dart_NativeArguments args) {
   std::string routeName =
       UIDartState::Current()->window()->client()->DefaultRouteName();
@@ -119,6 +102,23 @@ void _RespondToPlatformMessage(Dart_NativeArguments args) {
 
 }  // namespace
 
+Dart_Handle ToByteData(const std::vector<uint8_t>& buffer) {
+  Dart_Handle data_handle =
+      Dart_NewTypedData(Dart_TypedData_kByteData, buffer.size());
+  if (Dart_IsError(data_handle))
+    return data_handle;
+
+  Dart_TypedData_Type type;
+  void* data = nullptr;
+  intptr_t num_bytes = 0;
+  FXL_CHECK(!Dart_IsError(
+      Dart_TypedDataAcquireData(data_handle, &type, &data, &num_bytes)));
+
+  memcpy(data, buffer.data(), num_bytes);
+  Dart_TypedDataReleaseData(data_handle);
+  return data_handle;
+}
+
 WindowClient::~WindowClient() {}
 
 Window::Window(WindowClient* client) : client_(client) {}

engine/src/flutter/lib/ui/window/window.h

@@ -21,6 +21,8 @@ class DartLibraryNatives;
 namespace blink {
 class Scene;
 
+Dart_Handle ToByteData(const std::vector<uint8_t>& buffer);
+
 class WindowClient {
  public:
   virtual std::string DefaultRouteName() = 0;