peer: use tip mediantime for CSV timeout.

Using wallclock is gauche (and I saw it fail once in tests), so fix that
FIXME now it's easy.

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

daemon/chaintopology.c

@@ -516,7 +516,27 @@ u32 get_last_mediantime(struct lightningd_state *dstate,
 	last_mediantime(topo->root, w, &mediantime);
 	return mediantime;
 }
-	
+
+u32 get_tip_mediantime(struct lightningd_state *dstate)
+{
+	struct topology *topo = dstate->topology;
+	size_t i, longest = 0;
+	u32 mediantime;
+
+	mediantime = topo->tips[longest]->mediantime;
+
+	for (i = 1; i < tal_count(topo->tips); i++) {
+		if (topo->tips[i]->height > topo->tips[longest]->height) {
+			longest = i;
+			mediantime = topo->tips[longest]->mediantime;
+		} else if (topo->tips[i]->height == topo->tips[longest]->height) {
+			if (topo->tips[i]->mediantime > mediantime)
+				mediantime = topo->tips[i]->mediantime;
+		}
+	}
+	return mediantime;
+}
+
 void setup_topology(struct lightningd_state *dstate)
 {
 	dstate->topology = tal(dstate, struct topology);

daemon/chaintopology.h

@@ -14,6 +14,10 @@ size_t get_tx_depth(struct lightningd_state *dstate, const struct txwatch *w);
  * Assumes the depth is > 0! */
 u32 get_last_mediantime(struct lightningd_state *dstate,
 			const struct sha256_double *txid);
+
+/* Get mediantime of the tip; if more than one, pick greatest time. */
+u32 get_tip_mediantime(struct lightningd_state *dstate);
+
 void setup_topology(struct lightningd_state *dstate);
 
 #endif /* LIGHTNING_DAEMON_CRYPTOPKT_H */

daemon/peer.c

@@ -848,17 +848,18 @@ static void commit_tx_depth(struct peer *peer, int depth,
 		return;
 
 	mediantime = get_last_mediantime(peer->dstate, txid);
-
 	assert(mediantime);
-	
-	/* FIXME: We should really use bitcoin time here. */
-	if (controlled_time().ts.tv_sec > mediantime
+
+	if (get_tip_mediantime(peer->dstate) > mediantime
 	    + rel_locktime_to_seconds(&peer->them.locktime)) {
 		/* Free this watch; we're done */
 		peer->cur_commit.watch = tal_free(peer->cur_commit.watch);
 		state_event(peer, ptr2int(canspend), NULL);
 	} else
-		log_debug(peer->log, "... still CSV locked");
+		log_debug(peer->log, "... still CSV locked (mediantime %u, need %u + %u)",
+			  get_tip_mediantime(peer->dstate),
+			  mediantime,
+			  rel_locktime_to_seconds(&peer->them.locktime));
 }
 
 /* We should map back from commit_tx permutation to figure out what happened. */