fix(lightning): improve error message for unsupported LND partial self-payments

cashu/lightning/lnd_grpc/lnd_grpc.py

@@ -306,9 +306,14 @@ async def pay_partial_invoice(
                     break
         except AioRpcError as e:
             logger.error(f"QueryRoute or SendToRouteV2 failed: {e}")
+            err_msg = str(e)
+            if settings.mint_lnd_allow_self_payment and (
+                "target not found" in err_msg.lower() or "no route" in err_msg.lower()
+            ):
+                err_msg += " (Note: partial self-payments are unsupported by LND's QueryRoutes API)"
             return PaymentResponse(
                 result=PaymentResult.FAILED,
-                error_message=str(e),
+                error_message=err_msg,
             )
 
         assert route and response

cashu/lightning/lndrest.py

@@ -323,6 +323,11 @@ async def pay_partial_invoice(
             route_data = route.json()
             if route.is_error or route_data.get("message"):
                 error_message = route_data.get("message") or route.text
+                if settings.mint_lnd_allow_self_payment and (
+                    "target not found" in error_message.lower()
+                    or "no route" in error_message.lower()
+                ):
+                    error_message += " (Note: partial self-payments are unsupported by LND's QueryRoutes API)"
                 return PaymentResponse(
                     result=PaymentResult.FAILED, error_message=error_message
                 )

tests/lightning/test_lnd_partial_mocked.py

@@ -0,0 +1,131 @@
+from types import SimpleNamespace
+
+import grpc.aio
+import pytest
+from httpx import Response
+
+from cashu.core.base import Amount, MeltQuote, MeltQuoteState, Unit
+from cashu.lightning.base import PaymentResult
+from cashu.lightning.lnd_grpc.lnd_grpc import LndRPCWallet
+from cashu.lightning.lndrest import LndRestWallet
+
+
+def _quote(request: str, amount: int = 1) -> MeltQuote:
+    return MeltQuote(
+        quote="q1",
+        method="bolt11",
+        request=request,
+        checking_id="checking-1",
+        unit="sat",
+        amount=amount,
+        fee_reserve=1,
+        state=MeltQuoteState.unpaid,
+    )
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "enabled, error_contains", [(True, "QueryRoutes API"), (False, "target not found")]
+)
+async def test_lndrest_pay_partial_invoice_self_payment(
+    monkeypatch, enabled, error_contains
+):
+    wallet = object.__new__(LndRestWallet)
+    wallet.unit = Unit.sat
+    wallet.supports_mpp = True
+    wallet.endpoint = "http://localhost:8080"
+    wallet.macaroon = "macaroon"
+    wallet.cert = None
+
+    class MockClient:
+        async def post(self, url, **kwargs):
+            if "/v1/graph/routes/" in url:
+                return Response(status_code=500, json={"message": "target not found"})
+            return Response(status_code=200, json={})
+
+    wallet.client = MockClient()
+
+    monkeypatch.setattr(
+        "cashu.lightning.lndrest.bolt11.decode",
+        lambda request: SimpleNamespace(
+            amount_msat=2000,
+            tags=SimpleNamespace(get=lambda x: SimpleNamespace(data="fake_pubkey")),
+        ),
+    )
+    monkeypatch.setattr(
+        "cashu.lightning.lndrest.settings.mint_lnd_allow_self_payment",
+        enabled,
+    )
+
+    result = await wallet.pay_partial_invoice(
+        _quote("lnbc1fake", amount=1),
+        amount=Amount(unit=Unit.sat, amount=1),
+        fee_limit_msat=1000,
+    )
+    assert result.result == PaymentResult.FAILED
+    assert error_contains in result.error_message
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "enabled, error_contains", [(True, "QueryRoutes API"), (False, "target not found")]
+)
+async def test_lndrpc_pay_partial_invoice_self_payment(
+    monkeypatch, enabled, error_contains
+):
+    wallet = object.__new__(LndRPCWallet)
+    wallet.unit = Unit.sat
+    wallet.supports_mpp = True
+    wallet.endpoint = "localhost:10009"
+    wallet.combined_creds = None
+
+    class MockStub:
+        def __init__(self, channel):
+            pass
+
+        async def QueryRoutes(self, request):
+            raise grpc.aio.AioRpcError(
+                code=grpc.StatusCode.UNKNOWN,
+                initial_metadata=None,
+                trailing_metadata=None,
+                details="target not found",
+            )
+
+    class MockChannel:
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, exc_type, exc, tb):
+            return False
+
+    monkeypatch.setattr(
+        "cashu.lightning.lnd_grpc.lnd_grpc.grpc.aio.secure_channel",
+        lambda *args, **kwargs: MockChannel(),
+    )
+    monkeypatch.setattr(
+        "cashu.lightning.lnd_grpc.lnd_grpc.lightningstub.LightningStub",
+        MockStub,
+    )
+    monkeypatch.setattr(
+        "cashu.lightning.lnd_grpc.lnd_grpc.routerstub.RouterStub",
+        MockStub,
+    )
+    monkeypatch.setattr(
+        "cashu.lightning.lnd_grpc.lnd_grpc.bolt11.decode",
+        lambda request: SimpleNamespace(
+            amount_msat=2000,
+            tags=SimpleNamespace(get=lambda x: SimpleNamespace(data="fake_pubkey")),
+        ),
+    )
+    monkeypatch.setattr(
+        "cashu.lightning.lnd_grpc.lnd_grpc.settings.mint_lnd_allow_self_payment",
+        enabled,
+    )
+
+    result = await wallet.pay_partial_invoice(
+        _quote("lnbc1fake", amount=1),
+        amount=Amount(unit=Unit.sat, amount=1),
+        fee_limit_msat=1000,
+    )
+    assert result.result == PaymentResult.FAILED
+    assert error_contains in result.error_message