stream: Add subscriber_count field.

Fixes #34246.

Add subscriber_count field to Stream model to track number of
non-deactivated users subscribed to the channel.

zerver/lib/test_classes.py

@@ -5,7 +5,7 @@ import re
 import shutil
 import subprocess
 import tempfile
-from collections.abc import Callable, Collection, Iterator, Mapping, Sequence
+from collections.abc import Callable, Collection, Iterable, Iterator, Mapping, Sequence
 from contextlib import contextmanager
 from typing import TYPE_CHECKING, Any, Union, cast
 from unittest import TestResult, mock, skipUnless
@@ -1453,6 +1453,31 @@ Output:
         self.assertEqual(stream.recipient_id, message.recipient_id)
         self.assertEqual(stream.name, stream_name)
 
+    def assert_stream_subscriber_count(
+        self,
+        counts_before: dict[int, int],
+        counts_after: dict[int, int],
+        expected_difference: int,
+    ) -> None:
+        # Normally they should always be equal,
+        # but just in case this was called in some test where user/s streams have changed
+        # and we forgot to update streams,
+        # so this assertion catches that.
+        self.assertEqual(
+            set(counts_before),
+            set(counts_after),
+            msg="Different streams! You should compare subscriber_count for the same streams.",
+        )
+
+        for stream_id, count_before in counts_before.items():
+            self.assertEqual(
+                count_before + expected_difference,
+                counts_after[stream_id],
+                msg=f"""
+                stream of ID ({stream_id}) should have a subscriber_count of {count_before + expected_difference}.
+                """,
+            )
+
     def webhook_fixture_data(self, type: str, action: str, file_type: str = "json") -> str:
         fn = os.path.join(
             os.path.dirname(__file__),
@@ -2242,6 +2267,20 @@ class ZulipTestCase(ZulipTestCaseMixin, TestCase):
         with self.captureOnCommitCallbacks(execute=True):
             handle_missedmessage_emails(user_profile_id, message_ids)
 
+    def build_streams_subscriber_count(self, streams: Iterable[Stream]) -> dict[int, int]:
+        """
+        Callers MUST pass a new db-fetched version of streams each time.
+        """
+        return {stream.id: stream.subscriber_count for stream in streams}
+
+    def fetch_streams_subscriber_count(self, stream_ids: set[int]) -> dict[int, int]:
+        return self.build_streams_subscriber_count(streams=Stream.objects.filter(id__in=stream_ids))
+
+    def fetch_other_streams_subscriber_count(self, stream_ids: set[int]) -> dict[int, int]:
+        return self.build_streams_subscriber_count(
+            streams=Stream.objects.exclude(id__in=stream_ids)
+        )
+
 
 def get_row_ids_in_all_tables() -> Iterator[tuple[str, set[int]]]:
     all_models = apps.get_models(include_auto_created=True)