Implement Last-Modified for a number of API resources

src/pretix/api/views/__init__.py

@@ -1,3 +1,8 @@
+from calendar import timegm
+
+from django.db.models import Max
+from django.http import HttpResponse
+from django.utils.http import http_date, parse_http_date_safe
 from rest_framework.filters import OrderingFilter
 
 
@@ -21,3 +26,34 @@ class RichOrderingFilter(OrderingFilter):
             return queryset.order_by(*ordering)
 
         return queryset
+
+
+class ConditionalListView:
+
+    def list(self, request, **kwargs):
+        if_modified_since = request.META.get('HTTP_IF_MODIFIED_SINCE')
+        if if_modified_since:
+            if_modified_since = parse_http_date_safe(if_modified_since)
+        if_unmodified_since = request.META.get('HTTP_IF_UNMODIFIED_SINCE')
+        if if_unmodified_since:
+            if_unmodified_since = parse_http_date_safe(if_unmodified_since)
+        lmd = request.event.logentry_set.filter(
+            content_type__model=self.queryset.model._meta.model_name,
+            content_type__app_label=self.queryset.model._meta.app_label,
+        ).aggregate(
+            m=Max('datetime')
+        )['m']
+        if lmd:
+            lmd_ts = timegm(lmd.utctimetuple())
+            print(lmd_ts, if_modified_since)
+
+        if if_unmodified_since and lmd and lmd_ts > if_unmodified_since:
+            return HttpResponse(status=412)
+
+        if if_modified_since and lmd and lmd_ts <= if_modified_since:
+            return HttpResponse(status=304)
+
+        resp = super().list(request, **kwargs)
+        if lmd:
+            resp['Last-Modified'] = http_date(lmd_ts)
+        return resp