topic: Return JsonableError for race condition in topic mute.

To avoid an uncaught IntegrityError causing a 500 HTTP response in a
race between two processes trying to mute a topic, we catch the
integrity error and raise the error exception with status 400 we'd
have gotten if the second request had been a bit later.

Fixes #21011.

zerver/tests/test_muting_topics.py

@@ -113,6 +113,23 @@ class MutedTopicsTests(ZulipTestCase):
                 topic_name="Verona3",
             )
 
+        # Verify the error handling for the database level
+        # IntegrityError we'll get with a race between two processes
+        # trying to mute the topic.  To do this, we patch the
+        # topic_is_muted function to always return False when trying
+        # to mute a topic that is already muted.
+        add_topic_mute(
+            user_profile=user,
+            stream_id=stream.id,
+            recipient_id=stream.recipient.id,
+            topic_name="Verona3",
+            date_muted=datetime(2020, 1, 1, tzinfo=timezone.utc),
+        )
+
+        with mock.patch("zerver.views.muting.topic_is_muted", return_value=False):
+            result = self.api_patch(user, url, data)
+            self.assert_json_error(result, "Topic already muted")
+
     def test_remove_muted_topic(self) -> None:
         user = self.example_user("hamlet")
         realm = user.realm

zerver/views/muting.py

@@ -1,6 +1,7 @@
 import datetime
 from typing import Optional
 
+from django.db import IntegrityError
 from django.http import HttpRequest, HttpResponse
 from django.utils.timezone import now as timezone_now
 from django.utils.translation import gettext as _
@@ -39,7 +40,10 @@ def mute_topic(
     if topic_is_muted(user_profile, stream.id, topic_name):
         raise JsonableError(_("Topic already muted"))
 
-    do_mute_topic(user_profile, stream, topic_name, date_muted)
+    try:
+        do_mute_topic(user_profile, stream, topic_name, date_muted)
+    except IntegrityError:
+        raise JsonableError(_("Topic already muted"))
 
 
 def unmute_topic(