MINOR: mux-quic: realign Tx buffer if possible

A major reorganization of QUIC MUX sending has been implemented. Now
data transfer occur over a single QCS buffer. This has improve
performance but at the cost of restrictions on snd_buf. Indeed, buffer
instances are now shared from stream callback snd_buf up to quic-conn
layer.

As such, snd_buf cannot manipulate freely already present data buffer.
In particular, realign has been completely removed by the previous
patches.

This commit reintroduces a partial realign support. This is only done if
the buffer contains only unsent data, via a new MUX function
qcc_realign_stream_txbuf() which is called during snd_buf.

include/haproxy/mux_quic.h

@@ -25,6 +25,7 @@ int qcc_notify_buf(struct qcc *qcc);
 
 struct buffer *qcc_get_stream_rxbuf(struct qcs *qcs);
 struct buffer *qcc_get_stream_txbuf(struct qcs *qcs, int *err);
+int qcc_realign_stream_txbuf(const struct qcs *qcs, struct buffer *out);
 int qcc_release_stream_txbuf(struct qcs *qcs);
 int qcc_stream_can_send(const struct qcs *qcs);
 void qcc_reset_stream(struct qcs *qcs, int err);

src/h3.c

@@ -1900,8 +1900,14 @@ static int h3_resp_data_send(struct qcs *qcs, struct htx *htx,
 	if (fsize > count)
 		fsize = count;
 
-	/* TODO buffer can be realign only if no data waiting for ACK. */
-	outbuf = b_make(b_tail(res), b_contig_space(res), 0, 0);
+	while (1) {
+		b_reset(&outbuf);
+		outbuf = b_make(b_tail(res), b_contig_space(res), 0, 0);
+		if (b_size(&outbuf) > hsize || !b_space_wraps(res))
+			break;
+		if (qcc_realign_stream_txbuf(qcs, res))
+			break;
+	}
 
 	/* Not enough room for headers and at least one data byte, try to
 	 * release the current buffer and allocate a new one. If not possible,
@@ -2082,7 +2088,12 @@ static size_t h3_nego_ff(struct qcs *qcs, size_t count)
 
 	/* h3 DATA headers : 1-byte frame type + varint frame length */
 	hsize = 1 + QUIC_VARINT_MAX_SIZE;
-	/* TODO buffer can be realign only if no data waiting for ACK. */
+	while (1) {
+		if (b_contig_space(res) >= hsize || !b_space_wraps(res))
+			break;
+		if (qcc_realign_stream_txbuf(qcs, res))
+			break;
+	}
 
 	/* Not enough room for headers and at least one data byte, block the
 	 * stream. It is expected that the stream connector layer will subscribe

src/mux_quic.c

@@ -1019,6 +1019,21 @@ static uint64_t qcs_prep_bytes(const struct qcs *qcs)
 	return b_data(out) - diff;
 }
 
+/* Try to realign <out> buffer for <qcs> stream. This is done only if there is
+ * no data waiting for ACK.
+ *
+ * Returns 0 if realign was performed else non-zero.
+ */
+int qcc_realign_stream_txbuf(const struct qcs *qcs, struct buffer *out)
+{
+	if (qcs_prep_bytes(qcs) == b_data(out)) {
+		b_slow_realign(out, trash.area, b_data(out));
+		return 0;
+	}
+
+	return 1;
+}
+
 /* Release the current <qcs> Tx buffer. This is useful if space left is not
  * enough anymore. A new instance can then be allocated to continue sending.
  *