src: render N-API weak callbacks as cleanup hooks

Since worker threads are complete Node.js environments, including the
ability to load native addons, and since those native addons can
allocate resources to be freed when objects go out of scope, and since,
upon worker thread exit, the engine does not invoke the weak callbacks
responsible for freeing resources which still have references, this
modification introduces tracking for weak references such that a list
of outstanding weak references is maintained. This list is traversed
during environment teardown. The callbacks for the remaining weak
references are called.

This change is also relevant for Node.js embedder scenarios, because in
those cases the process also outlives the `node::Environment` and
therefore weak callbacks should also be rendered as environment cleanup
hooks to ensure proper cleanup after native addons. This changes
introduces the means by which this can be accomplished.

A benchmark is included which measures the time it takes to execute the
weak reference callback for a given number of weak references.

Re: https://github.com/tc39/proposal-weakrefs/issues/125#issuecomment-535832130
PR-URL: https://github.com/nodejs/node/pull/28428
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

3 files changed, 109 insertions, 0 deletions

diff --git a/benchmark/napi/ref/addon.c b/benchmark/napi/ref/addon.c
new file mode 100644
index 0000000000..3fb8de603d
--- /dev/null
+++ b/benchmark/napi/ref/addon.c
@@ -0,0 +1,82 @@
+#include <stdlib.h>
+#define NAPI_EXPERIMENTAL
+#include <node_api.h>
+
+#define NAPI_CALL(env, call)                          \
+  do {                                                \
+    napi_status status = (call);                      \
+    if (status != napi_ok) {                          \
+      napi_throw_error((env), NULL, #call " failed"); \
+      return NULL;                                    \
+    }                                                 \
+  } while (0)
+
+static napi_value
+GetCount(napi_env env, napi_callback_info info) {
+  napi_value result;
+  size_t* count;
+
+  NAPI_CALL(env, napi_get_instance_data(env, (void**)&count));
+  NAPI_CALL(env, napi_create_uint32(env, *count, &result));
+
+  return result;
+}
+
+static napi_value
+SetCount(napi_env env, napi_callback_info info) {
+  size_t* count;
+
+  NAPI_CALL(env, napi_get_instance_data(env, (void**)&count));
+
+  // Set the count to zero irrespective of what is passed into the setter.
+  *count = 0;
+
+  return NULL;
+}
+
+static void
+IncrementCounter(napi_env env, void* data, void* hint) {
+  size_t* count = data;
+  (*count) = (*count) + 1;
+}
+
+static napi_value
+NewWeak(napi_env env, napi_callback_info info) {
+  napi_value result;
+  void* instance_data;
+
+  NAPI_CALL(env, napi_create_object(env, &result));
+  NAPI_CALL(env, napi_get_instance_data(env, &instance_data));
+  NAPI_CALL(env, napi_add_finalizer(env,
+                                    result,
+                                    instance_data,
+                                    IncrementCounter,
+                                    NULL,
+                                    NULL));
+
+  return result;
+}
+
+static void
+FreeCount(napi_env env, void* data, void* hint) {
+  free(data);
+}
+
+/* napi_value */
+NAPI_MODULE_INIT(/* napi_env env, napi_value exports */) {
+  napi_property_descriptor props[] = {
+    { "count", NULL, NULL, GetCount, SetCount, NULL, napi_enumerable, NULL },
+    { "newWeak", NULL, NewWeak, NULL, NULL, NULL, napi_enumerable, NULL }
+  };
+
+  size_t* count = malloc(sizeof(*count));
+  *count = 0;
+
+  NAPI_CALL(env, napi_define_properties(env,
+                                        exports,
+                                        sizeof(props) / sizeof(*props),
+                                        props));
+  NAPI_CALL(env, napi_set_instance_data(env, count, FreeCount, NULL));
+
+  return exports;
+}
diff --git a/benchmark/napi/ref/binding.gyp b/benchmark/napi/ref/binding.gyp
new file mode 100644
index 0000000000..d641e99efd
--- /dev/null
+++ b/benchmark/napi/ref/binding.gyp
@@ -0,0 +1,10 @@
+{
+  'targets': [
+    {
+      'target_name': 'addon',
+      'sources': [
+        'addon.c'
+      ]
+    }
+  ]
+}
diff --git a/benchmark/napi/ref/index.js b/benchmark/napi/ref/index.js
new file mode 100644
index 0000000000..3a5e198827
--- /dev/null
+++ b/benchmark/napi/ref/index.js
@@ -0,0 +1,17 @@
+'use strict';
+const common = require('../../common');
+const addon = require(`./build/${common.buildType}/addon`);
+const bench = common.createBenchmark(main, { n: [1e7] });
+
+function callNewWeak() {
+  addon.newWeak();
+}
+
+function main({ n }) {
+  addon.count = 0;
+  bench.start();
+  while (addon.count < n) {
+    callNewWeak();
+  }
+  bench.end(n);
+}