Use LockFile/UnlockFile for Settings to port to Windows

client/settings.h

Index: client/settings.h
diff --git a/client/settings.h b/client/settings.h
index bab5186a5351374f5c0449a54c6cd61605b56d95..f528bc1c6659dd959c0ec60eb0f17749fb26ad98 100644
--- a/client/settings.h
+++ b/client/settings.h
@@ -21,6 +21,7 @@
 
 #include "base/basictypes.h"
 #include "base/files/file_path.h"
+#include "base/move.h"
 #include "util/file/file_io.h"
 #include "util/misc/initialization_state_dcheck.h"
 #include "util/misc/uuid.h"
@@ -95,13 +96,37 @@ class Settings {
  private:
   struct Data;
 
+  class ScopedLockedFileHandle {
+    MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(ScopedLockedFileHandle)
+
+    public:
+      ScopedLockedFileHandle();
+      ScopedLockedFileHandle(FileHandle file, FileLocking locking);
+      ScopedLockedFileHandle(ScopedLockedFileHandle&& rvalue)
+          : handle_(rvalue.handle_.release()) {}
+      ScopedLockedFileHandle& operator=(ScopedLockedFileHandle&& rvalue) {
+        handle_.reset(rvalue.handle_.release());
+        return *this;
+      }
+      ~ScopedLockedFileHandle();
+
+      FileHandle get() { return handle_.get(); }
+      bool is_valid() const { return handle_.is_valid(); }
+      void reset();
+
+    private:
+      void FreeIfNecessary();
+
+      ScopedFileHandle handle_;
+  };
+
   // Opens the settings file for reading. On error, logs a message and returns
   // the invalid handle.
-  ScopedFileHandle OpenForReading();
+  ScopedLockedFileHandle OpenForReading();
 
   // Opens the settings file for reading and writing. On error, logs a message
   // and returns the invalid handle.
-  ScopedFileHandle OpenForReadingAndWriting();
+  ScopedLockedFileHandle OpenForReadingAndWriting();
 
   // Opens the settings file and reads the data. If that fails, an error will
   // be logged and the settings will be recovered and re-initialized. If that
@@ -111,7 +136,7 @@ class Settings {
   // Opens the settings file for writing and reads the data. If reading fails,
   // recovery is attempted. Returns the opened file handle on success, or the
   // invalid file handle on failure, with an error logged.
-  ScopedFileHandle OpenForWritingAndReadSettings(Data* out_data);
+  ScopedLockedFileHandle OpenForWritingAndReadSettings(Data* out_data);
 
   // Reads the settings from |handle|. Logs an error and returns false on
   // failure. This does not perform recovery.