http2: remove redundant write indirection

`nghttp2_stream_write_t` was not a necessary redirection layer
and came with the cost of one additional allocation per stream write.

Also, having both `nghttp2_stream_write` and `nghttp2_stream_write_t`
as identifiers did not help with readability.

    $ ./node benchmark/compare.js --new ./node --old ./node-before --runs 100 --filter write --set streams=20 --set length=1024 --set size=16 http2 | Rscript benchmark/compare.R
    [00:26:09|% 100| 1/1 files | 200/200 runs | 1/1 configs]: Done
                                                                         improvement confidence     p.value
     http2/write.js benchmarker="h2load" size=16 length=1024 streams=20      0.58 %         ** 0.005535241

src/node_http2.cc

@@ -1369,30 +1369,6 @@ inline void Http2Stream::AddChunk(const uint8_t* data, size_t len) {
   data_chunks_.emplace(uv_buf_init(buf, len));
 }
 
-// The Http2Stream class is a subclass of StreamBase. The DoWrite method
-// receives outbound chunks of data to send as outbound DATA frames. These
-// are queued in an internal linked list of uv_buf_t structs that are sent
-// when nghttp2 is ready to serialize the data frame.
-int Http2Stream::DoWrite(WriteWrap* req_wrap,
-                         uv_buf_t* bufs,
-                         size_t count,
-                         uv_stream_t* send_handle) {
-  CHECK(!this->IsDestroyed());
-  session_->SetChunksSinceLastWrite();
-
-  nghttp2_stream_write_t* req = new nghttp2_stream_write_t;
-  req->data = req_wrap;
-
-  auto AfterWrite = [](nghttp2_stream_write_t* req, int status) {
-    WriteWrap* wrap = static_cast<WriteWrap*>(req->data);
-    wrap->Done(status);
-    delete req;
-  };
-  req_wrap->Dispatched();
-  Write(req, bufs, count, AfterWrite);
-  return 0;
-}
-
 
 inline void Http2Stream::Close(int32_t code) {
   CHECK(!this->IsDestroyed());
@@ -1447,7 +1423,7 @@ inline void Http2Stream::Destroy() {
     // we still have qeueued outbound writes.
     while (!stream->queue_.empty()) {
       nghttp2_stream_write* head = stream->queue_.front();
-      head->cb(head->req, UV_ECANCELED);
+      head->req_wrap->Done(UV_ECANCELED);
       delete head;
       stream->queue_.pop();
     }
@@ -1616,26 +1592,32 @@ inline int Http2Stream::ReadStop() {
   return 0;
 }
 
+// The Http2Stream class is a subclass of StreamBase. The DoWrite method
+// receives outbound chunks of data to send as outbound DATA frames. These
+// are queued in an internal linked list of uv_buf_t structs that are sent
+// when nghttp2 is ready to serialize the data frame.
+//
 // Queue the given set of uv_but_t handles for writing to an
-// nghttp2_stream. The callback will be invoked once the chunks
-// of data have been flushed to the underlying nghttp2_session.
+// nghttp2_stream. The WriteWrap's Done callback will be invoked once the
+// chunks of data have been flushed to the underlying nghttp2_session.
 // Note that this does *not* mean that the data has been flushed
 // to the socket yet.
-inline int Http2Stream::Write(nghttp2_stream_write_t* req,
-                              const uv_buf_t bufs[],
-                              unsigned int nbufs,
-                              nghttp2_stream_write_cb cb) {
+inline int Http2Stream::DoWrite(WriteWrap* req_wrap,
+                                uv_buf_t* bufs,
+                                size_t nbufs,
+                                uv_stream_t* send_handle) {
   CHECK(!this->IsDestroyed());
+  CHECK_EQ(send_handle, nullptr);
   Http2Scope h2scope(this);
+  session_->SetChunksSinceLastWrite();
+  req_wrap->Dispatched();
   if (!IsWritable()) {
-    if (cb != nullptr)
-      cb(req, UV_EOF);
+    req_wrap->Done(UV_EOF);
     return 0;
   }
   DEBUG_HTTP2STREAM2(this, "queuing %d buffers to send", id_, nbufs);
   nghttp2_stream_write* item = new nghttp2_stream_write;
-  item->cb = cb;
-  item->req = req;
+  item->req_wrap = req_wrap;
   item->nbufs = nbufs;
   item->bufs.AllocateSufficientStorage(nbufs);
   memcpy(*(item->bufs), bufs, nbufs * sizeof(*bufs));
@@ -1824,7 +1806,7 @@ ssize_t Http2Stream::Provider::Stream::OnRead(nghttp2_session* handle,
       stream->queue_offset_ = 0;
     }
     if (stream->queue_index_ == head->nbufs) {
-      head->cb(head->req, 0);
+      head->req_wrap->Done(0);
       delete head;
       stream->queue_.pop();
       stream->queue_offset_ = 0;

src/node_http2.h

@@ -91,8 +91,6 @@ void inline debug_vfprintf(const char* format, ...) {
 
 #define MAX_BUFFER_COUNT 16
 
-struct nghttp2_stream_write_t;
-
 enum nghttp2_session_type {
   NGHTTP2_SESSION_SERVER,
   NGHTTP2_SESSION_CLIENT
@@ -127,15 +125,9 @@ enum nghttp2_stream_options {
   STREAM_OPTION_GET_TRAILERS = 0x2,
 };
 
-// Callbacks
-typedef void (*nghttp2_stream_write_cb)(
-    nghttp2_stream_write_t* req,
-    int status);
-
 struct nghttp2_stream_write {
   unsigned int nbufs = 0;
-  nghttp2_stream_write_t* req = nullptr;
-  nghttp2_stream_write_cb cb = nullptr;
+  WriteWrap* req_wrap = nullptr;
   MaybeStackBuffer<uv_buf_t, MAX_BUFFER_COUNT> bufs;
 };
 
@@ -146,11 +138,6 @@ struct nghttp2_header {
 };
 
 
-struct nghttp2_stream_write_t {
-  void* data;
-  int status;
-};
-
 // Unlike the HTTP/1 implementation, the HTTP/2 implementation is not limited
 // to a fixed number of known supported HTTP methods. These constants, therefore
 // are provided strictly as a convenience to users and are exposed via the
@@ -558,13 +545,6 @@ class Http2Stream : public AsyncWrap,
 
   Http2Session* session() { return session_; }
 
-  // Queue outbound chunks of data to be sent on this stream
-  inline int Write(
-      nghttp2_stream_write_t* req,
-      const uv_buf_t bufs[],
-      unsigned int nbufs,
-      nghttp2_stream_write_cb cb);
-
   inline bool HasDataChunks(bool ignore_eos = false);
 
   inline void AddChunk(const uint8_t* data, size_t len);