qt wizard: make "GoBack" unroll the call stack to avoid stack overflow

fixes #6069

electrum/base_wizard.py

@@ -113,18 +113,21 @@ class BaseWizard(Logger):
     def can_go_back(self):
         return len(self._stack) > 1
 
-    def go_back(self):
+    def go_back(self, *, rerun_previous: bool = True) -> None:
         if not self.can_go_back():
             return
         # pop 'current' frame
         self._stack.pop()
-        # pop 'previous' frame
-        stack_item = self._stack.pop()
+        prev_frame = self._stack[-1]
         # try to undo side effects since we last entered 'previous' frame
-        # FIXME only self.storage is properly restored
-        self.data = copy.deepcopy(stack_item.db_data)
-        # rerun 'previous' frame
-        self.run(stack_item.action, *stack_item.args, **stack_item.kwargs)
+        # FIXME only self.data is properly restored
+        self.data = copy.deepcopy(prev_frame.db_data)
+
+        if rerun_previous:
+            # pop 'previous' frame
+            self._stack.pop()
+            # rerun 'previous' frame
+            self.run(prev_frame.action, *prev_frame.args, **prev_frame.kwargs)
 
     def reset_stack(self):
         self._stack = []

electrum/gui/qt/installwizard.py

@@ -96,19 +96,27 @@ def wizard_dialog(func):
     def func_wrapper(*args, **kwargs):
         run_next = kwargs['run_next']
         wizard = args[0]  # type: InstallWizard
-        wizard.back_button.setText(_('Back') if wizard.can_go_back() else _('Cancel'))
-        try:
-            out = func(*args, **kwargs)
-            if type(out) is not tuple:
-                out = (out,)
-            run_next(*out)
-        except GoBack:
-            if wizard.can_go_back():
-                wizard.go_back()
-                return
-            else:
-                wizard.close()
+        while True:
+            wizard.back_button.setText(_('Back') if wizard.can_go_back() else _('Cancel'))
+            # current dialog
+            try:
+                out = func(*args, **kwargs)
+                if type(out) is not tuple:
+                    out = (out,)
+            except GoBack:
+                if not wizard.can_go_back():
+                    wizard.close()
+                # to go back from the current dialog, we just let the caller unroll the stack:
                 raise
+            # next dialog
+            try:
+                run_next(*out)
+            except GoBack:
+                # 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)
+            else:
+                break
     return func_wrapper