relay: Precalculate frame properties before writing to sendCh

Previously, we wrote a frame to sendCh, then checked properties on the
frame. After writing to sendCh, the receiver may write out the frame and
release the frame back to the pool, before we've had a chance to run.

This meant we could be deleting the wrong relay item (or not deleting
anything), which manifested itself as a timeout error frame being sent
after a call response frame.

relay.go

@@ -197,14 +197,16 @@ func (r *Relayer) Relay(f *Frame) error {
 
 // Receive receives frames intended for this connection.
 func (r *Relayer) Receive(f *Frame, fType frameType) {
+	id := f.Header.ID
+
 	// If we receive a response frame, we expect to find that ID in our outbound.
 	// If we receive a request frame, we expect to find that ID in our inbound.
 	items := r.receiverItems(fType)
 
-	item, ok := items.Get(f.Header.ID)
+	item, ok := items.Get(id)
 	if !ok {
 		r.logger.WithFields(
-			LogField{"ID", f.Header.ID},
+			LogField{"id", id},
 		).Warn("Received a frame without a RelayItem.")
 		return
 	}
@@ -221,12 +223,17 @@ func (r *Relayer) Receive(f *Frame, fType frameType) {
 		}
 	}
 
+	// When we write the frame to sendCh, we lose ownership of the frame, and it
+	// may be released to the frame pool at any point.
+	finished := finishesCall(f)
+
 	// TODO: Add some sort of timeout here to avoid blocking forever on a
 	// stalled connection.
 	r.conn.sendCh <- f
-	if finishesCall(f) {
+
+	if finished {
 		items := r.receiverItems(fType)
-		r.finishRelayItem(items, f.Header.ID)
+		r.finishRelayItem(items, id)
 	}
 }