src: add AliasedStruct utility

For http2 (and eventually QUIC) we have a struct that is backed by a v8::BackingStore and exposed to the JavaScript side as an ArrayBuffer and TypedArray. This is similar to AliasedBuffer except that it is fronted by a struct on the C++ side. ```c++ struct foo { uint32_t ex1; uint32_t ex2; }; AliasedStruct<foo> foo_; foo_->ex1 = 1; foo_->ex2 = 2; foo_.GetArrayBuffer(); ``` Signed-off-by: James M Snell <jasnell@gmail.com>
nodejs · jasnell · Apr 11, 2020 · Apr 12, 2020 · Apr 13, 2020 · Apr 13, 2020
commit 63c95030179e8b583d02b0f75fb0b29af2dfbd1b
diff --git a/node.gyp b/node.gyp
@@ -631,6 +631,7 @@
         'src/uv.cc',
         # headers to make for a more pleasant IDE experience
         'src/aliased_buffer.h',
+        'src/aliased_struct.h',
         'src/async_wrap.h',
         'src/async_wrap-inl.h',
         'src/base_object.h',

diff --git a/src/aliased_struct.h b/src/aliased_struct.h
@@ -0,0 +1,73 @@
+#ifndef SRC_ALIASED_STRUCT_H_
+#define SRC_ALIASED_STRUCT_H_
+
+#if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#include "node_internals.h"
+#include "v8.h"
+#include <memory>
+
+namespace node {
+
+template <typename T,
+          typename = std::enable_if_t<std::is_object<T>::value>>
+class AliasedStruct {
+ public:
+  explicit AliasedStruct(v8::Isolate* isolate) : isolate_(isolate) {
+    const v8::HandleScope handle_scope(isolate);
+
+    store_ = v8::ArrayBuffer::NewBackingStore(isolate, sizeof(T));
+    ptr_ = new (store_->Data()) T;
+    DCHECK_NOT_NULL(ptr_);
+
+    v8::Local<v8::ArrayBuffer> buffer = v8::ArrayBuffer::New(isolate, store_);
+    buffer_ = v8::Global<v8::ArrayBuffer>(isolate, buffer);
+  }
+
+  AliasedStruct(const AliasedStruct& that)
+     : isolate_(that.isolate_),
+       store_(that.store_),
+       ptr_(that.ptr_) {
+    buffer_ = v8::Global<v8::ArrayBuffer>(that.isolate_, that.GetArrayBuffer());
+  }
+
+  ~AliasedStruct() {
+    if (ptr_ != nullptr) ptr_->~T();
+  }
+
+  AliasedStruct& operator=(AliasedStruct&& that) noexcept {
+    this->~AliasedStruct();
+    isolate_ = that.isolate_;
+    store_ = that.store_;
+    ptr_ = that.ptr_;
+
+    buffer_.Reset(isolate_, that.buffer_.Get(isolate_));
+
+    that.ptr_ = nullptr;
+    that.store_.reset();
+    that.buffer_.Reset();
+    return *this;
+  }
+
+  v8::Local<v8::ArrayBuffer> GetArrayBuffer() const {
+    return buffer_.Get(isolate_);
+  }
+
+  T* Data() const { return ptr_; }
+
+  T* operator*() const { return ptr_; }
+
+  T* operator->() const { return ptr_; }
+
+ private:
+  v8::Isolate* isolate_;
+  std::shared_ptr<v8::BackingStore> store_;
+  T* ptr_;
+  v8::Global<v8::ArrayBuffer> buffer_;
+};
+
+}  // namespace node
+
+#endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#endif  // SRC_ALIASED_STRUCT_H_
diff --git a/src/node_http2.cc b/src/node_http2.cc
@@ -18,7 +18,6 @@ namespace node {
 using v8::Array;
 using v8::ArrayBuffer;
 using v8::ArrayBufferView;
-using v8::BackingStore;
 using v8::Boolean;
 using v8::Context;
 using v8::Float64Array;
@@ -471,6 +470,7 @@ Http2Session::Http2Session(Http2State* http2_state,
                            Local<Object> wrap,
                            nghttp2_session_type type)
     : AsyncWrap(http2_state->env(), wrap, AsyncWrap::PROVIDER_HTTP2SESSION),
+      js_fields_(http2_state->env()->isolate()),
       session_type_(type),
       http2_state_(http2_state) {
   MakeWeak();
@@ -518,14 +518,8 @@ Http2Session::Http2Session(Http2State* http2_state,
   outgoing_storage_.reserve(1024);
   outgoing_buffers_.reserve(32);
 
-  // Make the js_fields_ property accessible to JS land.
-  js_fields_store_ =
-      ArrayBuffer::NewBackingStore(env()->isolate(), sizeof(SessionJSFields));
-  js_fields_ = new(js_fields_store_->Data()) SessionJSFields;
-
-  Local<ArrayBuffer> ab = ArrayBuffer::New(env()->isolate(), js_fields_store_);
   Local<Uint8Array> uint8_arr =
-      Uint8Array::New(ab, 0, kSessionUint8FieldCount);
+      Uint8Array::New(js_fields_.GetArrayBuffer(), 0, kSessionUint8FieldCount);
   USE(wrap->Set(env()->context(), env()->fields_string(), uint8_arr));
 }
 
@@ -536,7 +530,6 @@ Http2Session::~Http2Session() {
   // current_nghttp2_memory_ check passes.
   session_.reset();
   CHECK_EQ(current_nghttp2_memory_, 0);
-  js_fields_->~SessionJSFields();
 }
 
 std::string Http2Session::diagnostic_name() const {

diff --git a/src/node_http2.h b/src/node_http2.h
@@ -7,6 +7,7 @@
 #include <cstdint>
 #include "nghttp2/nghttp2.h"
 
+#include "aliased_struct.h"
 #include "node_http2_state.h"
 #include "node_http_common.h"
 #include "node_mem.h"
@@ -823,8 +824,7 @@ class Http2Session : public AsyncWrap,
   Nghttp2SessionPointer session_;
 
   // JS-accessible numeric fields, as indexed by SessionUint8Fields.
-  SessionJSFields* js_fields_ = nullptr;
-  std::shared_ptr<v8::BackingStore> js_fields_store_;
+  AliasedStruct<SessionJSFields> js_fields_;
 
   // The session type: client or server
   nghttp2_session_type session_type_;