[linux] Allow engine flags via environment vars (flutter#21497)

Adds the ability to pass engine switches temporarily at runtime via
environment variables. Uses the same approach recently added to
Windows and macOS.

This is enabled only for debug/profile to avoid potential issues with
tampering with released applications, 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).

Fixes flutter/flutter#60393

Author: stuartmorgan-g

ci/licenses_golden/licenses_flutter

@@ -1279,6 +1279,7 @@ FILE: ../../../flutter/shell/platform/linux/fl_binary_messenger.cc
 FILE: ../../../flutter/shell/platform/linux/fl_binary_messenger_private.h
 FILE: ../../../flutter/shell/platform/linux/fl_binary_messenger_test.cc
 FILE: ../../../flutter/shell/platform/linux/fl_dart_project.cc
+FILE: ../../../flutter/shell/platform/linux/fl_dart_project_private.h
 FILE: ../../../flutter/shell/platform/linux/fl_dart_project_test.cc
 FILE: ../../../flutter/shell/platform/linux/fl_engine.cc
 FILE: ../../../flutter/shell/platform/linux/fl_engine_private.h

shell/platform/linux/BUILD.gn

@@ -69,7 +69,16 @@ config("relative_flutter_linux_headers") {
 }
 
 source_set("flutter_linux_sources") {
-  public = _public_headers
+  public = _public_headers + [
+             "fl_binary_messenger_private.h",
+             "fl_dart_project_private.h",
+             "fl_engine_private.h",
+             "fl_method_call_private.h",
+             "fl_method_channel_private.h",
+             "fl_method_codec_private.h",
+             "fl_plugin_registrar_private.h",
+             "fl_standard_message_codec_private.h",
+           ]
 
   configs += [ "//flutter/shell/platform/linux/config:gtk" ]
 
@@ -109,6 +118,7 @@ source_set("flutter_linux_sources") {
 
   deps = [
     "//flutter/shell/platform/common/cpp:common_cpp_input",
+    "//flutter/shell/platform/common/cpp:common_cpp_switches",
     "//flutter/shell/platform/embedder:embedder_headers",
     "//third_party/rapidjson",
   ]

shell/platform/linux/fl_dart_project.cc

@@ -6,6 +6,12 @@
 
 #include <gmodule.h>
 
+#include <string>
+#include <vector>
+
+#include "flutter/shell/platform/common/cpp/engine_switches.h"
+#include "flutter/shell/platform/linux/fl_dart_project_private.h"
+
 struct _FlDartProject {
   GObject parent_instance;
 
@@ -91,3 +97,15 @@ G_MODULE_EXPORT const gchar* fl_dart_project_get_icu_data_path(
   g_return_val_if_fail(FL_IS_DART_PROJECT(self), nullptr);
   return self->icu_data_path;
 }
+
+GPtrArray* fl_dart_project_get_switches(FlDartProject* self) {
+  GPtrArray* switches = g_ptr_array_new_with_free_func(g_free);
+  std::vector<std::string> env_switches = flutter::GetSwitchesFromEnvironment();
+  for (const auto& env_switch : env_switches) {
+    g_ptr_array_add(switches, g_strdup(env_switch.c_str()));
+  }
+  if (self->enable_mirrors) {
+    g_ptr_array_add(switches, g_strdup("--dart-flags=--enable_mirrors=true"));
+  }
+  return switches;
+}

shell/platform/linux/fl_dart_project_private.h

@@ -0,0 +1,26 @@
+// 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.
+
+#ifndef FLUTTER_SHELL_PLATFORM_LINUX_FL_DART_PROJECT_PRIVATE_H_
+#define FLUTTER_SHELL_PLATFORM_LINUX_FL_DART_PROJECT_PRIVATE_H_
+
+#include <glib-object.h>
+
+#include "flutter/shell/platform/linux/public/flutter_linux/fl_dart_project.h"
+
+G_BEGIN_DECLS
+
+/**
+ * fl_dart_project_get_switches:
+ * @project: an #FlDartProject.
+ *
+ * Determines the engine switches that should be passed to the Flutter engine.
+ *
+ * Returns: an array of switches to pass to the Flutter engine.
+ */
+GPtrArray* fl_dart_project_get_switches(FlDartProject* project);
+
+G_END_DECLS
+
+#endif  // FLUTTER_SHELL_PLATFORM_LINUX_FL_DART_PROJECT_PRIVATE_H_

shell/platform/linux/fl_dart_project_test.cc

@@ -3,10 +3,14 @@
 // found in the LICENSE file.
 
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_dart_project.h"
-#include "gtest/gtest.h"
 
 #include <gmodule.h>
 
+#include <cstdlib>
+
+#include "flutter/shell/platform/linux/fl_dart_project_private.h"
+#include "gtest/gtest.h"
+
 TEST(FlDartProjectTest, GetPaths) {
   g_autoptr(FlDartProject) project = fl_dart_project_new();
   g_autofree gchar* exe_path = g_file_read_link("/proc/self/exe", nullptr);
@@ -33,3 +37,31 @@ TEST(FlDartProjectTest, EnableMirrors) {
   EXPECT_TRUE(fl_dart_project_get_enable_mirrors(project));
   G_GNUC_END_IGNORE_DEPRECATIONS
 }
+
+TEST(FlDartProjectTest, SwitchesEmpty) {
+  g_autoptr(FlDartProject) project = fl_dart_project_new();
+
+  // Clear the main environment variable, since test order is not guaranteed.
+  unsetenv("FLUTTER_ENGINE_SWITCHES");
+
+  g_autoptr(GPtrArray) switches = fl_dart_project_get_switches(project);
+
+  EXPECT_EQ(switches->len, 0U);
+}
+
+#ifndef FLUTTER_RELEASE
+TEST(FlDartProjectTest, Switches) {
+  g_autoptr(FlDartProject) project = fl_dart_project_new();
+
+  setenv("FLUTTER_ENGINE_SWITCHES", "2", 1);
+  setenv("FLUTTER_ENGINE_SWITCH_1", "abc", 1);
+  setenv("FLUTTER_ENGINE_SWITCH_2", "foo=\"bar, baz\"", 1);
+
+  g_autoptr(GPtrArray) switches = fl_dart_project_get_switches(project);
+  EXPECT_EQ(switches->len, 2U);
+  EXPECT_STREQ(static_cast<const char*>(g_ptr_array_index(switches, 0)),
+               "--abc");
+  EXPECT_STREQ(static_cast<const char*>(g_ptr_array_index(switches, 1)),
+               "--foo=\"bar, baz\"");
+}
+#endif  // !FLUTTER_RELEASE

shell/platform/linux/fl_engine.cc

@@ -3,17 +3,19 @@
 // found in the LICENSE file.
 
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_engine.h"
-#include "flutter/shell/platform/linux/fl_engine_private.h"
+
+#include <gmodule.h>
+
+#include <cstring>
 
 #include "flutter/shell/platform/linux/fl_binary_messenger_private.h"
+#include "flutter/shell/platform/linux/fl_dart_project_private.h"
+#include "flutter/shell/platform/linux/fl_engine_private.h"
 #include "flutter/shell/platform/linux/fl_plugin_registrar_private.h"
 #include "flutter/shell/platform/linux/fl_renderer.h"
 #include "flutter/shell/platform/linux/fl_renderer_headless.h"
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_plugin_registry.h"
 
-#include <gmodule.h>
-#include <cstring>
-
 static constexpr int kMicrosecondsPerNanosecond = 1000;
 
 // Unique number associated with platform tasks.
@@ -373,15 +375,11 @@ gboolean fl_engine_start(FlEngine* self, GError** error) {
   custom_task_runners.platform_task_runner = &platform_task_runner;
 
   g_autoptr(GPtrArray) command_line_args =
-      g_ptr_array_new_with_free_func(g_free);
-  g_ptr_array_add(command_line_args, g_strdup("flutter"));
-  G_GNUC_BEGIN_IGNORE_DEPRECATIONS
-  gboolean enable_mirrors = fl_dart_project_get_enable_mirrors(self->project);
-  G_GNUC_END_IGNORE_DEPRECATIONS
-  if (enable_mirrors) {
-    g_ptr_array_add(command_line_args,
-                    g_strdup("--dart-flags=--enable_mirrors=true"));
-  }
+      fl_dart_project_get_switches(self->project);
+  // FlutterProjectArgs expects 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 switches are used.
+  g_ptr_array_insert(command_line_args, 0, g_strdup("flutter"));
 
   FlutterProjectArgs args = {};
   args.struct_size = sizeof(FlutterProjectArgs);