xpay: add regtest for blinded path fees

When the paying node is the entry point to the blinded path it will send
the wrong amount to the next peer. Instead of sending AMT_MSAT it will
send AMT_MSAT+FEES_MSAT.

        # we pay fees to ourselves
        htlcs = l1.rpc.listhtlcs()["htlcs"]
        assert len(htlcs) == 1
        assert htlcs[0]["payment_hash"] == b12_decode["invoice_payment_hash"]
>       assert htlcs[0]["amount_msat"] == AMT_MSAT
E       assert 15000 == 10000

Changelog-None

Signed-off-by: Lagrang3 <lagrang3@protonmail.com>

Author: Lagrang3

tests/test_xpay.py

@@ -1072,3 +1072,38 @@ def mock_getblockhash(req):
     # Now let it catch up, and it will retry, and succeed.
     l1.daemon.rpcproxy.mock_rpc('getblockhash')
     fut.result(TIMEOUT)
+
+
+@pytest.mark.xfail(strict=True)
+def test_blinded_path_fees(node_factory):
+    """Test that we don't send the amount+fees to our direct peer (we should
+    only send the required amount) when the sending node is the entry point in
+    the blinded path."""
+    AMT_MSAT = 10000
+    FEES_MSAT = 5000
+    l1, l2 = node_factory.get_nodes(
+        2, opts={"may_reconnect": True, "fee-base": FEES_MSAT, "fee-per-satoshi": 0}
+    )
+    node_factory.join_nodes([l1, l2], wait_for_announce=True)
+
+    offer = l2.rpc.offer(amount="any", fronting_nodes=[l1.info["id"]])["bolt12"]
+    b12 = l1.rpc.fetchinvoice(offer, AMT_MSAT)["invoice"]
+
+    b12_decode = l1.rpc.decode(b12)
+    assert b12_decode["invoice_amount_msat"] == AMT_MSAT
+    assert len(b12_decode["invoice_paths"]) == 1
+    assert b12_decode["invoice_paths"][0]["first_node_id"] == l1.info["id"]
+    assert b12_decode["invoice_paths"][0]["payinfo"]["fee_base_msat"] == FEES_MSAT
+    assert b12_decode["invoice_paths"][0]["payinfo"]["fee_proportional_millionths"] == 0
+
+    ret = l1.rpc.xpay(invstring=b12)
+    assert ret["failed_parts"] == 0
+    assert ret["successful_parts"] == 1
+    assert ret["amount_msat"] == AMT_MSAT
+    assert ret["amount_sent_msat"] == AMT_MSAT + FEES_MSAT
+
+    # we pay fees to ourselves
+    htlcs = l1.rpc.listhtlcs()["htlcs"]
+    assert len(htlcs) == 1
+    assert htlcs[0]["payment_hash"] == b12_decode["invoice_payment_hash"]
+    assert htlcs[0]["amount_msat"] == AMT_MSAT