PPAPI: Disallow blocking callbacks while handling a blocking message

ppapi/api/pp_errors.idl

 /* 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.
  */
 
 /**
  * This file defines an enumeration of all PPAPI error codes.
  */
 
 /**
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    * Indicates an attempt to make a PPAPI call on a thread without previously
    * registering a message loop via PPB_MessageLoop.AttachToCurrentThread.
    * Without this registration step, no PPAPI calls are supported.
    */
   PP_ERROR_NO_MESSAGE_LOOP = -51,
   /**
    * Indicates that the requested operation is not permitted on the current
    * thread.
    */
   PP_ERROR_WRONG_THREAD   = -52,
+  /**
+   * Indicates that a null completion callback was used on a thread handling a
+   * blocking message from JavaScript. Null completion callbacks "block until
+   * complete", which could cause the main JavaScript thread to be blocked
+   * excessively.
+   */
+  PP_ERROR_WOULD_BLOCK_THREAD   = -53,
 
   /**
    * This value indicates that the connection was closed. For TCP sockets, it
    * corresponds to a TCP FIN.
    */
   PP_ERROR_CONNECTION_CLOSED = -100,
   /**
    * This value indicates that the connection was reset. For TCP sockets, it
    * corresponds to a TCP RST.
    */
@@ skipping 33 matching lines @@
   /**
    * This value indicates that the message was too large for the transport.
    */
   PP_ERROR_MESSAGE_TOO_BIG = -109,
   /**
    * This value indicates that the host name could not be resolved.
    */
   PP_ERROR_NAME_NOT_RESOLVED = -110
 };