lightningd: make sure gossipd knows before we update blockheight.

Without this, we can get spurious failures from lnprototest, which
is waiting for lightningd to acknowledge the blockheight in getinfo:

```
2021-12-21T00:56:21.460Z DEBUG   lightningd: Adding block 109: 57a7bd3ade3a88a899e5b442075e9722ccec07e0205f9a913b304a3e2e038e26
2021-12-21T00:56:21.470Z DEBUG   lightningd: Adding block 110: 11a280eb76f588e92e20c39999be9d2baff016c3c6bac1837b649a270570b7dd
2021-12-21T00:56:21.479Z DEBUG   lightningd: Adding block 111: 02977fc9529b2ab4e0a805c4bc1bcfbff5a4e6577a8b31266341d22e204a1d27
2021-12-21T00:56:21.487Z DEBUG   lightningd: Adding block 112: 2402f31c5ddfc9e847e8bbfb7df084d29a5d5d936a4358c109f2f4cf9ea8d828
2021-12-21T00:56:21.496Z DEBUG   lightningd: Adding block 113: 5a561fe9423b4df33f004fc09985ee3ef38364d692a56a8b27ecbc6098a16d39
2021-12-21T00:56:21.505Z DEBUG   lightningd: Adding block 114: 4502f5ec23c89177872846848848322e8fa6c3fb6f5eb361194e4cd47596dfe9
2021-12-21T00:56:21.511Z DEBUG   02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9-gossipd: Ignoring future channel_announcment for 109x1x0 (current block 108)
```

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

gossipd/gossipd.c

@@ -1178,6 +1178,9 @@ static void new_blockheight(struct daemon *daemon, const u8 *msg)
 		tal_arr_remove(&daemon->deferred_txouts, i);
 		i--;
 	}
+
+	daemon_conn_send(daemon->master,
+			 take(towire_gossipd_new_blockheight_reply(NULL)));
 }
 
 #if DEVELOPER
@@ -1499,6 +1502,7 @@ static struct io_plan *recv_req(struct io_conn *conn,
 	case WIRE_GOSSIPD_DEV_COMPACT_STORE_REPLY:
 	case WIRE_GOSSIPD_GOT_ONIONMSG_TO_US:
 	case WIRE_GOSSIPD_ADDGOSSIP_REPLY:
+	case WIRE_GOSSIPD_NEW_BLOCKHEIGHT_REPLY:
 		break;
 	}
 

gossipd/gossipd_wire.csv

@@ -74,6 +74,9 @@ msgdata,gossipd_dev_compact_store_reply,success,bool,
 msgtype,gossipd_new_blockheight,3026
 msgdata,gossipd_new_blockheight,blockheight,u32,
 
+# gossipd: got it!
+msgtype,gossipd_new_blockheight_reply,3126
+
 msgtype,gossipd_got_onionmsg_to_us,3145
 msgdata,gossipd_got_onionmsg_to_us,obs2,bool,
 msgdata,gossipd_got_onionmsg_to_us,node_alias,pubkey,

gossipd/test/run-onion_message.c

@@ -338,6 +338,9 @@ u8 *towire_gossipd_got_onionmsg_to_us(const tal_t *ctx UNNEEDED, bool obs2 UNNEE
 /* Generated stub for towire_gossipd_init_reply */
 u8 *towire_gossipd_init_reply(const tal_t *ctx UNNEEDED)
 { fprintf(stderr, "towire_gossipd_init_reply called!\n"); abort(); }
+/* Generated stub for towire_gossipd_new_blockheight_reply */
+u8 *towire_gossipd_new_blockheight_reply(const tal_t *ctx UNNEEDED)
+{ fprintf(stderr, "towire_gossipd_new_blockheight_reply called!\n"); abort(); }
 /* Generated stub for towire_gossipd_new_peer_reply */
 u8 *towire_gossipd_new_peer_reply(const tal_t *ctx UNNEEDED, bool success UNNEEDED, const struct gossip_state *gs UNNEEDED)
 { fprintf(stderr, "towire_gossipd_new_peer_reply called!\n"); abort(); }

lightningd/gossip_control.c

@@ -1,5 +1,6 @@
 #include "config.h"
 #include <ccan/err/err.h>
+#include <ccan/ptrint/ptrint.h>
 #include <common/json_command.h>
 #include <common/json_helpers.h>
 #include <common/json_tok.h>
@@ -132,6 +133,7 @@ static unsigned gossip_msg(struct subd *gossip, const u8 *msg, const int *fds)
 	case WIRE_GOSSIPD_DEV_COMPACT_STORE_REPLY:
 	case WIRE_GOSSIPD_GET_STRIPPED_CUPDATE_REPLY:
 	case WIRE_GOSSIPD_ADDGOSSIP_REPLY:
+	case WIRE_GOSSIPD_NEW_BLOCKHEIGHT_REPLY:
 		break;
 
 	case WIRE_GOSSIPD_GOT_ONIONMSG_TO_US:
@@ -144,14 +146,32 @@ static unsigned gossip_msg(struct subd *gossip, const u8 *msg, const int *fds)
 	return 0;
 }
 
+static void gossipd_new_blockheight_reply(struct subd *gossipd,
+					  const u8 *reply,
+					  const int *fds UNUSED,
+					  void *blockheight)
+{
+	if (!fromwire_gossipd_new_blockheight_reply(reply)) {
+		/* Shouldn't happen! */
+		log_broken(gossipd->ld->log,
+			   "Invalid new_blockheight_reply from gossipd: %s",
+			   tal_hex(tmpctx, reply));
+		return;
+	}
+
+	/* Now, finally update getinfo's blockheight */
+	gossipd->ld->blockheight = ptr2int(blockheight);
+}
+
 void gossip_notify_new_block(struct lightningd *ld, u32 blockheight)
 {
 	/* Only notify gossipd once we're synced. */
 	if (!topology_synced(ld->topology))
 		return;
 
-	subd_send_msg(ld->gossip,
-		      take(towire_gossipd_new_blockheight(NULL, blockheight)));
+	subd_req(ld->gossip, ld->gossip,
+		 take(towire_gossipd_new_blockheight(NULL, blockheight)),
+		 -1, 0, gossipd_new_blockheight_reply, int2ptr(blockheight));
 }
 
 static void gossip_topology_synced(struct chain_topology *topo, void *unused)

lightningd/lightningd.c

@@ -215,6 +215,7 @@ static struct lightningd *new_lightningd(const tal_t *ctx)
 
 	/*~ This is detailed in chaintopology.c */
 	ld->topology = new_topology(ld, ld->log);
+	ld->blockheight = 0;
 	ld->daemon_parent_fd = -1;
 	ld->proxyaddr = NULL;
 	ld->always_use_proxy = false;

lightningd/lightningd.h

@@ -167,6 +167,9 @@ struct lightningd {
 	/* Our chain topology. */
 	struct chain_topology *topology;
 
+	/* Blockheight (as acknowledged by gossipd) */
+	u32 blockheight;
+
 	/* HTLCs in flight. */
 	struct htlc_in_map htlcs_in;
 	struct htlc_out_map htlcs_out;

lightningd/peer_control.c

@@ -1749,7 +1749,7 @@ static struct command_result *json_getinfo(struct command *cmd,
         json_array_end(response);
     }
     json_add_string(response, "version", version());
-    json_add_num(response, "blockheight", get_block_height(cmd->ld->topology));
+    json_add_num(response, "blockheight", cmd->ld->blockheight);
     json_add_string(response, "network", chainparams->network_name);
     json_add_amount_msat_compat(response,
 				wallet_total_forward_fees(cmd->ld->wallet),