Some bug fixes related to change in txbuilder

Author: cffls

pycardano/txbuilder.py

@@ -751,6 +751,13 @@ def build(
             for v in self.withdrawals.values():
                 selected_amount.coin += v
 
+        can_merge_change = False
+        if merge_change:
+            for o in self.outputs:
+                if o.address == change_address:
+                    can_merge_change = True
+                    break
+
         selected_amount.coin -= self._get_total_key_deposit()
 
         requested_amount = Value()
@@ -771,14 +778,17 @@ def build(
 
         unfulfilled_amount = requested_amount - trimmed_selected_amount
 
-        if change_address is not None:
+        if change_address is not None and not can_merge_change:
             # If change address is provided and remainder is smaller than minimum ADA required in change,
             # we need to select additional UTxOs available from the address
-            unfulfilled_amount.coin = max(
-                0,
-                unfulfilled_amount.coin
-                + min_lovelace(selected_amount - trimmed_selected_amount, self.context),
-            )
+            if unfulfilled_amount.coin < 0:
+                unfulfilled_amount.coin = max(
+                    0,
+                    unfulfilled_amount.coin
+                    + min_lovelace(
+                        selected_amount - trimmed_selected_amount, self.context
+                    ),
+                )
         else:
             unfulfilled_amount.coin = max(0, unfulfilled_amount.coin)
 
@@ -808,6 +818,7 @@ def build(
                         [TransactionOutput(None, unfulfilled_amount)],
                         self.context,
                         include_max_fee=False,
+                        respect_min_utxo=not can_merge_change,
                     )
                     for s in selected:
                         selected_amount += s.output.amount

test/pycardano/test_txbuilder.py

@@ -1,6 +1,7 @@
 from dataclasses import replace
 from test.pycardano.test_key import SK
 from test.pycardano.util import chain_context
+from unittest.mock import patch
 
 import pytest
 
@@ -924,3 +925,82 @@ def test_tx_builder_merge_change_to_zero_amount_output(chain_context):
     }
 
     assert expected == tx_body.to_primitive()
+
+
+def test_tx_builder_small_utxo_input(chain_context):
+    with patch.object(chain_context, "utxos") as mock_utxos:
+        mock_utxos.return_value = [
+            UTxO(
+                TransactionInput.from_primitive(
+                    [
+                        "41cb004bec7051621b19b46aea28f0657a586a05ce2013152ea9b9f1a5614cc7",
+                        1,
+                    ]
+                ),
+                TransactionOutput.from_primitive(
+                    [
+                        "addr1qytqt3v9ej3kzefxcy8f59h9atf2knracnj5snkgtaea6p4r8g3mu652945v3gldw7v88dn5lrfudx0un540ak9qt2kqhfjl0d",
+                        2991353,
+                    ]
+                ),
+            )
+        ]
+        builder = TransactionBuilder(chain_context)
+        address = Address.from_primitive(
+            "addr1qytqt3v9ej3kzefxcy8f59h9atf2knracnj5snkgtaea6p4r8g3mu652945v3gldw7v88dn5lrfudx0un540ak9qt2kqhfjl0d"
+        )
+        builder.add_input_address(address)
+
+        builder.add_output(
+            TransactionOutput(
+                Address.from_primitive(
+                    "addr1qyady0evsaxqsfmz0z8rvmq62fmuas5w8n4m8z6qcm4wrt3e8dlsen8n464ucw69acfgdxgguscgfl5we3rwts4s57ashysyee"
+                ),
+                Value.from_primitive(
+                    [
+                        1000000,
+                    ]
+                ),
+            )
+        )
+        signed_tx = builder.build(change_address=address)
+
+
+def test_tx_builder_merge_change_to_output_3(chain_context):
+    with patch.object(chain_context, "utxos") as mock_utxos:
+        mock_utxos.return_value = [
+            UTxO(
+                TransactionInput.from_primitive(
+                    [
+                        "41cb004bec7051621b19b46aea28f0657a586a05ce2013152ea9b9f1a5614cc7",
+                        1,
+                    ]
+                ),
+                TransactionOutput.from_primitive(
+                    [
+                        "addr1qytqt3v9ej3kzefxcy8f59h9atf2knracnj5snkgtaea6p4r8g3mu652945v3gldw7v88dn5lrfudx0un540ak9qt2kqhfjl0d",
+                        2991353,
+                    ]
+                ),
+            )
+        ]
+        builder = TransactionBuilder(chain_context)
+        address = Address.from_primitive(
+            "addr1qytqt3v9ej3kzefxcy8f59h9atf2knracnj5snkgtaea6p4r8g3mu652945v3gldw7v88dn5lrfudx0un540ak9qt2kqhfjl0d"
+        )
+        builder.add_input_address(address)
+
+        builder.add_output(
+            TransactionOutput(
+                Address.from_primitive(
+                    "addr1qytqt3v9ej3kzefxcy8f59h9atf2knracnj5snkgtaea6p4r8g3mu652945v3gldw7v88dn5lrfudx0un540ak9qt2kqhfjl0d"
+                ),
+                Value.from_primitive(
+                    [
+                        1000000,
+                    ]
+                ),
+            )
+        )
+        tx = builder.build(change_address=address, merge_change=True)
+        assert len(tx.outputs) == 1