n-api: add generic finalizer callback

Add `napi_add_finalizer()`, which provides the ability to attach data
to an arbitrary object and be notified when that object is garbage-
collected so as to have an opportunity to delete the data previously
attached.

This differs from `napi_wrap()` in that it does not use up the private
slot on the object, and is therefore neither removable, nor retrievable
after the call to `napi_add_finalizer()`. It is assumed that the data
is accessible by other means, yet it must be tied to the lifetime of
the object. This is the case for data passed to a dynamically created
function which is itself heap-allocated and must therefore be freed
along with the function.

Fixes: nodejs/abi-stable-node#313

Author: Gabriel Schulhof

doc/api/n-api.md

@@ -3208,6 +3208,11 @@ JavaScript functions from native code. One can either call a function
 like a regular JavaScript function call, or as a constructor
 function.
 
+Any non-`NULL` data which is passed to this API via the `data` field of the
+`napi_property_descriptor` items can be associated with `object` and freed
+whenever `object` is garbage-collected by passing both `object` and the data to
+[`napi_add_finalizer`][].
+
 ### napi_call_function
 <!-- YAML
 added: v8.0.0
@@ -3345,6 +3350,11 @@ myaddon.sayHello();
 The string passed to `require()` is the name of the target in `binding.gyp`
 responsible for creating the `.node` file.
 
+Any non-`NULL` data which is passed to this API via the `data` parameter can
+be associated with the resulting JavaScript function (which is returned in the
+`result` parameter) and freed whenever the function is garbage-collected by
+passing both the JavaScript function and the data to [`napi_add_finalizer`][].
+
 JavaScript `Function`s are described in
 [Section 19.2](https://tc39.github.io/ecma262/#sec-function-objects)
 of the ECMAScript Language Specification.
@@ -3551,6 +3561,12 @@ case, to prevent the function value from being garbage-collected, create a
 persistent reference to it using [`napi_create_reference`][] and ensure the
 reference count is kept >= 1.
 
+Any non-`NULL` data which is passed to this API via the `data` parameter or via
+the `data` field of the `napi_property_descriptor` array items can be associated
+with the resulting JavaScript constructor (which is returned in the `result`
+parameter) and freed whenever the class is garbage-collected by passing both
+the JavaScript function and the data to [`napi_add_finalizer`][].
+
 ### napi_wrap
 <!-- YAML
 added: v8.0.0
@@ -3655,6 +3671,47 @@ object `js_object` using `napi_wrap()` and removes the wrapping. If a finalize
 callback was associated with the wrapping, it will no longer be called when the
 JavaScript object becomes garbage-collected.
 
+### napi_add_finalizer
+<!-- YAML
+added: v8.0.0
+napiVersion: 1
+-->
+```C
+napi_status napi_add_finalizer(napi_env env,
+                               napi_value js_object,
+                               void* native_object,
+                               napi_finalize finalize_cb,
+                               void* finalize_hint,
+                               napi_ref* result);
+```
+
+ - `[in] env`: The environment that the API is invoked under.
+ - `[in] js_object`: The JavaScript object to which the native data will be
+   attached.
+ - `[in] native_object`: The native data that will be attached to the JavaScript
+   object.
+ - `[in] finalize_cb`: Native callback that will be used to free the
+   native data when the JavaScript object is ready for garbage-collection.
+ - `[in] finalize_hint`: Optional contextual hint that is passed to the
+   finalize callback.
+ - `[out] result`: Optional reference to the JavaScript object.
+
+Returns `napi_ok` if the API succeeded.
+
+Adds a `napi_finalize` callback which will be called when the JavaScript object
+in `js_object` is ready for garbage collection. This API is similar to
+`napi_wrap()` except that
+* the native data cannot be retrieved later using `napi_unwrap()`,
+* nor can it be removed later using `napi_remove_wrap()`, and
+* the API can be called multiple times with different data items in order to
+  attach each of them to the JavaScript object.
+
+*Caution*: The optional returned reference (if obtained) should be deleted via
+[`napi_delete_reference`][] ONLY in response to the finalize callback
+invocation. If it is deleted before then, then the finalize callback may never
+be invoked. Therefore, when obtaining a reference a finalize callback is also
+required in order to enable correct disposal of the reference.
+
 ## Simple Asynchronous Operations
 
 Addon modules often need to leverage async helpers from libuv as part of their
@@ -4529,6 +4586,7 @@ This API may only be called from the main thread.
 [Working with JavaScript Values]: #n_api_working_with_javascript_values
 [Working with JavaScript Values - Abstract Operations]: #n_api_working_with_javascript_values_abstract_operations
 
+[`napi_add_finalizer`]: #n_api_napi_add_finalizer
 [`napi_async_init`]: #n_api_napi_async_init
 [`napi_cancel_async_work`]: #n_api_napi_cancel_async_work
 [`napi_close_escapable_handle_scope`]: #n_api_napi_close_escapable_handle_scope

src/node_api.cc

@@ -1157,6 +1157,63 @@ class ThreadSafeFunction : public node::AsyncResource {
   bool handles_closing;
 };
 
+enum WrapType {
+  retrievable,
+  anonymous
+};
+
+template <WrapType wrap_type> static inline
+napi_status Wrap(napi_env env,
+                 napi_value js_object,
+                 void* native_object,
+                 napi_finalize finalize_cb,
+                 void* finalize_hint,
+                 napi_ref* result) {
+  NAPI_PREAMBLE(env);
+  CHECK_ARG(env, js_object);
+
+  v8::Isolate* isolate = env->isolate;
+  v8::Local<v8::Context> context = isolate->GetCurrentContext();
+
+  v8::Local<v8::Value> value = v8impl::V8LocalValueFromJsValue(js_object);
+  RETURN_STATUS_IF_FALSE(env, value->IsObject(), napi_invalid_arg);
+  v8::Local<v8::Object> obj = value.As<v8::Object>();
+
+  if (wrap_type == retrievable) {
+    // If we've already wrapped this object, we error out.
+    RETURN_STATUS_IF_FALSE(env,
+        !obj->HasPrivate(context, NAPI_PRIVATE_KEY(context, wrapper))
+            .FromJust(),
+        napi_invalid_arg);
+  } else if (wrap_type == anonymous) {
+    // If no finalize callback is provided, we error out.
+    CHECK_ARG(env, finalize_cb);
+  }
+
+  v8impl::Reference* reference = nullptr;
+  if (result != nullptr) {
+    // The returned reference should be deleted via napi_delete_reference()
+    // ONLY in response to the finalize callback invocation. (If it is deleted
+    // before then, then the finalize callback will never be invoked.)
+    // Therefore a finalize callback is required when returning a reference.
+    CHECK_ARG(env, finalize_cb);
+    reference = v8impl::Reference::New(
+        env, obj, 0, false, finalize_cb, native_object, finalize_hint);
+    *result = reinterpret_cast<napi_ref>(reference);
+  } else {
+    // Create a self-deleting reference.
+    reference = v8impl::Reference::New(env, obj, 0, true, finalize_cb,
+        native_object, finalize_cb == nullptr ? nullptr : finalize_hint);
+  }
+
+  if (wrap_type == retrievable) {
+    CHECK(obj->SetPrivate(context, NAPI_PRIVATE_KEY(context, wrapper),
+          v8::External::New(isolate, reference)).FromJust());
+  }
+
+  return GET_RETURN_STATUS(env);
+}
+
 }  // end of namespace v8impl
 
 // Intercepts the Node-V8 module registration callback. Converts parameters
@@ -2859,41 +2916,12 @@ napi_status napi_wrap(napi_env env,
                       napi_finalize finalize_cb,
                       void* finalize_hint,
                       napi_ref* result) {
-  NAPI_PREAMBLE(env);
-  CHECK_ARG(env, js_object);
-
-  v8::Isolate* isolate = env->isolate;
-  v8::Local<v8::Context> context = isolate->GetCurrentContext();
-
-  v8::Local<v8::Value> value = v8impl::V8LocalValueFromJsValue(js_object);
-  RETURN_STATUS_IF_FALSE(env, value->IsObject(), napi_invalid_arg);
-  v8::Local<v8::Object> obj = value.As<v8::Object>();
-
-  // If we've already wrapped this object, we error out.
-  RETURN_STATUS_IF_FALSE(env,
-      !obj->HasPrivate(context, NAPI_PRIVATE_KEY(context, wrapper)).FromJust(),
-      napi_invalid_arg);
-
-  v8impl::Reference* reference = nullptr;
-  if (result != nullptr) {
-    // The returned reference should be deleted via napi_delete_reference()
-    // ONLY in response to the finalize callback invocation. (If it is deleted
-    // before then, then the finalize callback will never be invoked.)
-    // Therefore a finalize callback is required when returning a reference.
-    CHECK_ARG(env, finalize_cb);
-    reference = v8impl::Reference::New(
-        env, obj, 0, false, finalize_cb, native_object, finalize_hint);
-    *result = reinterpret_cast<napi_ref>(reference);
-  } else {
-    // Create a self-deleting reference.
-    reference = v8impl::Reference::New(env, obj, 0, true, finalize_cb,
-        native_object, finalize_cb == nullptr ? nullptr : finalize_hint);
-  }
-
-  CHECK(obj->SetPrivate(context, NAPI_PRIVATE_KEY(context, wrapper),
-        v8::External::New(isolate, reference)).FromJust());
-
-  return GET_RETURN_STATUS(env);
+  return v8impl::Wrap<v8impl::retrievable>(env,
+                                           js_object,
+                                           native_object,
+                                           finalize_cb,
+                                           finalize_hint,
+                                           result);
 }
 
 napi_status napi_unwrap(napi_env env, napi_value obj, void** result) {
@@ -4138,3 +4166,17 @@ napi_ref_threadsafe_function(napi_env env, napi_threadsafe_function func) {
   CHECK(func != nullptr);
   return reinterpret_cast<v8impl::ThreadSafeFunction*>(func)->Ref();
 }
+
+napi_status napi_add_finalizer(napi_env env,
+                               napi_value js_object,
+                               void* native_object,
+                               napi_finalize finalize_cb,
+                               void* finalize_hint,
+                               napi_ref* result) {
+  return v8impl::Wrap<v8impl::anonymous>(env,
+                                         js_object,
+                                         native_object,
+                                         finalize_cb,
+                                         finalize_hint,
+                                         result);
+}

src/node_api.h

@@ -695,6 +695,12 @@ NAPI_EXTERN napi_status napi_get_value_bigint_words(napi_env env,
                                                     int* sign_bit,
                                                     size_t* word_count,
                                                     uint64_t* words);
+NAPI_EXTERN napi_status napi_add_finalizer(napi_env env,
+                                           napi_value js_object,
+                                           void* native_object,
+                                           napi_finalize finalize_cb,
+                                           void* finalize_hint,
+                                           napi_ref* result);
 #endif  // NAPI_EXPERIMENTAL
 
 EXTERN_C_END

test/addons-napi/test_general/testFinalizer.js

@@ -0,0 +1,33 @@
+'use strict';
+// Flags: --expose-gc
+
+const common = require('../../common');
+const test_general = require(`./build/${common.buildType}/test_general`);
+const assert = require('assert');
+
+let finalized = {};
+const callback = common.mustCall(2);
+
+// Add two items to be finalized and ensure the callback is called for each.
+test_general.addFinalizerOnly(finalized, callback);
+test_general.addFinalizerOnly(finalized, callback);
+
+// Ensure attached items cannot be retrieved.
+common.expectsError(() => test_general.unwrap(finalized),
+                    { type: Error, message: 'Invalid argument' });
+
+// Ensure attached items cannot be removed.
+common.expectsError(() => test_general.removeWrap(finalized),
+                    { type: Error, message: 'Invalid argument' });
+finalized = null;
+global.gc();
+
+// Add an item to an object that is already wrapped, and ensure that its
+// finalizer as well as the wrap finalizer gets called.
+let finalizeAndWrap = {};
+test_general.wrap(finalizeAndWrap);
+test_general.addFinalizerOnly(finalizeAndWrap, common.mustCall());
+finalizeAndWrap = null;
+global.gc();
+assert.strictEqual(test_general.derefItemWasCalled(), true,
+                   'finalize callback was called');

test/addons-napi/test_general/test_general.c

@@ -1,3 +1,4 @@
+#define NAPI_EXPERIMENTAL
 #include <node_api.h>
 #include <stdlib.h>
 #include "../common.h"
@@ -177,6 +178,17 @@ static napi_value wrap(napi_env env, napi_callback_info info) {
   return NULL;
 }
 
+static napi_value unwrap(napi_env env, napi_callback_info info) {
+  size_t argc = 1;
+  napi_value wrapped;
+  void* data;
+
+  NAPI_CALL(env, napi_get_cb_info(env, info, &argc, &wrapped, NULL, NULL));
+  NAPI_CALL(env, napi_unwrap(env, wrapped, &data));
+
+  return NULL;
+}
+
 static napi_value remove_wrap(napi_env env, napi_callback_info info) {
   size_t argc = 1;
   napi_value wrapped;
@@ -232,6 +244,33 @@ static napi_value testNapiRun(napi_env env, napi_callback_info info) {
   return result;
 }
 
+static void finalizer_only_callback(napi_env env, void* data, void* hint) {
+  napi_ref js_cb_ref = data;
+  napi_value js_cb, undefined;
+  NAPI_CALL_RETURN_VOID(env, napi_get_reference_value(env, js_cb_ref, &js_cb));
+  NAPI_CALL_RETURN_VOID(env, napi_get_undefined(env, &undefined));
+  NAPI_CALL_RETURN_VOID(env,
+      napi_call_function(env, undefined, js_cb, 0, NULL, NULL));
+  NAPI_CALL_RETURN_VOID(env, napi_delete_reference(env, js_cb_ref));
+}
+
+static napi_value add_finalizer_only(napi_env env, napi_callback_info info) {
+  size_t argc = 2;
+  napi_value argv[2];
+  napi_ref js_cb_ref;
+
+  NAPI_CALL(env, napi_get_cb_info(env, info, &argc, argv, NULL, NULL));
+  NAPI_CALL(env, napi_create_reference(env, argv[1], 1, &js_cb_ref));
+  NAPI_CALL(env,
+      napi_add_finalizer(env,
+                         argv[0],
+                         js_cb_ref,
+                         finalizer_only_callback,
+                         NULL,
+                         NULL));
+  return NULL;
+}
+
 static napi_value Init(napi_env env, napi_value exports) {
   napi_property_descriptor descriptors[] = {
     DECLARE_NAPI_PROPERTY("testStrictEquals", testStrictEquals),
@@ -246,7 +285,9 @@ static napi_value Init(napi_env env, napi_value exports) {
     DECLARE_NAPI_PROPERTY("testNapiErrorCleanup", testNapiErrorCleanup),
     DECLARE_NAPI_PROPERTY("testNapiTypeof", testNapiTypeof),
     DECLARE_NAPI_PROPERTY("wrap", wrap),
+    DECLARE_NAPI_PROPERTY("unwrap", unwrap),
     DECLARE_NAPI_PROPERTY("removeWrap", remove_wrap),
+    DECLARE_NAPI_PROPERTY("addFinalizerOnly", add_finalizer_only),
     DECLARE_NAPI_PROPERTY("testFinalizeWrap", test_finalize_wrap),
     DECLARE_NAPI_PROPERTY("finalizeWasCalled", finalize_was_called),
     DECLARE_NAPI_PROPERTY("derefItemWasCalled", deref_item_was_called),