(fix) qt coin selection: signatures for coins would persist in memory

Scenario: select some UTXOs in the 'Coins' tab. Create a tx and sign it.
Close the tx dialog without broadcasting/etc (cancel tx).
Signatures would remain for selected UTXOs.
Create new tx -> invalid sigs.

electrum/gui/qt/utxo_list.py

@@ -25,6 +25,7 @@
 
 from typing import Optional, List, Dict, Sequence, Set
 from enum import IntEnum
+import copy
 
 from PyQt5.QtCore import Qt
 from PyQt5.QtGui import QStandardItemModel, QStandardItem, QFont
@@ -140,7 +141,8 @@ class UTXOList(MyTreeView):
     def get_spend_list(self) -> Optional[Sequence[PartialTxInput]]:
         if self._spend_set is None:
             return None
-        return [self.utxo_dict[x] for x in self._spend_set]
+        utxos = [self.utxo_dict[x] for x in self._spend_set]
+        return copy.deepcopy(utxos)  # copy so that side-effects don't affect utxo_dict
 
     def _maybe_reset_spend_list(self, current_wallet_utxos: Sequence[PartialTxInput]) -> None:
         if self._spend_set is None:

electrum/transaction.py

@@ -1344,6 +1344,12 @@ class PartialTxInput(TxInput, PSBTSection):
             self._is_p2sh_segwit = calc_if_p2sh_segwit_now()
         return self._is_p2sh_segwit
 
+    def already_has_some_signatures(self) -> bool:
+        """Returns whether progress has been made towards completing this input."""
+        return (self.part_sigs
+                or self.script_sig is not None
+                or self.witness is not None)
+
 
 class PartialTxOutput(TxOutput, PSBTSection):
     def __init__(self, *args, **kwargs):

electrum/wallet.py

@@ -978,6 +978,9 @@ class Abstract_Wallet(AddressSynchronizer):
                                   outputs: List[PartialTxOutput], fee=None,
                                   change_addr: str = None, is_sweep=False) -> PartialTransaction:
 
+        if any([c.already_has_some_signatures() for c in coins]):
+            raise Exception("Some inputs already contain signatures!")
+
         # prevent side-effect with '!'
         outputs = copy.deepcopy(outputs)