muting: Handle the case of a race muting the same user twice.

zerver/tests/test_muting_users.py

@@ -77,6 +77,14 @@ class MutedUsersTests(ZulipTestCase):
         result = self.api_post(hamlet, url)
         self.assert_json_error(result, "User already muted")
 
+        # Verify the error handling for the database level
+        # IntegrityError we'll get with a race between two processes
+        # trying to mute the user.  To do this, we patch the
+        # get_mute_object function to always return None.
+        with mock.patch("zerver.views.muting.get_mute_object", return_value=None):
+            result = self.api_post(hamlet, url)
+            self.assert_json_error(result, "User already muted")
+
     def _test_add_muted_user_valid_data(self, deactivate_user: bool = False) -> None:
         hamlet = self.example_user("hamlet")
         self.login_user(hamlet)

zerver/views/muting.py

@@ -106,7 +106,11 @@ def mute_user(request: HttpRequest, user_profile: UserProfile, muted_user_id: in
     if get_mute_object(user_profile, muted_user) is not None:
         raise JsonableError(_("User already muted"))
 
+    try:
         do_mute_user(user_profile, muted_user, date_muted)
+    except IntegrityError:
+        raise JsonableError(_("User already muted"))
+
     return json_success(request)