qt tx dialog: show_qr to warn if QR code is missing data

When exporting a tx as qr code, the prev txs are omitted to save space.
This causes problems with offline signers: software electrum signers will
just warn and then proceed, but hw devices will typically error.

electrum/gui/kivy/uix/dialogs/tx_dialog.py

@@ -332,7 +332,7 @@ class TxDialog(Factory.Popup):
 
     def show_qr(self):
         original_raw_tx = str(self.tx)
-        qr_data = self.tx.to_qr_data()
+        qr_data = self.tx.to_qr_data()[0]
         self.app.qr_dialog(_("Raw Transaction"), qr_data, text_for_clipboard=original_raw_tx)
 
     def remove_local_tx(self):

electrum/gui/qml/qetxdetails.py

@@ -5,7 +5,7 @@ from PyQt5.QtCore import pyqtProperty, pyqtSignal, pyqtSlot, QObject
 from electrum.i18n import _
 from electrum.logging import get_logger
 from electrum.util import format_time, AddTransactionException, TxMinedInfo
-from electrum.transaction import tx_from_any
+from electrum.transaction import tx_from_any, Transaction
 from electrum.network import Network
 
 from .qewallet import QEWallet
@@ -31,7 +31,7 @@ class QETxDetails(QObject, QtEventListener):
         self._rawtx = ''
         self._label = ''
 
-        self._tx = None
+        self._tx = None  # type: Optional[Transaction]
 
         self._status = ''
         self._amount = QEAmount()
@@ -396,6 +396,6 @@ class QETxDetails(QObject, QtEventListener):
     def getSerializedTx(self, for_qr=False):
         tx = self._tx
         if for_qr:
-            return tx.to_qr_data()
+            return tx.to_qr_data()[0]
         else:
             return str(tx)

electrum/gui/qml/qewallet.py

@@ -733,6 +733,6 @@ class QEWallet(AuthMixin, QObject, QtEventListener):
     def getSerializedTx(self, txid, for_qr=False):
         tx = self.wallet.db.get_transaction(txid)
         if for_qr:
-            return tx.to_qr_data()
+            return tx.to_qr_data()[0]
         else:
             return str(tx)

electrum/gui/qt/transaction_dialog.py

@@ -595,9 +595,15 @@ class TxDialog(QDialog, MessageBoxMixin):
     def show_qr(self, *, tx: Transaction = None):
         if tx is None:
             tx = self.tx
-        qr_data = tx.to_qr_data()
+        qr_data, is_complete = tx.to_qr_data()
+        help_text = None
+        if not is_complete:
+            help_text = _(
+                """Warning: Some data (prev txs / "full utxos") was left """
+                """out of the QR code as it would not fit. This might cause issues if signing offline. """
+                """As a workaround, try exporting the tx as file or text instead.""")
         try:
-            self.main_window.show_qrcode(qr_data, 'Transaction', parent=self)
+            self.main_window.show_qrcode(qr_data, 'Transaction', parent=self, help_text=help_text)
         except qrcode.exceptions.DataOverflowError:
             self.show_error(_('Failed to display QR code.') + '\n' +
                             _('Transaction is too large in size.'))

electrum/transaction.py

@@ -938,14 +938,19 @@ class Transaction:
         else:
             return nVersion + txins + txouts + nLocktime
 
-    def to_qr_data(self) -> str:
-        """Returns tx as data to be put into a QR code. No side-effects."""
+    def to_qr_data(self) -> Tuple[str, bool]:
+        """Returns (serialized_tx, is_complete). The tx is serialized to be put inside a QR code. No side-effects.
+        As space in a QR code is limited, some data might have to be omitted. This is signalled via is_complete=False.
+        """
+        is_complete = True
         tx = copy.deepcopy(self)  # make copy as we mutate tx
         if isinstance(tx, PartialTransaction):
             # this makes QR codes a lot smaller (or just possible in the first place!)
+            # note: will not apply if all inputs are taproot, due to new sighash.
             tx.convert_all_utxos_to_witness_utxos()
+            is_complete = False
         tx_bytes = tx.serialize_as_bytes()
-        return base_encode(tx_bytes, base=43)
+        return base_encode(tx_bytes, base=43), is_complete
 
     def txid(self) -> Optional[str]:
         if self._cached_txid is None: