Add Persistent<T>::Reset which disposes the handle and redirects it to point to another object.

BUG=
R=dcarney@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/14788013

Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14570 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

include/v8.h

@@ -690,6 +690,11 @@ template <class T> class Persistent // NOLINT
   // TODO(dcarney): remove before cutover
   V8_INLINE(uint16_t WrapperClassId(Isolate* isolate) const);
 
+  /**
+   * Disposes the current contents of the handle and replaces it.
+   */
+  V8_INLINE(void Reset(Isolate* isolate, const Handle<T>& other));
+
 #ifndef V8_USE_UNSAFE_HANDLES
 
 #ifndef V8_ALLOW_ACCESS_TO_PERSISTENT_IMPLICIT
@@ -5379,6 +5384,22 @@ void Persistent<T>::SetWrapperClassId(uint16_t class_id) {
   SetWrapperClassId(Isolate::GetCurrent(), class_id);
 }
 
+template <class T>
+void Persistent<T>::Reset(Isolate* isolate, const Handle<T>& other) {
+  Dispose(isolate);
+#ifdef V8_USE_UNSAFE_HANDLES
+  *this = *New(isolate, other);
+#else
+  if (other.IsEmpty()) {
+    this->val_ = NULL;
+    return;
+  }
+  internal::Object** p = reinterpret_cast<internal::Object**>(other.val_);
+  this->val_ = reinterpret_cast<T*>(
+      V8::GlobalizeReference(reinterpret_cast<internal::Isolate*>(isolate), p));
+#endif
+}
+
 template <class T>
 void Persistent<T>::SetWrapperClassId(Isolate* isolate, uint16_t class_id) {
   typedef internal::Internals I;

test/cctest/test-api.cc

@@ -2487,6 +2487,58 @@ THREADED_TEST(GlobalHandle) {
 }
 
 
+THREADED_TEST(ResettingGlobalHandle) {
+  v8::Isolate* isolate = v8::Isolate::GetCurrent();
+  v8::internal::GlobalHandles* global_handles = NULL;
+  int initial_handle_count = 0;
+  v8::Persistent<String> global;
+  {
+    v8::HandleScope scope(isolate);
+    Local<String> str = v8_str("str");
+    global_handles =
+        reinterpret_cast<v8::internal::Isolate*>(isolate)->global_handles();
+    initial_handle_count = global_handles->NumberOfGlobalHandles();
+    global = v8::Persistent<String>::New(isolate, str);
+  }
+  CHECK_EQ(global->Length(), 3);
+  CHECK_EQ(global_handles->NumberOfGlobalHandles(), initial_handle_count + 1);
+  {
+    v8::HandleScope scope(isolate);
+    Local<String> str = v8_str("longer");
+    global.Reset(isolate, str);
+  }
+  CHECK_EQ(global->Length(), 6);
+  CHECK_EQ(global_handles->NumberOfGlobalHandles(), initial_handle_count + 1);
+  global.Dispose(isolate);
+  CHECK_EQ(global_handles->NumberOfGlobalHandles(), initial_handle_count);
+}
+
+
+THREADED_TEST(ResettingGlobalHandleToEmpty) {
+  v8::Isolate* isolate = v8::Isolate::GetCurrent();
+  v8::internal::GlobalHandles* global_handles = NULL;
+  int initial_handle_count = 0;
+  v8::Persistent<String> global;
+  {
+    v8::HandleScope scope(isolate);
+    Local<String> str = v8_str("str");
+    global_handles =
+        reinterpret_cast<v8::internal::Isolate*>(isolate)->global_handles();
+    initial_handle_count = global_handles->NumberOfGlobalHandles();
+    global = v8::Persistent<String>::New(isolate, str);
+  }
+  CHECK_EQ(global->Length(), 3);
+  CHECK_EQ(global_handles->NumberOfGlobalHandles(), initial_handle_count + 1);
+  {
+    v8::HandleScope scope(isolate);
+    Local<String> empty;
+    global.Reset(isolate, empty);
+  }
+  CHECK(global.IsEmpty());
+  CHECK_EQ(global_handles->NumberOfGlobalHandles(), initial_handle_count);
+}
+
+
 THREADED_TEST(LocalHandle) {
   v8::HandleScope scope(v8::Isolate::GetCurrent());
   v8::Local<String> local = v8::Local<String>::New(v8_str("str"));