Clarify file sharing flags in FML filesystem APIs on Windows

Use shared mode when requesting read access and exclusive mode for write access

fml/BUILD.gn

@@ -351,7 +351,10 @@ if (enable_unittests) {
     }
 
     if (is_win) {
-      sources += [ "platform/win/wstring_conversion_unittests.cc" ]
+      sources += [
+        "platform/win/file_win_unittests.cc",
+        "platform/win/wstring_conversion_unittests.cc",
+      ]
     }
 
     deps = [

fml/platform/win/file_win.cc

@@ -85,11 +85,11 @@ static DWORD GetDesiredAccessFlags(FilePermission permission) {
 static DWORD GetShareFlags(FilePermission permission) {
   switch (permission) {
     case FilePermission::kRead:
-      return FILE_SHARE_READ;
+      return FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE;
     case FilePermission::kWrite:
-      return FILE_SHARE_WRITE;
+      return 0;
     case FilePermission::kReadWrite:
-      return FILE_SHARE_READ | FILE_SHARE_WRITE;
+      return 0;
   }
   return FILE_SHARE_READ;
 }

fml/platform/win/file_win_unittests.cc

@@ -0,0 +1,38 @@
+// 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/file.h"
+
+#include <Fileapi.h>
+
+#include "flutter/fml/platform/win/wstring_conversion.h"
+#include "gtest/gtest.h"
+
+namespace fml {
+namespace testing {
+
+TEST(FileWin, OpenDirectoryShare) {
+  fml::ScopedTemporaryDirectory temp_dir;
+  auto temp_path = Utf8ToWideString({temp_dir.path()});
+  fml::UniqueFD handle(
+      CreateFile(temp_path.c_str(),
+                 GENERIC_WRITE,
+                 FILE_SHARE_READ | FILE_SHARE_WRITE,
+                 nullptr,
+                 OPEN_EXISTING,
+                 FILE_ATTRIBUTE_NORMAL | FILE_FLAG_BACKUP_SEMANTICS,
+                 nullptr
+      )
+  );
+  ASSERT_TRUE(handle.is_valid());
+
+  // Check that fml::OpenDirectory(FilePermission::kRead) works with a
+  // directory that has also been opened for writing.
+  auto dir =
+      fml::OpenDirectory(temp_dir.path().c_str(), false, fml::FilePermission::kRead);
+  ASSERT_TRUE(dir.is_valid());
+}
+
+}  // namespace testing
+}  // namespace fml