n-api: enable napi_wrap() to work with any object

Change to inserting an external object into the wrapper object's
prototype chain, instead of injecting an alternate `this` object
in the constructor callback adapter. The latter approach didn't
work in certain scenarios because the JSRT runtime still returned
the original `this` object.

And with this change, the setting and checking of the extension
flag, which distinguished wrapper objects from external objects,
can be removed.

Also removing the CallbackInfo.returnValue field which is not
used anymore (since we switched to direct return values).

PR-URL: nodejs#269
Reviewed-By: Taylor Woll <taylor.woll@microsoft.com>
Reviewed-By: Kyle Farnung <kfarnung@microsoft.com>

src/node_api_jsrt.cc

@@ -69,7 +69,6 @@ struct CallbackInfo {
   uint16_t argc;
   napi_value* argv;
   void* data;
-  napi_value returnValue;
 };
 
 struct napi_env__ {
@@ -219,30 +218,9 @@ class ExternalCallback {
     CallbackInfo cbInfo;
     cbInfo.thisArg = reinterpret_cast<napi_value>(arguments[0]);
     cbInfo.isConstructCall = isConstructCall;
-
-    if (isConstructCall) {
-      // For constructor callbacks, replace the 'this' arg with a new external
-      // object, to support wrapping a native object in the external object.
-      JsValueRef externalThis;
-      if (JsNoError == JsCreateExternalObject(
-        nullptr, jsrtimpl::ExternalData::Finalize, &externalThis)) {
-        // Copy the prototype from the default 'this' arg to the new
-        // 'external' this arg.
-        if (arguments[0] != nullptr) {
-          JsValueRef thisPrototype;
-          if (JsNoError == JsGetPrototype(arguments[0], &thisPrototype)) {
-            JsSetPrototype(externalThis, thisPrototype);
-          }
-        }
-
-        cbInfo.thisArg = reinterpret_cast<napi_value>(externalThis);
-      }
-    }
-
     cbInfo.argc = argumentCount - 1;
     cbInfo.argv = reinterpret_cast<napi_value*>(arguments + 1);
     cbInfo.data = externalCallback->_data;
-    cbInfo.returnValue = reinterpret_cast<napi_value>(undefinedValue);
 
     napi_value result = externalCallback->_cb(
       externalCallback->_env, reinterpret_cast<napi_callback_info>(&cbInfo));
@@ -986,21 +964,12 @@ napi_status napi_typeof(napi_env env, napi_value vv, napi_valuetype* result) {
     case JsError: *result = napi_object; break;
 
     default:
-      // An "external" value is represented in JSRT as an Object that has
-      // external data and DOES NOT allow extensions. (A wrapped object has
-      // external data and DOES allow extensions.)
       bool hasExternalData;
       if (JsHasExternalData(value, &hasExternalData) != JsNoError) {
         hasExternalData = false;
       }
 
-      bool isExtensionAllowed;
-      if (JsGetExtensionAllowed(value, &isExtensionAllowed) != JsNoError) {
-        isExtensionAllowed = false;
-      }
-
-      *result =
-        (hasExternalData && !isExtensionAllowed) ? napi_external : napi_object;
+      *result = hasExternalData ? napi_external : napi_object;
       break;
   }
   return napi_ok;
@@ -1385,17 +1354,47 @@ napi_status napi_wrap(napi_env env,
     env, native_object, finalize_cb, finalize_hint);
   if (externalData == nullptr) return napi_set_last_error(napi_generic_failure);
 
-  CHECK_JSRT(JsSetExternalData(value, externalData));
+  // Create an external object that will hold the external data pointer.
+  JsValueRef external;
+  CHECK_JSRT(JsCreateExternalObject(
+    externalData, jsrtimpl::ExternalData::Finalize, &external));
+
+  // Insert the external object into the value's prototype chain.
+  JsValueRef valuePrototype;
+  CHECK_JSRT(JsGetPrototype(value, &valuePrototype));
+  CHECK_JSRT(JsSetPrototype(external, valuePrototype));
+  CHECK_JSRT(JsSetPrototype(value, external));
 
   if (result != nullptr) {
-    napi_create_reference(env, js_object, 0, result);
+    CHECK_NAPI(napi_create_reference(env, js_object, 0, result));
   }
 
   return napi_ok;
 }
 
-napi_status napi_unwrap(napi_env env, napi_value jsObject, void** result) {
-  return napi_get_value_external(env, jsObject, result);
+napi_status napi_unwrap(napi_env env, napi_value js_object, void** result) {
+  JsValueRef value = reinterpret_cast<JsValueRef>(js_object);
+
+  // Search the object's prototype chain for the wrapper with external data.
+  // Usually the wrapper would be the first in the chain, but it is OK for
+  // other objects to be inserted in the prototype chain.
+  JsValueRef wrapper = value;
+  bool hasExternalData = false;
+  do {
+    CHECK_JSRT(JsGetPrototype(wrapper, &wrapper));
+    if (wrapper == JS_INVALID_REFERENCE) {
+      return  napi_invalid_arg;
+    }
+    CHECK_JSRT(JsHasExternalData(wrapper, &hasExternalData));
+  } while (!hasExternalData);
+
+  jsrtimpl::ExternalData* externalData;
+  CHECK_JSRT(JsGetExternalData(
+    wrapper, reinterpret_cast<void**>(&externalData)));
+
+  *result = (externalData != nullptr ? externalData->Data() : nullptr);
+
+  return napi_ok;
 }
 
 napi_status napi_create_external(napi_env env,
@@ -1413,7 +1412,6 @@ napi_status napi_create_external(napi_env env,
     externalData,
     jsrtimpl::ExternalData::Finalize,
     reinterpret_cast<JsValueRef*>(result)));
-  CHECK_JSRT(JsPreventExtension(*reinterpret_cast<JsValueRef*>(result)));
 
   return napi_ok;
 }