migrations: Repeat part of migration 0376.

The blockquote explains the motivation for this change in detail.

Fixes zulip#21608.

tools/semgrep.yml

@@ -40,6 +40,7 @@ rules:
         - zerver/migrations/0209_user_profile_no_empty_password.py
         - zerver/migrations/0260_missed_message_addresses_from_redis_to_db.py
         - zerver/migrations/0376_set_realmemoji_author_and_reupload_realmemoji.py
+        - zerver/migrations/0387_reupload_realmemoji_again.py
         - pgroonga/migrations/0002_html_escape_subject.py
 
   - id: logging-format

zerver/migrations/0387_reupload_realmemoji_again.py

@@ -0,0 +1,46 @@
+from django.conf import settings
+from django.db import migrations
+from django.db.backends.postgresql.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+from zerver.lib.queue import queue_json_publish
+
+
+def reupload_realm_emoji(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    """As detailed in https://github.com/zulip/zulip/issues/21608, it is
+    possible for the deferred_work queue from Zulip 4.x to have been
+    started up by puppet during the deployment before migrations were
+    run on Zulip 5.0.
+
+    This means that the deferred_work events produced by migration
+    0376 might have been processed and discarded without effect.
+
+    Since it's harmless to reupload a custom emoji a second time, we
+    fix this issue for the slice of servers that have already
+    installed 5.0 by repeating that part of the migration.
+    """
+
+    Realm = apps.get_model("zerver", "Realm")
+    if settings.TEST_SUITE:
+        # There are no custom emoji in the test suite data set, and
+        # the below code won't work because RabbitMQ isn't enabled for
+        # the test suite.
+        return
+
+    for realm_id in Realm.objects.order_by("id").values_list("id", flat=True):
+        event = {
+            "type": "reupload_realm_emoji",
+            "realm_id": realm_id,
+        }
+        queue_json_publish("deferred_work", event)
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zerver", "0386_fix_attachment_caches"),
+    ]
+
+    operations = [
+        migrations.RunPython(reupload_realm_emoji, reverse_code=migrations.RunPython.noop),
+    ]