MEDIUM: h2: enable connection polling for send when a cs wants to emit

A conn_stream indicates its intent to send by setting the WR_ENA flag
and calling mux->update_poll(). There's no synchronous write so the only
way to emit a response from a stream is to proceed this way. The sender
h2s is then queued into the h2c's send_list if it was not yet queued.

Once the connection is ready, it will enter its send() callback to visit
writers, calling their data->send_cb() callback to complete the operation
using mux->snd_buf().

Also we enable polling if the mux contains data and wasn't enabled. This
may happen just after a response has been transmitted using chk_snd().
It likely is incomplete for now and should probably be refined.

src/mux_h2.c

@@ -938,6 +938,36 @@ static struct conn_stream *h2_attach(struct connection *conn)
  */
 static void h2_update_poll(struct conn_stream *cs)
 {
+	struct h2s *h2s = cs->ctx;
+
+	if (!h2s)
+		return;
+
+	/* Note: the stream and stream-int code doesn't allow us to perform a
+	 * synchronous send() here unfortunately, because this code is called
+	 * as si_update() from the process_stream() context. This means that
+	 * we have to queue the current cs and defer its processing after the
+	 * connection's cs list is processed anyway.
+	 */
+
+	if (cs->flags & CS_FL_DATA_WR_ENA) {
+		if (LIST_ISEMPTY(&h2s->list)) {
+			if (LIST_ISEMPTY(&h2s->h2c->send_list) &&
+			    !h2s->h2c->mbuf->o && // not yet subscribed
+			    !(cs->conn->flags & CO_FL_SOCK_WR_SH))
+				conn_xprt_want_send(cs->conn);
+			LIST_ADDQ(&h2s->h2c->send_list, &h2s->list);
+		}
+	}
+	else if (!LIST_ISEMPTY(&h2s->list)) {
+		LIST_DEL(&h2s->list);
+		LIST_INIT(&h2s->list);
+		h2s->flags &= ~(H2_SF_BLK_MBUSY | H2_SF_BLK_MROOM | H2_SF_BLK_MFCTL);
+	}
+
+	/* this can happen from within si_chk_snd() */
+	if (h2s->h2c->mbuf->o && !(cs->conn->flags & CO_FL_XPRT_WR_ENA))
+		conn_xprt_want_send(cs->conn);
 }
 
 /*