Migrate string encoding conversions to FML (#31334)

We've implemented UTF-8/UTF-16 string encoding conversions in multiple
places, from FML to //flutter/shell/platform/common, to the individual
embedders. This migrates these conversions to FML and adds tests.

Windows APIs use wchar_t-based strings and as a result, we continue to
keep Windows-specific functions in fml/platform/win/wstring_conversion.h.

We break out string_conversions into its own source set since FML brings
with it some Dart dependencies (e.g. dart_timestamp_provider.cc) that
are unused by some targets such as uwptool.exe in the Windows UWP
embedding.

Issue: flutter/flutter#98061

Author: cbracken

ci/licenses_golden/licenses_flutter

@@ -285,7 +285,9 @@ FILE: ../../../flutter/fml/platform/win/message_loop_win.h
 FILE: ../../../flutter/fml/platform/win/native_library_win.cc
 FILE: ../../../flutter/fml/platform/win/paths_win.cc
 FILE: ../../../flutter/fml/platform/win/posix_wrappers_win.cc
+FILE: ../../../flutter/fml/platform/win/wstring_conversion.cc
 FILE: ../../../flutter/fml/platform/win/wstring_conversion.h
+FILE: ../../../flutter/fml/platform/win/wstring_conversion_unittests.cc
 FILE: ../../../flutter/fml/posix_wrappers.h
 FILE: ../../../flutter/fml/raster_thread_merger.cc
 FILE: ../../../flutter/fml/raster_thread_merger.h
@@ -294,6 +296,9 @@ FILE: ../../../flutter/fml/shared_thread_merger.cc
 FILE: ../../../flutter/fml/shared_thread_merger.h
 FILE: ../../../flutter/fml/size.h
 FILE: ../../../flutter/fml/status.h
+FILE: ../../../flutter/fml/string_conversion.cc
+FILE: ../../../flutter/fml/string_conversion.h
+FILE: ../../../flutter/fml/string_conversion_unittests.cc
 FILE: ../../../flutter/fml/synchronization/atomic_object.h
 FILE: ../../../flutter/fml/synchronization/count_down_latch.cc
 FILE: ../../../flutter/fml/synchronization/count_down_latch.h
@@ -2049,9 +2054,6 @@ FILE: ../../../flutter/shell/platform/windows/public/flutter_windows.h
 FILE: ../../../flutter/shell/platform/windows/sequential_id_generator.cc
 FILE: ../../../flutter/shell/platform/windows/sequential_id_generator.h
 FILE: ../../../flutter/shell/platform/windows/sequential_id_generator_unittests.cc
-FILE: ../../../flutter/shell/platform/windows/string_conversion.cc
-FILE: ../../../flutter/shell/platform/windows/string_conversion.h
-FILE: ../../../flutter/shell/platform/windows/string_conversion_unittests.cc
 FILE: ../../../flutter/shell/platform/windows/system_utils.h
 FILE: ../../../flutter/shell/platform/windows/system_utils_unittests.cc
 FILE: ../../../flutter/shell/platform/windows/system_utils_win32.cc

fml/BUILD.gn

@@ -104,7 +104,11 @@ source_set("fml") {
     sources += [ "backtrace_stub.cc" ]
   }
 
-  public_deps = [ ":command_line" ]
+  public_deps = [
+    ":build_config",
+    ":command_line",
+    ":string_conversion",
+  ]
 
   deps = [
     "//third_party/abseil-cpp/absl/debugging:symbolize",
@@ -217,11 +221,7 @@ source_set("fml") {
       "platform/win/native_library_win.cc",
       "platform/win/paths_win.cc",
       "platform/win/posix_wrappers_win.cc",
-      "platform/win/wstring_conversion.h",
     ]
-
-    # For wstring_conversion. See issue #50053.
-    defines = [ "_SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING" ]
   } else {
     sources += [
       "platform/posix/file_posix.cc",
@@ -233,6 +233,15 @@ source_set("fml") {
   }
 }
 
+source_set("build_config") {
+  sources = [ "build_config.h" ]
+
+  public_configs = [
+    "//flutter:config",
+    "//flutter/common:flutter_config",
+  ]
+}
+
 source_set("command_line") {
   sources = [
     "command_line.cc",
@@ -245,6 +254,30 @@ source_set("command_line") {
   ]
 }
 
+source_set("string_conversion") {
+  sources = [
+    "string_conversion.cc",
+    "string_conversion.h",
+  ]
+
+  if (is_win) {
+    sources += [
+      "platform/win/wstring_conversion.cc",
+      "platform/win/wstring_conversion.h",
+    ]
+
+    # TODO(cbracken): https://github.com/flutter/flutter/issues/50053
+    defines = [ "_SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING" ]
+  }
+
+  deps = [ ":build_config" ]
+
+  public_configs = [
+    "//flutter:config",
+    "//flutter/common:flutter_config",
+  ]
+}
+
 if (enable_unittests) {
   test_fixtures("fml_fixtures") {
     fixtures = []
@@ -286,6 +319,7 @@ if (enable_unittests) {
       "message_loop_unittests.cc",
       "paths_unittests.cc",
       "raster_thread_merger_unittests.cc",
+      "string_conversion_unittests.cc",
       "synchronization/count_down_latch_unittests.cc",
       "synchronization/semaphore_unittest.cc",
       "synchronization/sync_switch_unittest.cc",
@@ -307,6 +341,10 @@ if (enable_unittests) {
       ]
     }
 
+    if (is_win) {
+      sources += [ "platform/win/wstring_conversion_unittests.cc" ]
+    }
+
     deps = [
       ":fml_fixtures",
       "//flutter/fml",

fml/platform/win/errors_win.cc

@@ -40,7 +40,7 @@ std::string GetLastErrorMessage() {
   std::wstringstream stream;
   stream << message << " (" << last_error << ").";
 
-  return WideStringToString(stream.str());
+  return WideStringToUtf8(stream.str());
 }
 
 }  // namespace fml

fml/platform/win/file_win.cc

@@ -45,7 +45,7 @@ static std::string GetFullHandlePath(const fml::UniqueFD& handle) {
     // was cached
     if (result && memcmp(found.value().id.Identifier, info.FileId.Identifier,
                          sizeof(FILE_ID_INFO))) {
-      return WideStringToString(found.value().filename);
+      return WideStringToUtf8(found.value().filename);
     } else {
       fml::internal::os_win::UniqueFDTraits::RemoveCacheEntry(handle.get());
     }
@@ -59,7 +59,7 @@ static std::string GetFullHandlePath(const fml::UniqueFD& handle) {
   if (buffer_size == 0) {
     return {};
   }
-  return WideStringToString({buffer, buffer_size});
+  return WideStringToUtf8({buffer, buffer_size});
 #endif
 }
 
@@ -106,7 +106,7 @@ static DWORD GetShareFlags(FilePermission permission) {
 }
 
 static DWORD GetFileAttributesForUtf8Path(const char* absolute_path) {
-  return ::GetFileAttributes(StringToWideString(absolute_path).c_str());
+  return ::GetFileAttributes(Utf8ToWideString(absolute_path).c_str());
 }
 
 static DWORD GetFileAttributesForUtf8Path(const fml::UniqueFD& base_directory,
@@ -147,15 +147,15 @@ std::string CreateTemporaryDirectory() {
   stream << temp_dir_container << "\\" << uuid_str;
   auto temp_dir = stream.str();
 
-  auto dir_fd = OpenDirectory(WideStringToString(temp_dir).c_str(), true,
+  auto dir_fd = OpenDirectory(WideStringToUtf8(temp_dir).c_str(), true,
                               FilePermission::kReadWrite);
   if (!dir_fd.is_valid()) {
     FML_DLOG(ERROR) << "Could not get temporary directory file descriptor. "
                     << GetLastErrorMessage();
     return {};
   }
 
-  return WideStringToString(std::move(temp_dir));
+  return WideStringToUtf8(std::move(temp_dir));
 }
 
 fml::UniqueFD OpenFile(const fml::UniqueFD& base_directory,
@@ -173,7 +173,7 @@ fml::UniqueFD OpenFile(const char* path,
     return {};
   }
 
-  auto file_name = StringToWideString({path});
+  auto file_name = Utf8ToWideString({path});
 
   if (file_name.size() == 0) {
     return {};
@@ -216,7 +216,7 @@ fml::UniqueFD OpenDirectory(const char* path,
     return {};
   }
 
-  auto file_name = StringToWideString({path});
+  auto file_name = Utf8ToWideString({path});
 
   if (file_name.size() == 0) {
     return {};
@@ -314,7 +314,7 @@ bool IsFile(const std::string& path) {
 }
 
 bool UnlinkDirectory(const char* path) {
-  if (!::RemoveDirectory(StringToWideString(path).c_str())) {
+  if (!::RemoveDirectory(Utf8ToWideString(path).c_str())) {
     FML_DLOG(ERROR) << "Could not remove directory: '" << path << "'. "
                     << GetLastErrorMessage();
     return false;
@@ -324,7 +324,7 @@ bool UnlinkDirectory(const char* path) {
 
 bool UnlinkDirectory(const fml::UniqueFD& base_directory, const char* path) {
   if (!::RemoveDirectory(
-          StringToWideString(GetAbsolutePath(base_directory, path)).c_str())) {
+          Utf8ToWideString(GetAbsolutePath(base_directory, path)).c_str())) {
     FML_DLOG(ERROR) << "Could not remove directory: '" << path << "'. "
                     << GetLastErrorMessage();
     return false;
@@ -333,7 +333,7 @@ bool UnlinkDirectory(const fml::UniqueFD& base_directory, const char* path) {
 }
 
 bool UnlinkFile(const char* path) {
-  if (!::DeleteFile(StringToWideString(path).c_str())) {
+  if (!::DeleteFile(Utf8ToWideString(path).c_str())) {
     FML_DLOG(ERROR) << "Could not remove file: '" << path << "'. "
                     << GetLastErrorMessage();
     return false;
@@ -343,7 +343,7 @@ bool UnlinkFile(const char* path) {
 
 bool UnlinkFile(const fml::UniqueFD& base_directory, const char* path) {
   if (!::DeleteFile(
-          StringToWideString(GetAbsolutePath(base_directory, path)).c_str())) {
+          Utf8ToWideString(GetAbsolutePath(base_directory, path)).c_str())) {
     FML_DLOG(ERROR) << "Could not remove file: '" << path << "'. "
                     << GetLastErrorMessage();
     return false;
@@ -436,8 +436,8 @@ bool WriteAtomically(const fml::UniqueFD& base_directory,
 
   temp_file.reset();
 
-  if (!::MoveFile(StringToWideString(temp_file_path).c_str(),
-                  StringToWideString(file_path).c_str())) {
+  if (!::MoveFile(Utf8ToWideString(temp_file_path).c_str(),
+                  Utf8ToWideString(file_path).c_str())) {
     FML_DLOG(ERROR)
         << "Could not replace temp file at correct path. File path: "
         << file_path << ". Temp file path: " << temp_file_path << " "
@@ -451,8 +451,8 @@ bool WriteAtomically(const fml::UniqueFD& base_directory,
 bool VisitFiles(const fml::UniqueFD& directory, const FileVisitor& visitor) {
   std::string search_pattern = GetFullHandlePath(directory) + "\\*";
   WIN32_FIND_DATA find_file_data;
-  HANDLE find_handle = ::FindFirstFile(
-      StringToWideString(search_pattern).c_str(), &find_file_data);
+  HANDLE find_handle = ::FindFirstFile(Utf8ToWideString(search_pattern).c_str(),
+                                       &find_file_data);
 
   if (find_handle == INVALID_HANDLE_VALUE) {
     FML_DLOG(ERROR) << "Can't open the directory. Error: "
@@ -461,7 +461,7 @@ bool VisitFiles(const fml::UniqueFD& directory, const FileVisitor& visitor) {
   }
 
   do {
-    std::string filename = WideStringToString(find_file_data.cFileName);
+    std::string filename = WideStringToUtf8(find_file_data.cFileName);
     if (filename != "." && filename != "..") {
       if (!visitor(directory, filename)) {
         ::FindClose(find_handle);

fml/platform/win/native_library_win.cc

@@ -16,7 +16,7 @@ NativeLibrary::NativeLibrary(const char* path)
     return;
   }
 
-  handle_ = ::LoadLibrary(StringToWideString(path).c_str());
+  handle_ = ::LoadLibrary(Utf8ToWideString(path).c_str());
 }
 
 NativeLibrary::NativeLibrary(Handle handle, bool close_handle)

fml/platform/win/wstring_conversion.cc

@@ -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.
+
+#include "flutter/fml/platform/win/wstring_conversion.h"
+
+#include <codecvt>
+#include <locale>
+#include <string>
+
+namespace fml {
+
+using WideStringConverter =
+    std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>, wchar_t>;
+
+std::string WideStringToUtf8(const std::wstring_view str) {
+  WideStringConverter converter;
+  return converter.to_bytes(str.data());
+}
+
+std::wstring Utf8ToWideString(const std::string_view str) {
+  WideStringConverter converter;
+  return converter.from_bytes(str.data());
+}
+
+}  // namespace fml

fml/platform/win/wstring_conversion.h

@@ -5,24 +5,16 @@
 #ifndef FLUTTER_FML_PLATFORM_WIN_WSTRING_CONVERSION_H_
 #define FLUTTER_FML_PLATFORM_WIN_WSTRING_CONVERSION_H_
 
-#include <codecvt>
-#include <locale>
 #include <string>
 
 namespace fml {
 
-using WideStringConvertor =
-    std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>, wchar_t>;
+// Returns a UTF-8 encoded equivalent of a UTF-16 encoded input wide string.
+std::string WideStringToUtf8(const std::wstring_view str);
 
-inline std::wstring StringToWideString(const std::string& str) {
-  WideStringConvertor converter;
-  return converter.from_bytes(str);
-}
-
-inline std::string WideStringToString(const std::wstring& wstr) {
-  WideStringConvertor converter;
-  return converter.to_bytes(wstr);
-}
+// Returns a UTF-16 encoded wide string equivalent of a UTF-8 encoded input
+// string.
+std::wstring Utf8ToWideString(const std::string_view str);
 
 }  // namespace fml
 

fml/platform/win/wstring_conversion_unittests.cc

@@ -0,0 +1,37 @@
+// 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/fml/platform/win/wstring_conversion.h"
+
+#include "gtest/gtest.h"
+
+namespace fml {
+namespace testing {
+
+TEST(StringConversion, Utf16ToWideStringEmpty) {
+  EXPECT_EQ(Utf8ToWideString(""), L"");
+}
+
+TEST(StringConversion, Utf8ToWideStringAscii) {
+  EXPECT_EQ(Utf8ToWideString("abc123"), L"abc123");
+}
+
+TEST(StringConversion, Utf8ToWideStringUnicode) {
+  EXPECT_EQ(Utf8ToWideString("\xe2\x98\x83"), L"\x2603");
+}
+
+TEST(StringConversion, WideStringToUtf8Empty) {
+  EXPECT_EQ(WideStringToUtf8(L""), "");
+}
+
+TEST(StringConversion, WideStringToUtf8Ascii) {
+  EXPECT_EQ(WideStringToUtf8(L"abc123"), "abc123");
+}
+
+TEST(StringConversion, WideStringToUtf8Unicode) {
+  EXPECT_EQ(WideStringToUtf8(L"\x2603"), "\xe2\x98\x83");
+}
+
+}  // namespace testing
+}  // namespace fml

fml/string_conversion.cc

@@ -0,0 +1,35 @@
+// 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/fml/string_conversion.h"
+
+#include <codecvt>
+#include <locale>
+#include <string>
+
+#include "flutter/fml/build_config.h"
+
+#if defined(FML_OS_WIN)
+// TODO(naifu): https://github.com/flutter/flutter/issues/98074
+// Eliminate this workaround for a link error on Windows when the underlying
+// bug is fixed.
+std::locale::id std::codecvt<char16_t, char, _Mbstatet>::id;
+#endif  // defined(FML_OS_WIN)
+
+namespace fml {
+
+using Utf16StringConverter =
+    std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t>;
+
+std::string Utf16ToUtf8(const std::u16string_view string) {
+  Utf16StringConverter converter;
+  return converter.to_bytes(string.data());
+}
+
+std::u16string Utf8ToUtf16(const std::string_view string) {
+  Utf16StringConverter converter;
+  return converter.from_bytes(string.data());
+}
+
+}  // namespace fml