Offload disk accesses to WorkerPool in ExtensionProtocolHandler

net/url_request/url_request_simple_job.h

Index: net/url_request/url_request_simple_job.h
===================================================================
--- net/url_request/url_request_simple_job.h	(revision 145483)
+++ net/url_request/url_request_simple_job.h	(working copy)
@@ -9,6 +9,7 @@
 #include <string>
 
 #include "base/memory/weak_ptr.h"
+#include "net/base/completion_callback.h"
 #include "net/base/net_export.h"
 #include "net/url_request/url_request_job.h"
 
@@ -30,15 +31,25 @@
  protected:
   virtual ~URLRequestSimpleJob();
 
-  // subclasses must override the way response data is determined.
-  virtual bool GetData(std::string* mime_type,
-                       std::string* charset,
-                       std::string* data) const = 0;
+  // Subclasses must override the way response data is determined.
+  // The return value should be:
+  //  - OK if data is obtained;
+  //  - ERR_FAILED to indicate an error;

[darin (slow to review), 2012/07/13 04:26:50]

questions:

- it is not OK to report other failures?

- it seems like another thread does not need to necessarily be involved here. 
one could also implement GetData using
MessageLoop::current()->PostDelayedTask(...), right?  that would also be valid. 
i think the key aspect here is that GetData may complete asynchronously.

+  //  - ERR_IO_PENDING if processing in another thread is needed to finish
+  //    obtaining data. This is the only case when |callback| should be called
+  //    after completion of the operation. In other situations |callback|
+  //    should never be called.
+  virtual int GetData(std::string* mime_type,
+                      std::string* charset,
+                      std::string* data,
+                      const CompletionCallback& callback) const = 0;
 
  protected:
   void StartAsync();
 
  private:
+  void OnGetDataCompleted(int result);
+
   std::string mime_type_;
   std::string charset_;
   std::string data_;