remove last-screen file before restarting or exiting

unless we are restarting because of a snap refresh.

ths lead to all kinds of confusion after e.g. a ui crash after a snap refresh

subiquity/controllers/refresh.py

@@ -92,7 +92,7 @@ class RefreshController(SubiquityController):
             if change['status'] == 'Done':
                 # Clearly if we got here we didn't get restarted by
                 # snapd/systemctl (dry-run mode or logged in via SSH)
-                self.app.restart()
+                self.app.restart(remove_last_screen=False)
             if change['status'] not in ['Do', 'Doing']:
                 raise Exception("update failed")
             await asyncio.sleep(0.1)

subiquity/core.py

@@ -131,7 +131,9 @@ class Subiquity(Application):
         else:
             super().exit()
 
-    def restart(self):
+    def restart(self, remove_last_screen=True):
+        if remove_last_screen:
+            self._remove_last_screen()
         self.urwid_loop.screen.stop()
         cmdline = ['snap', 'run', 'subiquity']
         if self.opts.dry_run:
@@ -168,6 +170,7 @@ class Subiquity(Application):
             report = self.make_apport_report(
                 ErrorReportKind.UI, "Installer UI", interrupt=False, wait=True)
             print("report saved to {}".format(report.path))
+            self._remove_last_screen()
             raise
 
     def report_start_event(self, name, description, level="INFO"):

subiquitycore/core.py

@@ -485,10 +485,13 @@ class Application:
 
 # EventLoop -------------------------------------------------------------------
 
-    def exit(self):
+    def _remove_last_screen(self):
         state_path = os.path.join(self.state_dir, 'last-screen')
         if os.path.exists(state_path):
             os.unlink(state_path)
+
+    def exit(self):
+        self._remove_last_screen()
         self.aio_loop.stop()
 
     def run_scripts(self, scripts):