gossipd: suppress redundant local updates which we would generate.

This doesn't do anything for us now, since we actually tend to produce
DISABLE/ENABLE update pairs.  But the infrastructure is useful for the
next patch.

We also add more details to the trace message in the core update code.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

gossipd/gossip.c

@@ -1656,6 +1656,21 @@ static u8 *create_channel_update(const tal_t *ctx,
 	return update;
 }
 
+/* Return true if the only change would be the timestamp. */
+static bool update_redundant(const struct half_chan *hc,
+			   bool disable, u16 cltv_delta, u64 htlc_minimum_msat,
+			   u32 fee_base_msat, u32 fee_proportional_millionths)
+{
+	if (!is_halfchan_defined(hc))
+		return false;
+
+	return !(hc->flags & ROUTING_FLAGS_DISABLED) == !disable
+		&& hc->delay == cltv_delta
+		&& hc->htlc_minimum_msat == htlc_minimum_msat
+		&& hc->base_fee == fee_base_msat
+		&& hc->proportional_fee == fee_proportional_millionths;
+}
+
 static void handle_local_channel_update(struct peer *peer, const u8 *msg)
 {
 	struct short_channel_id scid;
@@ -1701,6 +1716,25 @@ static void handle_local_channel_update(struct peer *peer, const u8 *msg)
 		return;
 	}
 
+	/* Avoid redundant updates on public channels: on non-public channels
+	 * we'd need to consider pending updates, so don't bother. */
+	if (is_chan_public(chan)
+	    && update_redundant(&chan->half[direction],
+				disable, cltv_delta, htlc_minimum_msat,
+				fee_base_msat, fee_proportional_millionths)) {
+		status_trace("Suppressing redundant channel update for %s:(%u) %s %"PRIu64"/%u vs %u/%u",
+			     type_to_string(tmpctx, struct short_channel_id,
+					    &scid),
+			     direction,
+			     is_halfchan_defined(&chan->half[direction])
+			     ? (chan->half[direction].flags & ROUTING_FLAGS_DISABLED ? "DISABLED" : "ACTIVE")
+			     : "UNDEFINED",
+			     chan->half[direction].last_timestamp,
+			     (u32)time_now().ts.tv_sec,
+			     chan->half[direction].flags, disable);
+		return;
+	}
+
 	cupdate = create_channel_update(tmpctx, peer->daemon->rstate,
 					chan, direction,
 					disable, cltv_delta,

gossipd/routing.c

@@ -1207,11 +1207,14 @@ u8 *handle_channel_update(struct routing_state *rstate, const u8 *update,
 		return err;
 	}
 
-	status_trace("Received channel_update for channel %s(%d) now %s (from %s)",
+	status_trace("Received channel_update for channel %s(%d) now %s was %s (from %s)",
 		     type_to_string(tmpctx, struct short_channel_id,
 				    &short_channel_id),
 		     flags & 0x01,
 		     flags & ROUTING_FLAGS_DISABLED ? "DISABLED" : "ACTIVE",
+		     is_halfchan_defined(c)
+		     ? (c->flags & ROUTING_FLAGS_DISABLED ? "DISABLED" : "ACTIVE")
+		     : "UNDEFINED",
 		     source);
 
 	if (!routing_add_channel_update(rstate, serialized))