Revert "http2: streamline OnStreamRead streamline memory accounting"

This reverts commit 51ccf1b.

Fixes: #31089

Backport-PR-URL: #34845
PR-URL: #34315
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>

src/node_http2.cc

@@ -1840,11 +1840,7 @@ void Http2Session::OnStreamRead(ssize_t nread, const uv_buf_t& buf_) {
 
   statistics_.data_received += nread;
 
-  if (LIKELY(stream_buf_offset_ == 0)) {
-    // Shrink to the actual amount of used data.
-    buf.Resize(nread);
-    IncrementCurrentSessionMemory(nread);
-  } else {
+  if (UNLIKELY(stream_buf_offset_ > 0)) {
     // 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
@@ -1854,17 +1850,20 @@ void Http2Session::OnStreamRead(ssize_t nread, const uv_buf_t& buf_) {
     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);