PPAPI: Reduce string copying in SerializedVar.

ppapi/proxy/plugin_var_serialization_rules.cc

 // 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.
 
 #include "ppapi/proxy/plugin_var_serialization_rules.h"
 
 #include "base/logging.h"
 #include "ppapi/proxy/plugin_dispatcher.h"
 #include "ppapi/proxy/plugin_globals.h"
 #include "ppapi/proxy/plugin_resource_tracker.h"
 #include "ppapi/proxy/plugin_var_tracker.h"
 #include "ppapi/shared_impl/ppapi_globals.h"
 #include "ppapi/shared_impl/var.h"
 
 namespace ppapi {
 namespace proxy {
 
 PluginVarSerializationRules::PluginVarSerializationRules()
     : var_tracker_(PluginGlobals::Get()->plugin_var_tracker()) {
 }
 
 PluginVarSerializationRules::~PluginVarSerializationRules() {
 }
 
-PP_Var PluginVarSerializationRules::SendCallerOwned(const PP_Var& var,
-                                                    std::string* str_val) {
+PP_Var PluginVarSerializationRules::SendCallerOwned(
+    const PP_Var& var,
+    const std::string** str_ptr_out) {
   // Objects need special translations to get the IDs valid in the host.
   if (var.type == PP_VARTYPE_OBJECT)
     return var_tracker_->GetHostObject(var);
 
-  // Retrieve the string to use for IPC.
+  // Retrieve the pointer to the string in the tracker in order to send the
+  // string over IPC without unnecessary copies.
   if (var.type == PP_VARTYPE_STRING) {
     StringVar* string_var = StringVar::FromPPVar(var);
     if (string_var)
-      *str_val = string_var->value();
+      *str_ptr_out = string_var->ptr();
     else
       NOTREACHED() << "Trying to send unknown string over IPC.";
   }
   return var;
 }
 
 PP_Var PluginVarSerializationRules::BeginReceiveCallerOwned(
     const PP_Var& var,
-    const std::string* str_val,
+    scoped_ptr<std::string> str,
     Dispatcher* dispatcher) {
   if (var.type == PP_VARTYPE_STRING)
-    return StringVar::StringToPPVar(*str_val);
+    return StringVar::StringToPPVar(str.Pass());
 
   if (var.type == PP_VARTYPE_OBJECT) {
     DCHECK(dispatcher->IsPlugin());
     return var_tracker_->TrackObjectWithNoReference(
         var, static_cast<PluginDispatcher*>(dispatcher));
   }
 
   return var;
 }
 
 void PluginVarSerializationRules::EndReceiveCallerOwned(const PP_Var& var) {
   if (var.type == PP_VARTYPE_STRING) {
     // Destroy the string BeginReceiveCallerOwned created above.
     var_tracker_->ReleaseVar(var);
   } else if (var.type == PP_VARTYPE_OBJECT) {
     var_tracker_->StopTrackingObjectWithNoReference(var);
   }
 }
 
 PP_Var PluginVarSerializationRules::ReceivePassRef(const PP_Var& var,
-                                                   const std::string& str_val,
+                                                   scoped_ptr<std::string> str,
                                                    Dispatcher* dispatcher) {
   if (var.type == PP_VARTYPE_STRING)
-    return StringVar::StringToPPVar(str_val);
+    return StringVar::StringToPPVar(str.Pass());
 
   // Overview of sending an object with "pass ref" from the browser to the
   // plugin:
   //                                  Example 1             Example 2
   //                               Plugin   Browser      Plugin   Browser
   // Before send                      3        2            0        1
   // Browser calls BeginSendPassRef   3        2            0        1
   // Plugin calls ReceivePassRef      4        1            1        1
   // Browser calls EndSendPassRef     4        1            1        1
   //
   // In example 1 before the send, the plugin has 3 refs which are represented
   // as one ref in the browser (since the plugin only tells the browser when
   // it's refcount goes from 1 -> 0). The initial state is that the browser
   // plugin code started to return a value, which means it gets another ref
   // on behalf of the caller. This needs to be transferred to the plugin and
   // folded in to its set of refs it maintains (with one ref representing all
   // of them in the browser).
   if (var.type == PP_VARTYPE_OBJECT) {
     DCHECK(dispatcher->IsPlugin());
     return var_tracker_->ReceiveObjectPassRef(
         var, static_cast<PluginDispatcher*>(dispatcher));
   }
 
   // Other types are unchanged.
   return var;
 }
 
-PP_Var PluginVarSerializationRules::BeginSendPassRef(const PP_Var& var,
-                                                     std::string* str_val) {
+PP_Var PluginVarSerializationRules::BeginSendPassRef(
+    const PP_Var& var,
+    const std::string** str_ptr_out) {
   // Overview of sending an object with "pass ref" from the plugin to the
   // browser:
   //                                  Example 1             Example 2
   //                               Plugin   Browser      Plugin   Browser
   // Before send                      3        1            1        1
   // Plugin calls BeginSendPassRef    3        1            1        1
   // Browser calls ReceivePassRef     3        2            1        2
   // Plugin calls EndSendPassRef      2        2            0        1
   //
   // The plugin maintains one ref count in the browser on behalf of the
   // entire ref count in the plugin. When the plugin refcount goes to 0, it
   // will call the browser to deref the object. This is why in example 2
   // transferring the object ref to the browser involves no net change in the
   // browser's refcount.
 
   // Objects need special translations to get the IDs valid in the host.
   if (var.type == PP_VARTYPE_OBJECT)
     return var_tracker_->GetHostObject(var);
 
   if (var.type == PP_VARTYPE_STRING) {
+    // Get the pointer to the string that's in the tracker and return it, so we
+    // can avoid an extra copy of the string when serializing over IPC.
     StringVar* string_var = StringVar::FromPPVar(var);
     if (string_var)
-      *str_val = string_var->value();
+      *str_ptr_out = string_var->ptr();
     else
       NOTREACHED() << "Trying to send unknown string over IPC.";
   }
   return var;
 }
 
 void PluginVarSerializationRules::EndSendPassRef(const PP_Var& var,
                                                  Dispatcher* dispatcher) {
   // See BeginSendPassRef for an example of why we release our ref here.
   // The var we have in our inner class has been converted to a host object
   // by BeginSendPassRef. This means it's not a normal var valid in the plugin,
   // so we need to use the special ReleaseHostObject.
   if (var.type == PP_VARTYPE_OBJECT) {
     var_tracker_->ReleaseHostObject(
         static_cast<PluginDispatcher*>(dispatcher), var);
   } else if (var.type == PP_VARTYPE_STRING) {
     var_tracker_->ReleaseVar(var);
   }
 }
 
 void PluginVarSerializationRules::ReleaseObjectRef(const PP_Var& var) {
   var_tracker_->ReleaseVar(var);
 }
 
 }  // namespace proxy
 }  // namespace ppapi