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.

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);