Add synchronous callback support to important_file_writer.cc

base/files/important_file_writer.h

Index: base/files/important_file_writer.h
diff --git a/base/files/important_file_writer.h b/base/files/important_file_writer.h
index 6e8646af3dab1de60cdaba572ce059987932835c..6265869caf0d47aa6bdc744b96a959a0d5b1e84d 100644
--- a/base/files/important_file_writer.h
+++ b/base/files/important_file_writer.h
@@ -94,10 +94,18 @@ class BASE_EXPORT ImportantFileWriter : public NonThreadSafe {
   // Serialize data pending to be saved and execute write on backend thread.
   void DoScheduledWrite();
 
-  // Registers |on_next_successful_write| to be called once, on the next
-  // successful write event. Only one callback can be set at once.
-  void RegisterOnNextSuccessfulWriteCallback(
-      const Closure& on_next_successful_write);
+  // Registers |on_next_successful_write_reply| to be invoked on the sequence
+  // owning this ImportantFileWriter in reply to the next successful write.
+  // Only one such callback can be set at once.
+  void RegisterOnNextSuccessfulWriteReply(
+      const Closure& on_next_successful_write_reply);
+
+  // Registers |on_next_write_callback| to be synchronously invoked from
+  // WriteFileAtomically() on its next write (i.e. from |task_runner_|), with
+  // |success| indicating whether it succeeded or not.
+  // Only one such callback can be set at once.
+  void RegisterOnNextWriteSynchronousCallback(
+      const Callback<void(bool success)>& on_next_write_callback);
 
   TimeDelta commit_interval() const {
     return commit_interval_;
@@ -112,7 +120,10 @@ class BASE_EXPORT ImportantFileWriter : public NonThreadSafe {
   void ForwardSuccessfulWrite(bool result);
 
   // Invoked once and then reset on the next successful write event.
-  Closure on_next_successful_write_;
+  Closure on_next_successful_write_reply_;
+
+  // Invoked synchronously on the next write event.
+  Callback<void(bool success)> on_next_write_callback_;
 
   // Path being written to.
   const FilePath path_;