worker: add SharedArrayBuffer sharing

Logic is added to the `MessagePort` mechanism that
attaches hidden objects to those instances when they are transferred
that track their lifetime and maintain a reference count, to make
sure that memory is freed at the appropriate times.

Thanks to Stephen Belanger for reviewing this change in its original PR.

Refs: https://github.com/ayojs/ayo/pull/106

PR-URL: https://github.com/nodejs/node/pull/20876
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>

src/sharedarraybuffer_metadata.cc

@@ -0,0 +1,129 @@
+#include "sharedarraybuffer_metadata.h"
+#include "base_object.h"
+#include "base_object-inl.h"
+#include "node_errors.h"
+
+using v8::Context;
+using v8::Function;
+using v8::FunctionTemplate;
+using v8::Local;
+using v8::Maybe;
+using v8::MaybeLocal;
+using v8::Nothing;
+using v8::Object;
+using v8::SharedArrayBuffer;
+using v8::Value;
+
+namespace node {
+namespace worker {
+
+namespace {
+
+// Yield a JS constructor for SABLifetimePartner objects in the form of a
+// standard API object, that has a single field for containing the raw
+// SABLiftimePartner* pointer.
+Local<Function> GetSABLifetimePartnerConstructor(
+    Environment* env, Local<Context> context) {
+  Local<FunctionTemplate> templ;
+  templ = env->sab_lifetimepartner_constructor_template();
+  if (!templ.IsEmpty())
+    return templ->GetFunction(context).ToLocalChecked();
+
+  templ = BaseObject::MakeLazilyInitializedJSTemplate(env);
+  templ->SetClassName(FIXED_ONE_BYTE_STRING(env->isolate(),
+                                            "SABLifetimePartner"));
+  env->set_sab_lifetimepartner_constructor_template(templ);
+
+  return GetSABLifetimePartnerConstructor(env, context);
+}
+
+class SABLifetimePartner : public BaseObject {
+ public:
+  SABLifetimePartner(Environment* env,
+                     Local<Object> obj,
+                     SharedArrayBufferMetadataReference r)
+    : BaseObject(env, obj),
+      reference(r) {
+    MakeWeak();
+  }
+
+  SharedArrayBufferMetadataReference reference;
+};
+
+}  // anonymous namespace
+
+SharedArrayBufferMetadataReference
+SharedArrayBufferMetadata::ForSharedArrayBuffer(
+    Environment* env,
+    Local<Context> context,
+    Local<SharedArrayBuffer> source) {
+  Local<Value> lifetime_partner;
+
+  if (!source->GetPrivate(context,
+                          env->sab_lifetimepartner_symbol())
+                              .ToLocal(&lifetime_partner)) {
+    return nullptr;
+  }
+
+  if (lifetime_partner->IsObject() &&
+      env->sab_lifetimepartner_constructor_template()
+         ->HasInstance(lifetime_partner)) {
+    CHECK(source->IsExternal());
+    SABLifetimePartner* partner =
+        Unwrap<SABLifetimePartner>(lifetime_partner.As<Object>());
+    CHECK_NE(partner, nullptr);
+    return partner->reference;
+  }
+
+  if (source->IsExternal()) {
+    // If this is an external SharedArrayBuffer but we do not see a lifetime
+    // partner object, it was not us who externalized it. In that case, there
+    // is no way to serialize it, because it's unclear how the memory
+    // is actually owned.
+    THROW_ERR_TRANSFERRING_EXTERNALIZED_SHAREDARRAYBUFFER(env);
+    return nullptr;
+  }
+
+  SharedArrayBuffer::Contents contents = source->Externalize();
+  SharedArrayBufferMetadataReference r(new SharedArrayBufferMetadata(
+      contents.Data(), contents.ByteLength()));
+  if (r->AssignToSharedArrayBuffer(env, context, source).IsNothing())
+    return nullptr;
+  return r;
+}
+
+Maybe<bool> SharedArrayBufferMetadata::AssignToSharedArrayBuffer(
+    Environment* env, Local<Context> context,
+    Local<SharedArrayBuffer> target) {
+  CHECK(target->IsExternal());
+  Local<Function> ctor = GetSABLifetimePartnerConstructor(env, context);
+  Local<Object> obj;
+  if (!ctor->NewInstance(context).ToLocal(&obj))
+    return Nothing<bool>();
+
+  new SABLifetimePartner(env, obj, shared_from_this());
+  return target->SetPrivate(context,
+                            env->sab_lifetimepartner_symbol(),
+                            obj);
+}
+
+SharedArrayBufferMetadata::SharedArrayBufferMetadata(void* data, size_t size)
+  : data(data), size(size) { }
+
+SharedArrayBufferMetadata::~SharedArrayBufferMetadata() {
+  free(data);
+}
+
+MaybeLocal<SharedArrayBuffer> SharedArrayBufferMetadata::GetSharedArrayBuffer(
+    Environment* env, Local<Context> context) {
+  Local<SharedArrayBuffer> obj =
+      SharedArrayBuffer::New(env->isolate(), data, size);
+
+  if (AssignToSharedArrayBuffer(env, context, obj).IsNothing())
+    return MaybeLocal<SharedArrayBuffer>();
+
+  return obj;
+}
+
+}  // namespace worker
+}  // namespace node