htlc: Allow for exactly min_final_cltv_expiry cltv delta

We are announcing that we are willing to accept incoming payments with
current_height + min_final_cltv_expiry + slack, assuming that the
sender adds some slack. In particular we'd reject the payment if
slack=0 which is allowed by the spec.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

lightningd/peer_htlcs.c

@@ -359,9 +359,9 @@ static void handle_localpay(struct htlc_in *hin,
 	 * If the `cltv_expiry` is too low, the final node MUST fail the HTLC:
 	 */
 	if (get_block_height(ld->topology) + ld->config.cltv_final
-	    >= cltv_expiry) {
+	    > cltv_expiry) {
 		log_debug(hin->key.peer->log,
-			  "Expiry cltv %u too close to current %u + %u",
+			  "Expiry cltv too soon %u < %u + %u",
 			  cltv_expiry,
 			  get_block_height(ld->topology),
 			  ld->config.cltv_final);

tests/test_lightningd.py

@@ -1631,9 +1631,9 @@ class LightningDTests(BaseLightningDTests):
         # Takes 6 blocks to timeout (cltv-final + 1), but we also give grace period of 1 block.
         bitcoind.rpc.generate(5 + 1)
         assert not l1.daemon.is_in_log('hit deadline')
-        bitcoind.rpc.generate(1)
+        bitcoind.rpc.generate(2)
 
-        l1.daemon.wait_for_log('Offered HTLC 0 SENT_ADD_ACK_REVOCATION cltv {} hit deadline'.format(bitcoind.rpc.getblockcount()-1))
+        l1.daemon.wait_for_log('Offered HTLC 0 SENT_ADD_ACK_REVOCATION cltv .* hit deadline')
         l1.daemon.wait_for_log('sendrawtx exit 0')
         l1.bitcoin.rpc.generate(1)
         l1.daemon.wait_for_log('-> ONCHAIND_OUR_UNILATERAL')
@@ -1668,12 +1668,12 @@ class LightningDTests(BaseLightningDTests):
         # l1 will drop to chain, not reconnect.
         l1.daemon.wait_for_log('dev_disconnect: -WIRE_REVOKE_AND_ACK')
 
-        # Deadline HTLC expity minus 1/2 cltv-expiry delta (rounded up) (== cltv - 3).  ctlv is 5+1.
+        # Deadline HTLC expiry minus 1/2 cltv-expiry delta (rounded up) (== cltv - 3).  ctlv is 5+1.
         bitcoind.rpc.generate(2)
         assert not l2.daemon.is_in_log('hit deadline')
-        bitcoind.rpc.generate(1)
+        bitcoind.rpc.generate(2)
 
-        l2.daemon.wait_for_log('Fulfilled HTLC 0 SENT_REMOVE_COMMIT cltv {} hit deadline'.format(bitcoind.rpc.getblockcount()+3))
+        l2.daemon.wait_for_log('Fulfilled HTLC 0 SENT_REMOVE_COMMIT cltv .* hit deadline')
         l2.daemon.wait_for_log('sendrawtx exit 0')
         l2.bitcoin.rpc.generate(1)
         l2.daemon.wait_for_log('-> ONCHAIND_OUR_UNILATERAL')