[windows] Allow engine flags via environment vars (#21161)

Replaces the (temporary) compile-time option to pass engine switches
with the ability to pass them temporarily at runtime via environment
variables.

This is enabled only for debug/profile to avoid potential issues with
tampering with released applicaitons, but if there is a need for that in
the future it could be added (potentially with a whitelist, as is
currently used for Dart VM flags).

Windows portion of:
https://github.com/flutter/flutter/issues/38569
https://github.com/flutter/flutter/issues/60393

ci/licenses_golden/licenses_flutter

@@ -1360,6 +1360,7 @@ FILE: ../../../flutter/shell/platform/windows/cursor_handler.cc
 FILE: ../../../flutter/shell/platform/windows/cursor_handler.h
 FILE: ../../../flutter/shell/platform/windows/flutter_project_bundle.cc
 FILE: ../../../flutter/shell/platform/windows/flutter_project_bundle.h
+FILE: ../../../flutter/shell/platform/windows/flutter_project_bundle_unittests.cc
 FILE: ../../../flutter/shell/platform/windows/flutter_windows.cc
 FILE: ../../../flutter/shell/platform/windows/flutter_windows_engine.cc
 FILE: ../../../flutter/shell/platform/windows/flutter_windows_engine.h

shell/platform/windows/BUILD.gn

@@ -121,6 +121,7 @@ executable("flutter_windows_unittests") {
   testonly = true
 
   sources = [
+    "flutter_project_bundle_unittests.cc",
     "string_conversion_unittests.cc",
     "system_utils_unittests.cc",
     "testing/mock_win32_window.cc",

shell/platform/windows/client_wrapper/dart_project_unittests.cc

@@ -26,12 +26,6 @@ class DartProjectTest : public ::testing::Test {
   std::wstring GetProjectAotLibraryPath(const DartProject& project) {
     return project.aot_library_path();
   }
-
-  // Wrapper for accessing private engine_switches.
-  std::vector<std::string> GetProjectEngineSwitches(
-      const DartProject& project) {
-    return project.engine_switches();
-  }
 };
 
 TEST_F(DartProjectTest, StandardProjectFormat) {
@@ -41,12 +35,4 @@ TEST_F(DartProjectTest, StandardProjectFormat) {
   EXPECT_EQ(GetProjectAotLibraryPath(project), L"test\\app.so");
 }
 
-TEST_F(DartProjectTest, Switches) {
-  DartProject project(L"test");
-  std::vector<std::string> switches = {"--foo", "--bar"};
-  project.SetEngineSwitches(switches);
-  EXPECT_EQ(GetProjectEngineSwitches(project).size(), 2);
-  EXPECT_EQ(GetProjectEngineSwitches(project)[0], "--foo");
-}
-
 }  // namespace flutter

shell/platform/windows/client_wrapper/flutter_engine.cc

@@ -16,15 +16,6 @@ FlutterEngine::FlutterEngine(const DartProject& project) {
   c_engine_properties.assets_path = project.assets_path().c_str();
   c_engine_properties.icu_data_path = project.icu_data_path().c_str();
   c_engine_properties.aot_library_path = project.aot_library_path().c_str();
-  std::vector<const char*> engine_switches;
-  std::transform(
-      project.engine_switches().begin(), project.engine_switches().end(),
-      std::back_inserter(engine_switches),
-      [](const std::string& arg) -> const char* { return arg.c_str(); });
-  if (engine_switches.size() > 0) {
-    c_engine_properties.switches = &engine_switches[0];
-    c_engine_properties.switches_count = engine_switches.size();
-  }
 
   engine_ = FlutterDesktopEngineCreate(c_engine_properties);
 

shell/platform/windows/client_wrapper/include/flutter/dart_project.h

@@ -29,17 +29,6 @@ class DartProject {
 
   ~DartProject() = default;
 
-  // Switches to pass to the Flutter engine. See
-  // https://github.com/flutter/engine/blob/master/shell/common/switches.h
-  // for details. Not all switches will apply to embedding mode. Switches have
-  // not stability guarantee, and are subject to change without notice.
-  //
-  // Note: This WILL BE REMOVED in the future. If you call this, please see
-  // https://github.com/flutter/flutter/issues/38569.
-  void SetEngineSwitches(const std::vector<std::string>& switches) {
-    engine_switches_ = switches;
-  }
-
  private:
   // Accessors for internals are private, so that they can be changed if more
   // flexible options for project structures are needed later without it
@@ -52,9 +41,6 @@ class DartProject {
   const std::wstring& assets_path() const { return assets_path_; }
   const std::wstring& icu_data_path() const { return icu_data_path_; }
   const std::wstring& aot_library_path() const { return aot_library_path_; }
-  const std::vector<std::string>& engine_switches() const {
-    return engine_switches_;
-  }
 
   // The path to the assets directory.
   std::wstring assets_path_;
@@ -63,8 +49,6 @@ class DartProject {
   // The path to the AOT library. This will always return a path, but non-AOT
   // builds will not be expected to actually have a library at that path.
   std::wstring aot_library_path_;
-  // Switches to pass to the engine.
-  std::vector<std::string> engine_switches_;
 };
 
 }  // namespace flutter

shell/platform/windows/flutter_project_bundle.cc

@@ -4,8 +4,10 @@
 
 #include "flutter/shell/platform/windows/flutter_project_bundle.h"
 
+#include <cstdlib>
 #include <filesystem>
 #include <iostream>
+#include <sstream>
 
 #include "flutter/shell/platform/common/cpp/path_utils.h"
 
@@ -38,11 +40,6 @@ FlutterProjectBundle::FlutterProjectBundle(
       }
     }
   }
-
-  if (properties.switches_count > 0) {
-    switches_.insert(switches_.end(), &properties.switches[0],
-                     &properties.switches[properties.switches_count]);
-  }
 }
 
 bool FlutterProjectBundle::HasValidPaths() {
@@ -78,4 +75,30 @@ UniqueAotDataPtr FlutterProjectBundle::LoadAotData() {
 
 FlutterProjectBundle::~FlutterProjectBundle() {}
 
+const std::vector<std::string> FlutterProjectBundle::GetSwitches() {
+  std::vector<std::string> switches;
+  // Read engine switches from the environment in debug/profile.
+#ifndef FLUTTER_RELEASE
+  const char* switch_count_key = "FLUTTER_ENGINE_SWITCHES";
+  const int kMaxSwitchCount = 50;
+  const char* switch_count_string = std::getenv(switch_count_key);
+  int switch_count = std::min(
+      kMaxSwitchCount, switch_count_string ? atoi(switch_count_string) : 0);
+  for (int i = 1; i <= switch_count; ++i) {
+    std::ostringstream switch_key;
+    switch_key << "FLUTTER_ENGINE_SWITCH_" << i;
+    const char* switch_value = std::getenv(switch_key.str().c_str());
+    if (switch_value) {
+      std::ostringstream switch_value_as_flag;
+      switch_value_as_flag << "--" << switch_value;
+      switches.push_back(switch_value_as_flag.str());
+    } else {
+      std::cerr << switch_count << " keys expected from " << switch_count_key
+                << ", but " << switch_key.str() << " is missing." << std::endl;
+    }
+  }
+#endif  // !FLUTTER_RELEASE
+  return switches;
+}
+
 }  // namespace flutter

shell/platform/windows/flutter_project_bundle.h

@@ -43,7 +43,7 @@ class FlutterProjectBundle {
   const std::filesystem::path& icu_path() { return icu_path_; }
 
   // Returns any switches that should be passed to the engine.
-  const std::vector<std::string>& switches() { return switches_; }
+  const std::vector<std::string> GetSwitches();
 
   // Attempts to load AOT data for this bundle. The returned data must be
   // retained until any engine instance it is passed to has been shut down.
@@ -54,7 +54,6 @@ class FlutterProjectBundle {
  private:
   std::filesystem::path assets_path_;
   std::filesystem::path icu_path_;
-  std::vector<std::string> switches_;
 
   // Path to the AOT library file, if any.
   std::filesystem::path aot_library_path_;

shell/platform/windows/flutter_project_bundle_unittests.cc

+// 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/shell/platform/windows/flutter_project_bundle.h"
+#include "gtest/gtest.h"
+
+namespace flutter {
+namespace testing {
+
+TEST(FlutterProjectBundle, BasicPropertiesAbsolutePaths) {
+  FlutterDesktopEngineProperties properties = {};
+  properties.assets_path = L"C:\\foo\\flutter_assets";
+  properties.icu_data_path = L"C:\\foo\\icudtl.dat";
+
+  FlutterProjectBundle project(properties);
+
+  EXPECT_TRUE(project.HasValidPaths());
+  EXPECT_EQ(project.assets_path().string(), "C:\\foo\\flutter_assets");
+  EXPECT_EQ(project.icu_path().string(), "C:\\foo\\icudtl.dat");
+}
+
+TEST(FlutterProjectBundle, BasicPropertiesRelativePaths) {
+  FlutterDesktopEngineProperties properties = {};
+  properties.assets_path = L"foo\\flutter_assets";
+  properties.icu_data_path = L"foo\\icudtl.dat";
+
+  FlutterProjectBundle project(properties);
+
+  EXPECT_TRUE(project.HasValidPaths());
+  EXPECT_TRUE(project.assets_path().is_absolute());
+  EXPECT_EQ(project.assets_path().filename().string(), "flutter_assets");
+  EXPECT_TRUE(project.icu_path().is_absolute());
+  EXPECT_EQ(project.icu_path().filename().string(), "icudtl.dat");
+}
+
+TEST(FlutterProjectBundle, SwitchesEmpty) {
+  FlutterDesktopEngineProperties properties = {};
+  properties.assets_path = L"foo\\flutter_assets";
+  properties.icu_data_path = L"foo\\icudtl.dat";
+
+  // Clear the main environment variable, since test order is not guaranteed.
+  _putenv_s("FLUTTER_ENGINE_SWITCHES", "");
+
+  FlutterProjectBundle project(properties);
+
+  EXPECT_EQ(project.GetSwitches().size(), 0);
+}
+
+#ifndef FLUTTER_RELEASE
+TEST(FlutterProjectBundle, Switches) {
+  FlutterDesktopEngineProperties properties = {};
+  properties.assets_path = L"foo\\flutter_assets";
+  properties.icu_data_path = L"foo\\icudtl.dat";
+
+  _putenv_s("FLUTTER_ENGINE_SWITCHES", "2");
+  _putenv_s("FLUTTER_ENGINE_SWITCH_1", "abc");
+  _putenv_s("FLUTTER_ENGINE_SWITCH_2", "foo=\"bar, baz\"");
+
+  FlutterProjectBundle project(properties);
+
+  std::vector<std::string> switches = project.GetSwitches();
+  EXPECT_EQ(switches.size(), 2);
+  EXPECT_EQ(switches[0], "--abc");
+  EXPECT_EQ(switches[1], "--foo=\"bar, baz\"");
+}
+
+TEST(FlutterProjectBundle, SwitchesExtraValues) {
+  FlutterDesktopEngineProperties properties = {};
+  properties.assets_path = L"foo\\flutter_assets";
+  properties.icu_data_path = L"foo\\icudtl.dat";
+
+  _putenv_s("FLUTTER_ENGINE_SWITCHES", "1");
+  _putenv_s("FLUTTER_ENGINE_SWITCH_1", "abc");
+  _putenv_s("FLUTTER_ENGINE_SWITCH_2", "foo=\"bar, baz\"");
+
+  FlutterProjectBundle project(properties);
+
+  std::vector<std::string> switches = project.GetSwitches();
+  EXPECT_EQ(switches.size(), 1);
+  EXPECT_EQ(switches[0], "--abc");
+}
+
+TEST(FlutterProjectBundle, SwitchesMissingValues) {
+  FlutterDesktopEngineProperties properties = {};
+  properties.assets_path = L"foo\\flutter_assets";
+  properties.icu_data_path = L"foo\\icudtl.dat";
+
+  _putenv_s("FLUTTER_ENGINE_SWITCHES", "4");
+  _putenv_s("FLUTTER_ENGINE_SWITCH_1", "abc");
+  _putenv_s("FLUTTER_ENGINE_SWITCH_2", "foo=\"bar, baz\"");
+  _putenv_s("FLUTTER_ENGINE_SWITCH_4", "oops");
+
+  FlutterProjectBundle project(properties);
+
+  std::vector<std::string> switches = project.GetSwitches();
+  EXPECT_EQ(switches.size(), 3);
+  EXPECT_EQ(switches[0], "--abc");
+  EXPECT_EQ(switches[1], "--foo=\"bar, baz\"");
+  // The missing switch should be skipped, leaving SWITCH_4 as the third
+  // switch in the array.
+  EXPECT_EQ(switches[2], "--oops");
+}
+#endif  // !FLUTTER_RELEASE
+
+}  // namespace testing
+}  // namespace flutter

shell/platform/windows/flutter_windows_engine.cc

@@ -137,10 +137,10 @@ bool FlutterWindowsEngine::RunWithEntrypoint(const char* entrypoint) {
   // FlutterProjectArgs is expecting a full argv, so when processing it for
   // flags the first item is treated as the executable and ignored. Add a dummy
   // value so that all provided arguments are used.
+  std::vector<std::string> switches = project_->GetSwitches();
   std::vector<const char*> argv = {"placeholder"};
   std::transform(
-      project_->switches().begin(), project_->switches().end(),
-      std::back_inserter(argv),
+      switches.begin(), switches.end(), std::back_inserter(argv),
       [](const std::string& arg) -> const char* { return arg.c_str(); });
 
   // Configure task runners.

shell/platform/windows/public/flutter_windows.h

@@ -46,15 +46,6 @@ typedef struct {
   // containing the executable. This can be nullptr for a non-AOT build, as
   // it will be ignored in that case.
   const wchar_t* aot_library_path;
-
-  // The switches to pass to the Flutter engine.
-  //
-  // See: https://github.com/flutter/engine/blob/master/shell/common/switches.h
-  // for details. Not all arguments will apply to desktop.
-  const char** switches;
-
-  // The number of elements in |switches|.
-  size_t switches_count;
 } FlutterDesktopEngineProperties;
 
 // ========== View Controller ==========