http2: streamline OnStreamRead streamline memory accounting

* avoid consecutive decrement/increment session memory calls.
* only Resize the buffer when it is needed.
* flip `stream_buf_offset_` condition to the LIKELY case.
PR-URL: #30351
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: lundibundi

src/node_http2.cc

@@ -1862,7 +1862,11 @@ void Http2Session::OnStreamRead(ssize_t nread, const uv_buf_t& buf_) {
 
  statistics_.data_received += nread;
 
-if (UNLIKELY(stream_buf_offset_ > 0)) {
+if (LIKELY(stream_buf_offset_ == 0)) {
+// Shrink to the actual amount of used data.
+ buf.Resize(nread);
+IncrementCurrentSessionMemory(nread);
+ } else {
 // This is a very unlikely case, and should only happen if the ReadStart()
 // call in OnStreamAfterWrite() immediately provides data. If that does
 // happen, we concatenate the data we received with the already-stored
@@ -1871,20 +1875,18 @@ void Http2Session::OnStreamRead(ssize_t nread, const uv_buf_t& buf_) {
  AllocatedBuffer new_buf = env()->AllocateManaged(pending_len + nread);
 memcpy(new_buf.data(), stream_buf_.base + stream_buf_offset_, pending_len);
 memcpy(new_buf.data() + pending_len, buf.data(), nread);
+
+// The data in stream_buf_ is already accounted for, add nread received
+// bytes to session memory but remove the already processed
+// stream_buf_offset_ bytes.
+IncrementCurrentSessionMemory(nread - stream_buf_offset_);
+
  buf = std::move(new_buf);
  nread = buf.size();
  stream_buf_offset_ = 0;
  stream_buf_ab_.Reset();
-
-// We have now fully processed the stream_buf_ input chunk (by moving the
-// remaining part into buf, which will be accounted for below).
-DecrementCurrentSessionMemory(stream_buf_.len);
  }
 
-// Shrink to the actual amount of used data.
- buf.Resize(nread);
-IncrementCurrentSessionMemory(nread);
-
 // Remember the current buffer, so that OnDataChunkReceived knows the
 // offset of a DATA frame's data into the socket read buffer.
  stream_buf_ = uv_buf_init(buf.data(), nread);