Time machine mode [Z#23129725] (#3961)

Allows organizers to test their shop as if it were a different date and time.

Implemented using a time_machine_now() function which is used instead of regular now(), which can overlay the real date time with a value from a ContextVar, assigned from a session value in EventMiddleware.

For more information, see doc/development/implementation/timemachine.rst

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>
Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/base/timemachine.py

@@ -0,0 +1,85 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020 Raphael Michel and contributors
+# Copyright (C) 2020-2021 rami.io GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
+# <https://www.gnu.org/licenses/>.
+#
+import contextvars
+from contextlib import contextmanager
+
+from dateutil.parser import parse
+from django.utils.timezone import now
+
+timemachine_now_var = contextvars.ContextVar('timemachine_now', default=None)
+
+
+@contextmanager
+def time_machine_now_assigned_from_request(request):
+    if hasattr(request, 'event') and f'timemachine_now_dt:{request.event.pk}' in request.session and \
+            request.event.testmode and has_time_machine_permission(request, request.event):
+        request.now_dt = parse(request.session[f'timemachine_now_dt:{request.event.pk}'])
+        request.now_dt_is_fake = True
+    else:
+        request.now_dt = now()
+        request.now_dt_is_fake = False
+
+    try:
+        timemachine_now_var.set(request.now_dt if request.now_dt_is_fake else None)
+
+        yield
+    finally:
+        timemachine_now_var.set(None)
+
+
+def time_machine_now(default=False):
+    """
+    Return the datetime to use as current datetime for checking order restrictions in event
+    index and checkout flow.
+
+    :param default:   Value to return if time machine mode is disabled. By default the current datetime is used.
+    """
+    if default is False:
+        default = now()
+    return timemachine_now_var.get() or default
+
+
+@contextmanager
+def time_machine_now_assigned(now_dt):
+    """
+    Use this context manager to assign current datetime for time machine mode. Useful e.g. for background tasks.
+
+    :param now_dt: The datetime value to assign. May be `None` to disable time machine.
+    """
+    try:
+        timemachine_now_var.set(now_dt)
+        yield
+    finally:
+        timemachine_now_var.set(None)
+
+
+def has_time_machine_permission(request, event):
+    permission = 'can_change_event_settings'
+
+    return (
+        request.user.is_authenticated and
+        request.user.has_event_permission(request.organizer, request.event, permission, request=request)
+    ) or (
+        getattr(request, 'event_access_user', None) and
+        request.event_access_user.is_authenticated and
+        request.event_access_user.has_event_permission(request.organizer, request.event, permission, request=request)
+    )