user-groups: Remove deactivated users from groups

As newly required to support Zulip Server 10+; see zulip#5899.

Fixes: zulip#5899

src/user-groups/__tests__/userGroupsReducer-test.js

@@ -1,6 +1,7 @@
 /* @flow strict-local */
 
 import deepFreeze from 'deep-freeze';
+import invariant from 'invariant';
 
 import * as eg from '../../__tests__/lib/exampleData';
 import {
@@ -11,6 +12,7 @@ import {
   EVENT_USER_GROUP_REMOVE_MEMBERS,
 } from '../../actionConstants';
 import userGroupsReducer from '../userGroupsReducer';
+import eventToAction from '../../events/eventToAction';
 
 describe('userGroupsReducer', () => {
   describe('REGISTER_COMPLETE', () => {
@@ -192,4 +194,44 @@ describe('userGroupsReducer', () => {
       ).toEqual([{ ...group1, members: [user1.user_id, user4.user_id] }, group2]);
     });
   });
+
+  describe('realm_user op: update', () => {
+    test('a user is deactivated', () => {
+      const user1 = eg.makeUser();
+      const user2 = eg.makeUser();
+      const user3 = eg.makeUser();
+
+      const group1 = eg.makeUserGroup({ members: [user1.user_id, user2.user_id, user3.user_id] });
+      const group2 = eg.makeUserGroup({ members: [user2.user_id, user1.user_id] });
+      const group3 = eg.makeUserGroup({ members: [user1.user_id] });
+
+      const event = {
+        id: 0,
+        type: 'realm_user',
+        op: 'update',
+        person: { user_id: user1.user_id, is_active: false },
+      };
+
+      const prevUserGroupsState = deepFreeze([group1, group2, group3]);
+      const prevPerAccountState = eg.reduxStatePlus({
+        users: [user1, user2, user3],
+        userGroups: prevUserGroupsState,
+      });
+      const action = eventToAction(prevPerAccountState, event);
+
+      expect(action).not.toBeNull();
+      invariant(action != null, 'action not null');
+
+      const actualState = userGroupsReducer(prevUserGroupsState, action);
+
+      expect(actualState).toEqual([
+        { ...group1, members: [user2.user_id, user3.user_id] },
+        { ...group2, members: [user2.user_id] },
+
+        // A newly-empty group is not pruned; when a group is deactivated,
+        // we expect a user_group/remove event.
+        { ...group3, members: [] },
+      ]);
+    });
+  });
 });

src/user-groups/userGroupsReducer.js

@@ -2,13 +2,15 @@
 import type { UserGroupsState, PerAccountApplicableAction } from '../types';
 import {
   REGISTER_COMPLETE,
+  EVENT,
   EVENT_USER_GROUP_ADD,
   EVENT_USER_GROUP_REMOVE,
   EVENT_USER_GROUP_UPDATE,
   EVENT_USER_GROUP_ADD_MEMBERS,
   EVENT_USER_GROUP_REMOVE_MEMBERS,
   RESET_ACCOUNT_DATA,
 } from '../actionConstants';
+import { EventTypes } from '../api/eventTypes';
 import { NULL_ARRAY } from '../nullObjects';
 
 const initialState: UserGroupsState = NULL_ARRAY;
@@ -69,6 +71,32 @@ export default (
     case EVENT_USER_GROUP_REMOVE_MEMBERS:
       return eventUserGroupRemoveMembers(state, action);
 
+    case EVENT: {
+      const { event } = action;
+      switch (event.type) {
+        case EventTypes.realm_user: {
+          switch (event.op) {
+            case 'update': {
+              const { person } = event;
+              // TODO(flow) teach Flow that the `person.existingUser != null` is redundant
+              if (person.is_active === false && person.existingUser != null) {
+                const userId = person.existingUser.user_id;
+                return state.map(g => ({ ...g, members: g.members.filter(m => m !== userId) }));
+              }
+
+              return state;
+            }
+
+            default:
+              return state;
+          }
+        }
+
+        default:
+          return state;
+      }
+    }
+
     default:
       return state;
   }