qt wizard: tweak GoBack behaviour to recalc inputs to previous dialog

follow-up f13f46c555

When on dialog n user presses "Back",
- previously, we went back to when dialog n-1 appeared
- now, go back to just after dialog n-2 finishes
This way, any calculations between when dialog n-2 finishes and
dialog n-1 appears will rerun, potentially populating dialog n-1 differently.

Namely if the user presses back on the confirm_seed_dialog, we want to
go back to the show_seed_dialog but with a freshly generated seed.

electrum/base_wizard.py

@@ -60,6 +60,9 @@ class ScriptTypeNotSupported(Exception): pass
 class GoBack(Exception): pass
 
 
+class ReRunDialog(Exception): pass
+
+
 class ChooseHwDeviceAgain(Exception): pass
 
 

electrum/gui/qt/installwizard.py

@@ -19,7 +19,7 @@ from PyQt5.QtWidgets import (QWidget, QDialog, QLabel, QHBoxLayout, QMessageBox,
 from electrum.wallet import Wallet, Abstract_Wallet
 from electrum.storage import WalletStorage, StorageReadWriteError
 from electrum.util import UserCancelled, InvalidPassword, WalletFileException, get_new_wallet_name
-from electrum.base_wizard import BaseWizard, HWD_SETUP_DECRYPT_WALLET, GoBack
+from electrum.base_wizard import BaseWizard, HWD_SETUP_DECRYPT_WALLET, GoBack, ReRunDialog
 from electrum.i18n import _
 
 from .seed_dialog import SeedLayout, KeysLayout
@@ -97,6 +97,7 @@ def wizard_dialog(func):
         run_next = kwargs['run_next']
         wizard = args[0]  # type: InstallWizard
         while True:
+            #wizard.logger.debug(f"dialog stack. len: {len(wizard._stack)}. stack: {wizard._stack}")
             wizard.back_button.setText(_('Back') if wizard.can_go_back() else _('Cancel'))
             # current dialog
             try:
@@ -109,12 +110,25 @@ def wizard_dialog(func):
                 # to go back from the current dialog, we just let the caller unroll the stack:
                 raise
             # next dialog
+            try:
+                while True:
                     try:
                         run_next(*out)
-            except GoBack:
+                    except ReRunDialog:
+                        # restore state, and then let the loop re-run next
+                        wizard.go_back(rerun_previous=False)
+                    else:
+                        break
+            except GoBack as e:
                 # to go back from the next dialog, we ask the wizard to restore state
                 wizard.go_back(rerun_previous=False)
-                # and we re-run the current dialog (by continuing)
+                # and we re-run the current dialog
+                if wizard.can_go_back():
+                    # also rerun any calculations that might have populated the inputs to the current dialog,
+                    # by going back to just after the *previous* dialog finished
+                    raise ReRunDialog() from e
+                else:
+                    continue
             else:
                 break
     return func_wrapper