PPAPI: Make CompletionCallbacks work right on background threads.

ppapi/cpp/completion_callback.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PPAPI_CPP_COMPLETION_CALLBACK_H_
 #define PPAPI_CPP_COMPLETION_CALLBACK_H_
 
 #include "ppapi/c/pp_completion_callback.h"
 #include "ppapi/c/pp_errors.h"
 #include "ppapi/cpp/logging.h"
@@ skipping 81 matching lines @@
   /// IsOptional() is used to determine the setting of the
   /// <code>PP_COMPLETIONCALLBACK_FLAG_OPTIONAL</code> flag. This flag allows
   /// any method taking such callback to complete synchronously
   /// and not call the callback if the operation would not block. This is useful
   /// when performance is an issue, and the operation bandwidth should not be
   /// limited to the processing speed of the message loop.
   ///
   /// On synchronous method completion, the completion result will be returned
   /// by the method itself. Otherwise, the method will return
   /// PP_OK_COMPLETIONPENDING, and the callback will be invoked asynchronously
-  /// on the main thread of Pepper execution.
+  /// on the same thread where the PPB method was invoked.
   ///
   /// @return true if this callback is optional, otherwise false.
   bool IsOptional() const {
     return (cc_.func == NULL ||
             (cc_.flags & PP_COMPLETIONCALLBACK_FLAG_OPTIONAL) != 0);
   }
 
   /// The pp_completion_callback() function returns the underlying
   /// <code>PP_CompletionCallback</code>
   ///
@@ skipping 30 matching lines @@
   /// indicates that the callback has already been scheduled. Other
   /// non-positive values correspond to error codes from
   /// <code>pp_errors.h</code>. Positive values indicate additional information
   /// such as bytes read.
   ///
   /// @return <code>PP_OK_COMPLETIONPENDING</code> if the callback has been
   /// forced, result parameter otherwise.
   int32_t MayForce(int32_t result) const {
     if (result == PP_OK_COMPLETIONPENDING || IsOptional())
       return result;
+    // FIXME(dmichael): Use pp::MessageLoop here once it's out of Dev.
     Module::Get()->core()->CallOnMainThread(0, *this, result);
     return PP_OK_COMPLETIONPENDING;
   }
 
  protected:
   PP_CompletionCallback cc_;
 };
 
 /// A CompletionCallbackWithOutput defines a completion callback that
 /// additionally stores a pointer to some output data. Some C++ wrappers
@@ skipping 94 matching lines @@
 /// @return A <code>CompletionCallback</code> corresponding to a NULL callback.
 inline CompletionCallback BlockUntilComplete() {
   // Note: Explicitly inlined to avoid link errors when included into
   // ppapi_proxy and ppapi_cpp_objects.
   return CompletionCallback();
 }
 
 }  // namespace pp
 
 #endif  // PPAPI_CPP_COMPLETION_CALLBACK_H_