pay: specify the channel to send the first hop.

If we only specify the node_id, we get the "first" channel.

Closes: #5803
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: Plugins: `pay` uses the correct local channel for payments when there are multiple available (not just always the first!)

plugins/libplugin-pay.c

@@ -1573,6 +1573,7 @@ static struct command_result *payment_createonion_success(struct command *cmd,
 	json_add_amount_msat_only(req->js, "amount_msat", first->amount);
 	json_add_num(req->js, "delay", first->delay);
 	json_add_node_id(req->js, "id", &first->node_id);
+	json_add_short_channel_id(req->js, "channel", &first->scid);
 	json_object_end(req->js);
 
 	json_add_sha256(req->js, "payment_hash", p->payment_hash);

tests/test_pay.py

@@ -5305,7 +5305,6 @@ def test_payerkey(node_factory):
         assert n.rpc.decode(b12)['invreq_payer_id'] == k
 
 
-@pytest.mark.xfail(strict=True)
 def test_cln_sendpay_weirdness(bitcoind, node_factory):
     # 0. Setup