dual-fund: Fail if you try to buy a liquidity ad w/o dualfunding on

Fixes #5271

In-Collaboration-With: Base58 'n Coding Seminar Participants

Changelog-Changed: `fundchannel` now errors if you try to buy a liquidity ad but dont' have `experimental-dual-fund` enabled

plugins/spender/multifundchannel.c

@@ -1611,13 +1611,21 @@ connect_ok(struct command *cmd,
 			   json_tok_full_len(features_tok),
 			   json_tok_full(buf, features_tok));
 
+	dest->state = MULTIFUNDCHANNEL_CONNECTED;
+
 	/* Set the open protocol to use now */
 	if (feature_negotiated(plugin_feature_set(mfc->cmd->plugin),
 			       dest->their_features,
 			       OPT_DUAL_FUND))
 		dest->protocol = OPEN_CHANNEL;
+	else if (!amount_sat_zero(dest->request_amt) || !(!dest->rates))
+		/* Return an error */
+		fail_destination_msg(dest, FUNDING_V2_NOT_SUPPORTED,
+				     "Tried to buy a liquidity ad"
+				     " but we(?) don't have"
+				     " experimental-dual-fund"
+				     " enabled");
 
-	dest->state = MULTIFUNDCHANNEL_CONNECTED;
 	return connect_done(dest);
 }
 

tests/test_opening.py

@@ -1391,3 +1391,22 @@ def test_zeroconf_forward(node_factory, bitcoind):
     wait_for(lambda: len(l3.rpc.listchannels()['channels']) == 4)
     inv = l1.rpc.invoice(42, 'back1', 'desc')['bolt11']
     l3.rpc.pay(inv)
+
+
+@pytest.mark.openchannel('v1')
+def test_buy_liquidity_ad_no_v2(node_factory, bitcoind):
+    """ Test that you can't actually request amt for a
+    node that doesn' support v2 opens """
+
+    l1, l2, = node_factory.get_nodes(2)
+    amount = 500000
+    feerate = 2000
+
+    l1.fundwallet(amount * 100)
+    l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
+
+    # l1 leases a channel from l2
+    with pytest.raises(RpcError, match=r"Tried to buy a liquidity ad but we[(][?][)] don't have experimental-dual-fund enabled"):
+        l1.rpc.fundchannel(l2.info['id'], amount, request_amt=amount,
+                           feerate='{}perkw'.format(feerate),
+                           compact_lease='029a002d000000004b2003e8')