Moves swapState up, so it gets executed no matter what. (#2649)

* Moves swapState up, so it gets executed no matter what.

* Adds an explanation.

internal/transport/http2_server.go

@@ -1030,6 +1030,9 @@ func (t *http2Server) deleteStream(s *Stream, eosReceived bool) {
 // closeStream clears the footprint of a stream when the stream is not needed
 // any more.
 func (t *http2Server) closeStream(s *Stream, rst bool, rstCode http2.ErrCode, hdr *headerFrame, eosReceived bool) {
+	// Mark the stream as done
+	oldState := s.swapState(streamDone)
+
 	// In case stream sending and receiving are invoked in separate
 	// goroutines (e.g., bi-directional streaming), cancel needs to be
 	// called to interrupt the potential blocking on other goroutines.
@@ -1051,8 +1054,19 @@ func (t *http2Server) closeStream(s *Stream, rst bool, rstCode http2.ErrCode, hd
 		return
 	}
 
+	// We do the check here, because of the following scenario:
+	// 1. closeStream is called first with a trailer. A trailer item with a piggybacked cleanup item
+	// is put to control buffer.
+	// 2. Loopy writer is waiting on a stream quota. It will never get it because client errored at
+	// some point. So loopy can't act on trailer
+	// 3. Client sends a RST_STREAM due to the error. Then closeStream is called without a trailer as
+	// the result of the received RST_STREAM.
+	// If we do this check at the beginning of the closeStream, then we won't put a cleanup item in
+	// response to received RST_STREAM into the control buffer and outStream in loopy writer will
+	// never get cleaned up.
+
 	// If the stream is already done, don't send the trailer.
-	if s.swapState(streamDone) == streamDone {
+	if oldState == streamDone {
 		return
 	}