PPAPI: Reduce string copying in SerializedVar.

ppapi/proxy/var_serialization_rules.h

 // Copyright (c) 2011 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_PROXY_VAR_SERIALIZATION_RULES_H_
 #define PPAPI_PROXY_VAR_SERIALIZATION_RULES_H_
 
+#include "base/memory/scoped_ptr.h"
 #include "ppapi/c/pp_var.h"
 
 #include <string>
 
 namespace ppapi {
 namespace proxy {
 
 class Dispatcher;
 
 // Encapsulates the rules for serializing and deserializing vars to and from
 // the local process. The renderer and the plugin process each have separate
 // bookkeeping rules.
 class VarSerializationRules {
  public:
   virtual ~VarSerializationRules() {}
 
   // Caller-owned calls --------------------------------------------------------
   //
   // A caller-owned call is when doing a function call with a "normal" input
   // argument. The caller has a reference to the var, and the caller is
   // responsible for freeing that reference.
 
-  // Prepares the given var for sending to the callee. If the var is a string,
-  // the value of that string will be placed in *str_val. If the var is not
-  // a string, str_val will be untouched and may be NULL. The return value will
-  // be the var valid for the host process.
-  virtual PP_Var SendCallerOwned(const PP_Var& var, std::string* str_val) = 0;
+  // Prepares the given var for sending to the remote process. If the var is a
+  // string, *str_ptr_out will be set to point to the string in the tracker
+  // referenced by var. If the var is not a string, str_ptr_out will be
+  // untouched and may be NULL. The return value will be the var valid for the
+  // local process.
+  virtual PP_Var SendCallerOwned(const PP_Var& var,
+                                 const std::string** str_ptr_out) = 0;
 
   // When receiving a caller-owned variable, normally we don't have to do
   // anything. However, in the case of strings, we need to deserialize the
   // string from IPC, create a new PP_Var string in the local process, call the
   // function, and then destroy the temporary string. These two functions
-  // handle that process
+  // handle that process.
   //
   // BeginReceiveCallerOwned takes a var from IPC and an optional pointer to
   // the deserialized string (which will be used only when var is a
   // VARTYPE_STRING and may be NULL otherwise) and returns a new var
   // representing the input in the local process.
   //
-  // EndReceiveCallerOwned destroys the string created by Begin* and does
-  // nothing otherwise. It should be called with the result of Begin*.
+  // EndReceiveCallerOwned releases the reference count in the Var tracker for
+  // the object or string that was added to the tracker. (Note, if the recipient
+  // took a reference to the Var, it will remain in the tracker after
+  // EndReceiveCallerOwned).
   virtual PP_Var BeginReceiveCallerOwned(const PP_Var& var,
-                                         const std::string* str_val,
+                                         scoped_ptr<std::string> str,
                                          Dispatcher* dispatcher) = 0;
   virtual void EndReceiveCallerOwned(const PP_Var& var) = 0;
 
   // Passing refs -------------------------------------------------------------
   //
   // A pass-ref transfer is when ownership of a reference is passed from
   // one side to the other. Normally, this happens via return values and
   // output arguments, as for exceptions. The code generating the value
   // (the function returning it in the case of a return value) will AddRef
   // the var on behalf of the consumer of the value. Responsibility for
   // Release is on the consumer (the caller of the function in the case of a
   // return value).
 
   // Creates a var in the context of the local process from the given
   // deserialized var and deserialized string (which will be used only when var
   // is a VARTYPE_STRING and may be NULL otherwise). The input var/string
   // should be the result of calling SendPassRef in the remote process. The
-  // return value is the var valid in the plugin process.
+  // return value is the var valid in the host process for object vars.
+  // Otherwise, the return value is a var which is valid in the local process.
   virtual PP_Var ReceivePassRef(const PP_Var& var,
-                                const std::string& str_val,
+                                scoped_ptr<std::string> str,
                                 Dispatcher* dispatcher) = 0;
 
   // Prepares a var to be sent to the remote side. One local reference will
   // be passed to the remote side. Call Begin* before doing the send and End*
   // after doing the send. See SendCallerOwned for a description of the string.
   //
-  // The return value from BeginSendPassRef will be the var valid for the host
-  // process. This same value must be passed to EndSendPassRef.
-  virtual PP_Var BeginSendPassRef(const PP_Var& var, std::string* str_val) = 0;
+  // For object vars, the return value from BeginSendPassRef will be the var
+  // valid for the host process. Otherwise, it is a var that is valid in the
+  // local process. This same var must be passed to EndSendPassRef.
+  virtual PP_Var BeginSendPassRef(const PP_Var& var,
+                                  const std::string** str_ptr_out) = 0;
   virtual void EndSendPassRef(const PP_Var& var, Dispatcher* dispatcher) = 0;
 
   // ---------------------------------------------------------------------------
 
   virtual void ReleaseObjectRef(const PP_Var& var) = 0;
 
  protected:
   VarSerializationRules() {}
 };
 
 }  // namespace proxy
 }  // namespace ppapi
 
 #endif  // PPAPI_PROXY_VAR_SERIALIZATION_RULES_H_