use specific keyset for returned change (#351)

cashu/mint/ledger.py

@@ -315,7 +315,11 @@ async def mint(
         return promises
 
     async def melt(
-        self, proofs: List[Proof], invoice: str, outputs: Optional[List[BlindedMessage]]
+        self,
+        proofs: List[Proof],
+        invoice: str,
+        outputs: Optional[List[BlindedMessage]],
+        keyset: Optional[MintKeyset] = None,
     ) -> Tuple[bool, str, List[BlindedSignature]]:
         """Invalidates proofs and pays a Lightning invoice.
 
@@ -386,6 +390,7 @@ async def melt(
                     invoice_amount=invoice_amount,
                     ln_fee_msat=payment.fee_msat,
                     outputs=outputs,
+                    keyset=keyset,
                 )
 
         except Exception as e:

tests/test_wallet.py

@@ -243,10 +243,17 @@ async def test_melt(wallet1: Wallet):
     assert fee_reserve_sat == 2
     _, send_proofs = await wallet1.split_to_send(wallet1.proofs, total_amount)
 
-    await wallet1.pay_lightning(
+    melt_response = await wallet1.pay_lightning(
         send_proofs, invoice=invoice.bolt11, fee_reserve_sat=fee_reserve_sat
     )
 
+    assert melt_response.change
+    assert len(melt_response.change) == 1
+    # NOTE: we assume that we will get a token back from the same keyset as the ones we melted
+    # this could be wrong if we melted tokens from an old keyset but the returned ones are
+    # from a newer one.
+    assert melt_response.change[0].id == send_proofs[0].id
+
     # verify that proofs in proofs_used db have the same melt_id as the invoice in the db
     assert invoice.payment_hash
     invoice_db = await get_lightning_invoice(