channeld: give some tolerance for empty commitments.

The spec says not to send a commitment_signed without any changes, but LND
does this.  To understand why, you have to understand how LND works.  I
haven't read the code, but I'm pretty sure it works like this:

1. lnd slows down to do garbage collection, because it's in Go.
2. When an alert timer goes off, noticing it's not making process, it
   sends a twitter message to @Roasbeef.
3. @Roasbeef sshs into the user's machine and binary patches lnd to send
   a commitment_signed message.
4. Unfortunately he works so fast that various laws of causality are broken,
   meaning sometimes the commitment_signed is sent before any of thes
   other things happen.

I'm fairly sure that this will stop as @Roasbeef ages, or lnd introduces
some kind of causality enforcement fix.

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

CHANGELOG.md

@@ -36,7 +36,7 @@ changes.
 - `--bind-addr=<path>` fixed for nodes using local sockets (eg. testing).
 - Unannounced local channels were forgotten for routing on restart until reconnection occurred.
 - lightning-cli: arguments containing `"` now succeed, rather than causing JSON errors.
-- protocol: handle lnd sending more messages before `reestablish`; don't fail channel.
+- protocol: handle lnd sending more messages before `reestablish`; don't fail channel, and handle older lnd's spurious empty commitments.
 
 ### Security
 

channeld/channeld.c

@@ -157,6 +157,9 @@ struct peer {
 
 	/* Additional confirmations need for local lockin. */
 	u32 depth_togo;
+
+	/* Empty commitments.  Spec violation, but a minor one. */
+	u64 last_empty_commitment;
 };
 
 static u8 *create_channel_announcement(const tal_t *ctx, struct peer *peer);
@@ -1344,9 +1347,13 @@ static void handle_peer_commit_sig(struct peer *peer, const u8 *msg)
 		 *   - MUST NOT send a `commitment_signed` message that does not
 		 *     include any updates.
 		 */
-		peer_failed(&peer->cs,
-			    &peer->channel_id,
-			    "commit_sig with no changes");
+		status_trace("Oh hi LND! Empty commitment at #%"PRIu64,
+			     peer->next_index[LOCAL]);
+		if (peer->last_empty_commitment == peer->next_index[LOCAL] - 1)
+			peer_failed(&peer->cs,
+				    &peer->channel_id,
+				    "commit_sig with no changes (again!)");
+		peer->last_empty_commitment = peer->next_index[LOCAL];
 	}
 
 	/* We were supposed to check this was affordable as we go. */
@@ -2967,6 +2974,7 @@ int main(int argc, char *argv[])
 	peer->last_update_timestamp = 0;
 	/* We actually received it in the previous daemon, but near enough */
 	peer->last_recv = time_now();
+	peer->last_empty_commitment = 0;
 
 	/* We send these to HSM to get real signatures; don't have valgrind
 	 * complain. */