Open a window with an empty session

With general -> save-session on and only private windows open, we can easily get
a session file with "windows: []" in it. If we loaded such a file, we got no
windows at all when qutebrowser started.

Fixes #2664

qutebrowser/misc/sessions.py

@@ -390,6 +390,7 @@ class SessionManager(QObject):
                 data = yaml.load(f, Loader=YamlLoader)
         except (OSError, UnicodeDecodeError, yaml.YAMLError) as e:
             raise SessionError(e)
+
         log.sessions.debug("Loading session {} from {}...".format(name, path))
         for win in data['windows']:
             window = mainwindow.MainWindow(geometry=win['geometry'],
@@ -410,7 +411,9 @@ class SessionManager(QObject):
                 tabbed_browser.setCurrentIndex(tab_to_focus)
             if win.get('active', False):
                 QTimer.singleShot(0, tabbed_browser.activateWindow)
-        self.did_load = True
+
+        if data['windows']:
+            self.did_load = True
         if not name.startswith('_') and not temp:
             self._current = name
 

tests/end2end/test_invocations.py

@@ -285,3 +285,22 @@ def test_initial_private_browsing(request, quteproc_new):
 
     quteproc_new.send_cmd(':quit')
     quteproc_new.wait_for_quit()
+
+
+def test_loading_empty_session(tmpdir, request, quteproc_new):
+    """Make sure loading an empty session opens a window."""
+    session = tmpdir / 'session.yml'
+    session.write('windows: []')
+
+    args = _base_args(request.config) + ['--temp-basedir', '-r', str(session)]
+    quteproc_new.start(args)
+
+    quteproc_new.compare_session("""
+        windows:
+            - tabs:
+              - history:
+                - url: about:blank
+    """)
+
+    quteproc_new.send_cmd(':quit')
+    quteproc_new.wait_for_quit()