unread: Add tool for marking all messages as unread for testing.

This tool can save a lot of manual work in testing our unread counts
logic.

docs/pointer.md

@@ -108,3 +108,14 @@ use the product.  The algorithm is as follows:
 These two simple rules, combined with the pointer logic above, end up
 matching user expectations well for whether the product should treat
 them as having read a set of messages (or not).
+
+## Testing and development
+
+In a Zulip development environment, you can use `manage.py
+mark_all_messages_unread` to set every user's pointer to 0 and all
+messages as unread, for convenience in testing unread count related
+logic.
+
+It can be useful to combine this with `manage.py populate_db -n 3000`
+(which rebuilds the database with 3000 initial messages) to ensure a
+large number of messages are present.

zilencer/management/commands/mark_all_messages_unread.py

@@ -0,0 +1,19 @@
+from __future__ import absolute_import
+
+from typing import Any
+
+from django.conf import settings
+from django.core.management.base import BaseCommand
+from django.db.models import F
+from django.core.cache import cache
+from zerver.models import UserProfile, UserMessage
+
+class Command(BaseCommand):
+    help = """Script to mark all messages as unread."""
+
+    def handle(self, *args, **options):
+        # type: (*Any, **Any) -> None
+        assert settings.DEVELOPMENT
+        UserMessage.objects.all().update(flags=F('flags').bitand(~UserMessage.flags.read))
+        UserProfile.objects.all().update(pointer=0)
+        cache._cache.flush_all()