MINOR: quic/pacing: implement quic_pacer engine

Extend quic_pacer engine to support pacing emission. Several functions
are defined.
* quic_pacing_sent_done() to notify engine about an emission of one or
  several datagrams
* quic_pacing_expired() to check if emission should be delayed or can be
  conducted immediately

Author: a-denoyelle

include/haproxy/quic_cc-t.h

@@ -123,6 +123,7 @@ struct quic_cc_algo {
 	void (*state_trace)(struct buffer *buf, const struct quic_cc *cc);
 	void (*state_cli)(struct buffer *buf, const struct quic_cc_path *path);
 	void (*hystart_start_round)(struct quic_cc *cc, uint64_t pn);
+	uint (*pacing_rate)(const struct quic_cc *cc);
 };
 
 #endif /* USE_QUIC */

include/haproxy/quic_cc.h

@@ -107,6 +107,11 @@ static inline size_t quic_cc_path_prep_data(struct quic_cc_path *path)
 	return path->cwnd - path->prep_in_flight;
 }
 
+static inline uint quic_cc_default_pacing_rate(const struct quic_cc *cc)
+{
+	struct quic_cc_path *path = container_of(cc, struct quic_cc_path, cc);
+	return path->loss.srtt * 1000000 / (path->cwnd / path->mtu + 1);
+}
 
 #endif /* USE_QUIC */
 #endif /* _PROTO_QUIC_CC_H */

include/haproxy/quic_pacing-t.h

@@ -7,6 +7,7 @@
 struct quic_pacer {
 	struct list *frms;
 	const struct quic_cc *cc;
+	ullong next;
 };
 
 #endif /* _HAPROXY_QUIC_PACING_T_H */

include/haproxy/quic_pacing.h

@@ -12,6 +12,11 @@ static inline void quic_pacing_init(struct quic_pacer *pacer,
 {
 	pacer->cc = cc;
 	pacer->frms = frms;
+	pacer->next = 0;
 }
 
+int quic_pacing_expired(const struct quic_pacer *pacer);
+
+void quic_pacing_sent_done(struct quic_pacer *pacer);
+
 #endif /* _HAPROXY_QUIC_PACING_H */

src/quic_pacing.c

@@ -1 +1,15 @@
 #include <haproxy/quic_pacing.h>
+
+#include <haproxy/quic_tx.h>
+
+/* Returns true if <pacer> timer is expired and emission can be retried. */
+int quic_pacing_expired(const struct quic_pacer *pacer)
+{
+	return !pacer->next || pacer->next <= now_mono_time();
+}
+
+/* Notify <pacer> about an emission of one datagram. */
+void quic_pacing_sent_done(struct quic_pacer *pacer)
+{
+	pacer->next = now_mono_time() + pacer->cc->algo->pacing_rate(pacer->cc);
+}