psbt: allow insecure signing of legacy UTXOs without full previous tx

When "importing" a psbt, we accept witness utxos even for legacy inputs
(warning shown to user in gui).
When "exporting" a psbt, we follow the spec; except when exporting as a QR code,
in which case we include witness utxos for all inputs.
This makes QR codes for psbts with legacy inputs feasible, just like they
were before, with our custom tx serialization format (with the same risk,
of burning coins as miner fees).

electrum/gui/qt/transaction_dialog.py

@@ -31,7 +31,7 @@ import time
 from typing import TYPE_CHECKING, Callable
 
 from PyQt5.QtCore import QSize, Qt
-from PyQt5.QtGui import QTextCharFormat, QBrush, QFont
+from PyQt5.QtGui import QTextCharFormat, QBrush, QFont, QPixmap
 from PyQt5.QtWidgets import (QDialog, QLabel, QPushButton, QHBoxLayout, QVBoxLayout,
                              QTextEdit, QFrame, QAction, QToolButton, QMenu)
 import qrcode
@@ -45,8 +45,9 @@ from electrum.util import bfh
 from electrum.transaction import SerializationError, Transaction, PartialTransaction, PartialTxInput
 from electrum.logging import get_logger
 
-from .util import (MessageBoxMixin, read_QIcon, Buttons, CopyButton,
-                   MONOSPACE_FONT, ColorScheme, ButtonsLineEdit, text_dialog)
+from .util import (MessageBoxMixin, read_QIcon, Buttons, CopyButton, icon_path,
+                   MONOSPACE_FONT, ColorScheme, ButtonsLineEdit, text_dialog,
+                   char_width_in_lineedit)
 
 if TYPE_CHECKING:
     from .main_window import ElectrumWindow
@@ -241,6 +242,10 @@ class TxDialog(QDialog, MessageBoxMixin):
     def show_qr(self, *, tx: Transaction = None):
         if tx is None:
             tx = self.tx
+        tx = copy.deepcopy(tx)  # make copy as we mutate tx
+        if isinstance(tx, PartialTransaction):
+            # this makes QR codes a lot smaller (or just possible in the first place!)
+            tx.convert_all_utxos_to_witness_utxos()
         text = tx.serialize_as_bytes()
         text = base_encode(text, base=43)
         try:
@@ -389,6 +394,10 @@ class TxDialog(QDialog, MessageBoxMixin):
             feerate_warning = simple_config.FEERATE_WARNING_HIGH_FEE
             if fee_rate > feerate_warning:
                 fee_str += ' - ' + _('Warning') + ': ' + _("high fee") + '!'
+        if isinstance(self.tx, PartialTransaction):
+            risk_of_burning_coins = (can_sign and fee is not None
+                                     and self.tx.is_there_risk_of_burning_coins_as_fees())
+            self.fee_warning_icon.setVisible(risk_of_burning_coins)
         self.amount_label.setText(amount_str)
         self.fee_label.setText(fee_str)
         self.size_label.setText(size_str)
@@ -464,8 +473,24 @@ class TxDialog(QDialog, MessageBoxMixin):
         vbox_left.addWidget(self.date_label)
         self.amount_label = TxDetailLabel()
         vbox_left.addWidget(self.amount_label)
+
+        fee_hbox = QHBoxLayout()
         self.fee_label = TxDetailLabel()
-        vbox_left.addWidget(self.fee_label)
+        fee_hbox.addWidget(self.fee_label)
+        self.fee_warning_icon = QLabel()
+        pixmap = QPixmap(icon_path("warning"))
+        pixmap_size = round(2 * char_width_in_lineedit())
+        pixmap = pixmap.scaled(pixmap_size, pixmap_size, Qt.KeepAspectRatio, Qt.SmoothTransformation)
+        self.fee_warning_icon.setPixmap(pixmap)
+        self.fee_warning_icon.setToolTip(_("Warning") + ": "
+                                         + _("The fee could not be verified. Signing non-segwit inputs is risky:\n"
+                                             "if this transaction was maliciously modified before you sign,\n"
+                                             "you might end up paying a higher mining fee than displayed."))
+        self.fee_warning_icon.setVisible(False)
+        fee_hbox.addWidget(self.fee_warning_icon)
+        fee_hbox.addStretch(1)
+        vbox_left.addLayout(fee_hbox)
+
         vbox_left.addStretch(1)
         hbox_stats.addLayout(vbox_left, 50)