subd: clear transient billboard on start and shutdown.

Use NULL on the callback to mean "clear the slot", and call it.

We have do this in two places: the old daemon might die, or the new
daemon might start first.

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

lightningd/channel.c

@@ -326,11 +326,13 @@ void channel_set_billboard(struct channel *channel, bool perm, const char *str)
 		p = &channel->billboard.permanent[channel->state];
 	else
 		p = &channel->billboard.transient;
-	tal_free(*p);
+	*p = tal_free(*p);
 
-	*p = tal_fmt(channel, "%s:%s", channel_state_name(channel), str);
-	if (taken(str))
-		tal_free(str);
+	if (str) {
+		*p = tal_fmt(channel, "%s:%s", channel_state_name(channel), str);
+		if (taken(str))
+			tal_free(str);
+	}
 }
 
 void channel_fail_transient(struct channel *channel, const char *fmt, ...)

lightningd/opening_control.c

@@ -490,8 +490,9 @@ static void opening_channel_set_billboard(struct uncommitted_channel *uc,
 					  bool perm UNUSED,
 					  const char *happenings TAKES)
 {
-	tal_free(uc->transient_billboard);
-	uc->transient_billboard = tal_strdup(uc, happenings);
+	uc->transient_billboard = tal_free(uc->transient_billboard);
+	if (happenings)
+		uc->transient_billboard = tal_strdup(uc, happenings);
 }
 
 static void destroy_uncommitted_channel(struct uncommitted_channel *uc)

lightningd/subd.c

@@ -585,6 +585,8 @@ static void destroy_subd(struct subd *sd)
 		struct db *db = sd->ld->wallet->db;
 		bool outer_transaction;
 
+		/* Clear any transient messages in billboard */
+		sd->billboardcb(channel, false, NULL);
 		sd->channel = NULL;
 
 		/* We can be freed both inside msg handling, or spontaneously. */
@@ -695,6 +697,9 @@ static struct subd *new_subd(struct lightningd *ld,
 
 	log_debug(sd->log, "pid %u, msgfd %i", sd->pid, msg_fd);
 
+	/* Clear any old transient message. */
+	if (billboardcb)
+		billboardcb(sd->channel, false, NULL);
 	return sd;
 }