Fix Windows file checks of unicode paths

FileExists is currently implemeented using stat on windows, which
interprets the path in an environment-determined way rather than as
unicode. This replaces that implementation with wide-string-based
Win32 calls that are guaranteed to use unicode.

Fixes #49372

Author: stuartmorgan-g

fml/file_unittest.cc

@@ -10,6 +10,7 @@
 #include "flutter/fml/build_config.h"
 #include "flutter/fml/file.h"
 #include "flutter/fml/mapping.h"
+#include "flutter/fml/paths.h"
 #include "flutter/fml/unique_fd.h"
 
 static bool WriteStringToFile(const fml::UniqueFD& fd,
@@ -267,3 +268,31 @@ TEST(FileTest, EmptyMappingTest) {
 
   ASSERT_TRUE(fml::UnlinkFile(dir.fd(), "my_contents"));
 }
+
+TEST(FileTest, FileTestsWork) {
+  fml::ScopedTemporaryDirectory dir;
+  ASSERT_TRUE(dir.fd().is_valid());
+  const char* filename = "some.txt";
+  auto fd =
+      fml::OpenFile(dir.fd(), filename, true, fml::FilePermission::kWrite);
+  ASSERT_TRUE(fd.is_valid());
+  fd.reset();
+  ASSERT_TRUE(fml::FileExists(dir.fd(), filename));
+  ASSERT_TRUE(
+      fml::IsFile(fml::paths::JoinPaths({dir.path(), filename}).c_str()));
+  ASSERT_TRUE(fml::UnlinkFile(dir.fd(), filename));
+}
+
+TEST(FileTest, FileTestsSupportsUnicode) {
+  fml::ScopedTemporaryDirectory dir;
+  ASSERT_TRUE(dir.fd().is_valid());
+  const char* filename = u8"äëïöüテスト☃";
+  auto fd =
+      fml::OpenFile(dir.fd(), filename, true, fml::FilePermission::kWrite);
+  ASSERT_TRUE(fd.is_valid());
+  fd.reset();
+  ASSERT_TRUE(fml::FileExists(dir.fd(), filename));
+  ASSERT_TRUE(
+      fml::IsFile(fml::paths::JoinPaths({dir.path(), filename}).c_str()));
+  ASSERT_TRUE(fml::UnlinkFile(dir.fd(), filename));
+}

fml/platform/win/file_win.cc

@@ -8,7 +8,6 @@
 #include <Shlwapi.h>
 #include <fcntl.h>
 #include <limits.h>
-#include <sys/stat.h>
 
 #include <algorithm>
 #include <sstream>
@@ -18,10 +17,6 @@
 #include "flutter/fml/platform/win/errors_win.h"
 #include "flutter/fml/platform/win/wstring_conversion.h"
 
-#if defined(OS_WIN)
-#define S_ISREG(m) (((m)&S_IFMT) == S_IFREG)
-#endif
-
 namespace fml {
 
 static std::string GetFullHandlePath(const fml::UniqueFD& handle) {
@@ -80,6 +75,16 @@ static DWORD GetShareFlags(FilePermission permission) {
   return FILE_SHARE_READ;
 }
 
+static DWORD GetFileAttributesForUtf8Path(const char* absolute_path) {
+  return ::GetFileAttributes(ConvertToWString(absolute_path).c_str());
+}
+
+static DWORD GetFileAttributesForUtf8Path(const fml::UniqueFD& base_directory,
+                                          const char* path) {
+  std::string full_path = GetFullHandlePath(base_directory) + "\\" + path;
+  return GetFileAttributesForUtf8Path(full_path.c_str());
+}
+
 std::string CreateTemporaryDirectory() {
   // Get the system temporary directory.
   auto temp_dir_container = GetTemporaryDirectoryPath();
@@ -253,16 +258,17 @@ bool IsDirectory(const fml::UniqueFD& directory) {
 }
 
 bool IsDirectory(const fml::UniqueFD& base_directory, const char* path) {
-  std::string full_path = GetFullHandlePath(base_directory) + "\\" + path;
-  return ::GetFileAttributes(ConvertToWString(full_path.c_str()).c_str()) &
+  return GetFileAttributesForUtf8Path(base_directory, path) &
          FILE_ATTRIBUTE_DIRECTORY;
 }
 
 bool IsFile(const std::string& path) {
-  struct stat buf;
-  if (stat(path.c_str(), &buf) != 0)
+  DWORD attributes = GetFileAttributesForUtf8Path(path.c_str());
+  if (attributes == INVALID_FILE_ATTRIBUTES) {
     return false;
-  return S_ISREG(buf.st_mode);
+  }
+  return !(attributes &
+           (FILE_ATTRIBUTE_DIRECTORY | FILE_ATTRIBUTE_REPARSE_POINT));
 }
 
 bool UnlinkDirectory(const char* path) {
@@ -323,7 +329,8 @@ bool TruncateFile(const fml::UniqueFD& file, size_t size) {
 }
 
 bool FileExists(const fml::UniqueFD& base_directory, const char* path) {
-  return IsFile(GetAbsolutePath(base_directory, path).c_str());
+  return GetFileAttributesForUtf8Path(base_directory, path) !=
+         INVALID_FILE_ATTRIBUTES;
 }
 
 bool WriteAtomically(const fml::UniqueFD& base_directory,