MINOR: mux-quic: simplify empty emitted STREAM FIN handling

An empty STREAM frame can be emitted by QUIC MUX to notify about a
delayed FIN. This requires tedious comparison in qmux_ctrl_send() to
differentiate between the first empty frame and a retransmitted one.

Simplify this by unsubscribe from streamdesc layer when the QCS is
locally closed after FIN transmission. This prevents all future
retransmitted frames to be notify to the QCS instance, especially any
retransmitted empty FIN.

Author: a-denoyelle

src/mux_quic.c

@@ -563,15 +563,19 @@ static void qmux_ctrl_send(struct qc_stream_desc *stream, uint64_t data, uint64_
 	/* Real off MUST always be the greatest offset sent. */
 	BUG_ON(offset > qcs->tx.fc.off_real);
 
-	/* check if the STREAM frame has already been notified. It can happen
-	 * for retransmission. Special care must be taken to ensure an empty
-	 * STREAM frame with FIN set is not considered as retransmitted
+	/* Check if the STREAM frame has already been notified. An empty FIN
+	 * frame must not be considered retransmitted.
 	 */
-	if (offset + data < qcs->tx.fc.off_real || (!data && !(qcs->flags & QC_SF_FIN_STREAM))) {
+	if (data && offset + data <= qcs->tx.fc.off_real) {
 		TRACE_DEVEL("offset already notified", QMUX_EV_QCS_SEND, qcc->conn, qcs);
 		goto out;
 	}
 
+	/* An empty STREAM frame is only used to notify FIN. A retransmitted
+	 * empty FIN cannot be notified as QCS will be unsubscribed first.
+	 */
+	BUG_ON(!data && !(qcs->flags & QC_SF_FIN_STREAM));
+
 	qcs_idle_open(qcs);
 
 	diff = offset + data - qcs->tx.fc.off_real;
@@ -622,6 +626,9 @@ static void qmux_ctrl_send(struct qc_stream_desc *stream, uint64_t data, uint64_
 				/* Reset flag to not emit multiple FIN STREAM frames. */
 				qcs->flags &= ~QC_SF_FIN_STREAM;
 			}
+
+			/* Unsubscribe from streamdesc when everything sent. */
+			qc_stream_desc_sub_send(qcs->stream, NULL);
 		}
 	}