---
type: guide
title: Cross-Tab Role Change Sync
description: How an admin changing a user's role (or their own role) at /admin/players propagates the new role to every other open tab of the SPA, plus the LAST_ADMIN invariant that protects concurrent demotion.
tags: [guide, auth, session, role, broadcast-channel, storage-event, admin, last-admin, tester]
timestamp: 2026-07-23T08:42:24Z
---

# What this feature does

The [Cross-Tab Invalidation](/guides/cross-tab-invalidation.md) guide
describes how a logout in one tab instantly clears every other open tab.
Feature 913 adds a parallel, **non-destructive** channel: when an admin
changes any user's role (admin ↔ player) at `/admin/players`, every other
open tab of the same browser profile receives the change and updates its
in-memory `CurrentUser` — without forcing a re-login or a page reload.

Two concerns had to be solved together:

1. **UI propagation.** Without a broadcast, an admin who demoted another
   admin from `admin` to `player` would still see them as `admin` in every
   other tab, and conversely the demoted user would still see admin-only
   navigation entries until they refreshed.
2. **The LAST_ADMIN invariant.** The `[Admin Endpoints](/api/admin.md)`
   surface already rejects a role demotion that would leave the system
   with zero admins (`409 LAST_ADMIN`). Feature 913 also verifies that
   *concurrent* demotion requests cannot race past that check: if two
   admins try to demote each other at the same instant, exactly one
   request succeeds and the other receives `409 LAST_ADMIN`.

# Why a new concept doc

The change spans the pure contract module (`auth-session-events.pure.ts`),
the `AuthService` transport, the `UserStore`, the admin players component
(origin of the publish), and the home/shell component (consumer of the
notification). It also adds a backend test that pins down the concurrent
demotion invariant. Reading this doc together with
[Cross-Tab Invalidation](/guides/cross-tab-invalidation.md) and
[Admin — Challenges/Players area](/guides/admin-shell.md) is the fastest
way to understand the full wiring.

# Tester flows

> Open the SPA in **two browser tabs** of the same browser profile. Both
> tabs must be on authenticated routes. Sign in as **Admin A** in Tab A
> and **Admin B** (also an admin) in Tab B (or use two browser profiles
> and a regular user in Tab B — the broadcast only fires to tabs signed
> in as the *target* user, so a regular-user tab will receive the role
> update if it is the target).

## Self demotion (admin changes their own role)

1. Have two admin accounts (`admin1`, `admin2`).
2. Sign in as `admin1` in **Tab A** and **Tab B** (duplicate the tab).
3. In **Tab A**, navigate to `/admin/players`, find the row for
   `admin1`, and use the role dropdown to switch it from `admin` to
   `player`. The dropdown closes, the row updates, and the status line
   reads `Role updated to player.`
4. In **Tab B** the following happens **without any user interaction**:
   - The header user menu updates to show the player variant
     (no admin-only entries, no `Players` nav item).
   - If Tab B happens to be on `/admin/*`, a toast notification appears
     with the text `Your role was changed to "player" by another admin.
     You may need to refresh or log in again to continue.`
5. Confirm in DevTools → Application → Session Storage of **Tab B** that
   the persisted `hipctf.auth.v1` session blob now records `role: 'player'`.

## Admin demotes another admin (peer notification)

1. Sign in as `admin1` in **Tab A** and `admin2` in **Tab B**. Both are
   on `/admin/players`.
2. In **Tab A**, change `admin2`'s role from `admin` to `player`.
3. **Tab B** updates `admin2`'s row to `player` (the source of truth is
   still the server — the next `GET /admin/players` will reconcile any
   drift) and `admin2`'s own header in Tab B shows the player variant.
4. If `admin2` was viewing `/admin/*` at the moment of the change, Tab B
   shows the role-changed toast and the admin-only navigation items
   disappear.

## Demoting a player to player (no-op for peers)

Changing a regular player between equivalent rows (or assigning them a
role they already hold) still publishes a message, but the
`applyRoleChangeToUser` guard short-circuits when the role is unchanged,
so no toast is shown and no `sessionStorage` write occurs.

## Negative cases

| Action | Expected |
|---|---|
| Two admins open `/admin/players`; `admin1` demotes `admin2`, and at the same instant `admin2` demotes `admin1` | The server rejects one of the two requests with `409 LAST_ADMIN`; exactly one admin row remains. The successful demotion still publishes to other tabs; the rejected demotion returns an error toast (`You cannot demote the last admin`). |
| Admin demotes themselves while on a tab that is on `/login` (logged out) | No broadcast is processed — the receiving tab has no `currentUser`, so `handlePeerRoleChange` returns early. |
| A non-HIPCTF tab on the same browser profile writes `hipctf.role.invalidate.v1` to `localStorage` with garbage | The payload validator (`isStorageRoleChangeEvent`) ignores it — no state change. |
| Same tab echoes its own broadcast (test environment that delivers self-messages) | The `origin === crossTabOrigin` guard prevents self-processing and rebroadcast loops. |
| A third tab is signed in as a *different* user than the target | The `me.id !== msg.userId` guard prevents that tab from applying the change. |

# Visual / interactive elements

There is **one new UI affordance** — a toast — and the existing header
user-menu gains/losses admin-only entries dynamically.

| Element | Selector | After peer role change (target tab on `/admin/*`) |
|---|---|---|
| Notification toast | `[data-testid="notification-info"]` (see [Notifications](/guides/notifications.md)) | Visible with the role-changed message. Auto-dismisses per the existing toast policy. |
| Admin nav items | `[data-testid="shell-nav-admin"]` | Hidden when the new role is `player`. |
| User menu trigger | `[data-testid="user-menu-trigger"]` | Still visible; admin-only entries (`Players`, `Settings`) removed when role is `player`. |
| Players row | `[data-testid="admin-players-row"]` | Source-of-truth is still the server; a peer change re-renders the row on the next `GET /admin/players` call. |

# How it works (developer map)

## Pure contract module additions

`frontend/src/app/core/services/auth-session-events.pure.ts` now
additionally defines:

| Export | Purpose |
|---|---|
| `CROSS_TAB_ROLE_CHANNEL_NAME = 'hipctf.auth.role.v1'` | Stable `BroadcastChannel` name for role-change messages. Separate from the invalidation channel so the two transports cannot alias. |
| `CROSS_TAB_ROLE_STORAGE_KEY = 'hipctf.role.invalidate.v1'` | `localStorage` fallback key used when `BroadcastChannel` is unavailable. |
| `CROSS_TAB_ROLE_EVENT_TYPE = 'hipctf.session.role-changed'` | Marker for the discriminated-union guard. |
| `CrossTabRoleChangeMessage` | `{ type: 'role-changed'; origin: string; ts: number; userId: string; newRole: 'admin' \| 'player' }`. |
| `encodeRoleChangeMessage(origin, userId, newRole)` | Builds a message stamped with `Date.now()`. |
| `isCrossTabRoleChangeMessage(value)` | Type-guard used by both `BroadcastChannel` and `storage` listeners; rejects `session-invalidated` payloads so the two channels never double-fire. |
| `isStorageRoleChangeEvent(event)` | Guards `storage` events: only accepts the namespaced key with a parseable payload. |
| `applyRoleChangeToUser(user, newRole)` | Pure helper: returns the same reference when roles match, a new `{...user, role}` object otherwise, and `null` for a `null` user. |

The payload carries only the **target user id** and the **new role** —
no tokens, no session blob, no other user attributes.

## `AuthService` wiring additions

`frontend/src/app/core/services/auth.service.ts`

* `installCrossTabListener()` now opens a *second* `BroadcastChannel`
  named `hipctf.auth.role.v1` and subscribes a `'message'` handler
  (`onRoleBroadcastMessage`). The existing `'storage'` listener is
  extended to dispatch both `isStorageInvalidationEvent` **and**
  `isStorageRoleChangeEvent` — the storage event fires only in the
  tabs *other* than the writer, which is exactly the fan-out we want.
* `publishRoleChange(userId, newRole)` is the new public entry point.
  It encodes a message with the per-tab `crossTabOrigin` and posts it
  on both transports. Failures are swallowed (the storage fallback still
  fires from the other side). It deliberately does **not** clear local
  state — the originating tab is the one that just successfully called
  `PATCH /api/v1/admin/players/:id/role`.
* `applyPeerRoleChange(newRole)` updates the local `currentUser` signal
  in-place and writes the new user into `sessionStorage` so a subsequent
  hard reload hydrates with the updated role. It does **not** publish.
* `onPeerRoleChange(cb)` is the new subscriber API; returns an
  unsubscribe closure. Used by `HomeComponent`.
* `handlePeerRoleChange(raw)` is the single fan-in for both transport
  sources: it parses, drops self-origin messages, drops messages when
  there is no signed-in user, drops messages that target a different
  user id, then calls `applyPeerRoleChange` and notifies subscribers.
* `teardownCrossTabListener()` closes the role channel and clears
  `peerRoleChangeListeners`.

## `UserStore.setRole`

`frontend/src/app/core/services/user.store.ts` gains a `setRole(role)`
helper that updates the in-memory user signal and calls
`auth.applyPeerRoleChange(role)`. This is the path used when the
broadcast was received from a peer tab.

## Publish path — `AdminUsersComponent.onToggleRole`

`frontend/src/app/features/admin/admin-users.component.ts`

After the successful `PATCH /api/v1/admin/players/:id/role` response,
the component now:

1. Updates the local `rows` signal with the returned `updated` payload.
2. If the *currently signed-in* user matches `updated.id` **and** the
   role actually changed (`me.role !== updated.role`), it calls
   `auth.applyPeerRoleChange(updated.role)` to update the local user
   signal immediately, then `auth.publishRoleChange(updated.id, updated.role)`
   to fan the change out to every other tab.

The build fix captured in
`.kilo/plans/913-build-fix.md` adds the missing `AuthService` injection
so TypeScript can resolve `this.auth`.

## Consumer path — `HomeComponent.handlePeerRoleChange`

`frontend/src/app/features/home/home.component.ts`

* `ngOnInit()` calls `auth.onPeerRoleChange(handlePeerRoleChange)` and
  stores the unsubscribe handle in `peerRoleChangeUnsubscribe`.
* `ngOnDestroy()` calls the unsubscribe (in addition to the existing
  peer-invalidation unsubscribe).
* `handlePeerRoleChange(msg)`:
  * Returns early if there is no signed-in user, or the user id does not
    match the broadcast.
  * Calls `userStore.setRole(msg.newRole)` to update the signal and
    re-persist via `AuthService`.
  * If the current route is under `/admin/*`, shows an info toast:
    `Your role was changed to "<role>" by another admin. You may need to
    refresh or log in again to continue.`

## Backend invariant — LAST_ADMIN under concurrent demotion

The existing `PATCH /api/v1/admin/players/:id/role` handler runs inside a
transaction and re-checks the remaining admin count before committing,
so the LAST_ADMIN invariant already holds for a *sequential* demotion.
`tests/backend/admin-players.spec.ts` adds a **concurrent** demotion
test that:

1. Registers `original` as the first admin, then creates a second admin
   `bob` (so the system has two admins).
2. Concurrently fires `PATCH /api/v1/admin/players/original.id/role`
   from `original`'s session and
   `PATCH /api/v1/admin/players/bob.id/role` from `bob`'s session.
3. Asserts that exactly one request returns `200` with `role: 'player'`,
   the other returns `409` with body `code: 'LAST_ADMIN'`, and the
   database still has exactly one admin row (`bob`).

Because both demotions happen against the same `users` table inside the
DB-side transactional guard, the race resolves deterministically: the
first transaction commits with the last-admin check passing; the second
sees zero remaining admins and aborts with `409`.

## Test coverage

| File | What it pins down |
|---|---|
| `tests/frontend/auth-session-events.spec.ts` | Pure payload validation (`encodeRoleChangeMessage`, `isCrossTabRoleChangeMessage`, `isStorageRoleChangeEvent`), `applyRoleChangeToUser` no-op semantics, and rejection of `session-invalidated` payloads on the role channel. |
| `tests/backend/admin-players.spec.ts` | Concurrent two-admin demotion: one succeeds, the other gets `409 LAST_ADMIN`, exactly one admin remains in the DB. |

# See also

- [Cross-Tab Invalidation](/guides/cross-tab-invalidation.md) — sibling
  channel for session invalidation.
- [Authenticated Shell](/guides/authenticated-shell.md) — the consumer
  of peer events.
- [Admin Shell & Side Navigation](/guides/admin-shell.md) — the
  `/admin/players` page where role changes originate.
- [Admin Endpoints](/api/admin.md) — the `PATCH /admin/players/:id/role`
  surface and its `LAST_ADMIN` error code.
- [Users Table](/database/users.md) — the `role` column on the `user`
  table.
- [Frontend Structure](/architecture/frontend-structure.md)
- [REST API Overview](/api/rest-overview.md)