feat: let a display survive its network — as code, not as authorization

A display is the one surface expected to outlive its network: a television on
a wall in a building whose uplink is a consumer router, with nobody standing
in front of it. When that uplink drops and the screen reboots, the browser
shows its own error page — not the display shell, not an Ekklesia message,
nothing that recovers on its own, because there is no Ekklesia code left
running to recover.

Cache the shell and the declared code, in a service worker scoped to
`/display/`: the page, the host script, the module's display artifact, and
every contributed renderer that display actually mounted. All of them are code
whose bytes already confer no authority, and all of them are addressed by a
URL carrying an exact version.

Cache nothing else, ever. Not the descriptor, not `/api/v1/principal`, not a
module or platform API response, not the realtime stream, not a pairing
exchange, and not any cross-origin request — which is what a signed
object-delivery URL is. It is an allowlist rather than a deny list, so the
cost of forgetting a new endpoint is that code goes uncached rather than that
a permission decision gets stored.

The consequence is stated rather than papered over: a display cannot render
module content while offline. Mounting needs a descriptor, a descriptor is
authorization-bearing, and it is never cached — so a revoked device stays
revoked and cached code can never manufacture current authorization. What the
cache buys is that a kiosk rebooting mid-outage boots into Ekklesia's own
offline state and keeps retrying, that reconnecting mounts with nothing left
to download, and that a hiccup fetching 600 KB of artifact after the tiny
descriptor already resolved falls back to identical bytes instead of failing
the display.

Network-first everywhere, so a screen picks up new code the moment the server
returns and can never be trapped on an obsolete build — the failure mode a
cache-first worker produces on a screen nobody visits. Only a complete
same-origin 200 is stored. Cache names carry a policy version and superseded
generations are deleted; artifacts are pruned to the current version per
module, keeping every artifact belonging to that version because a display
artifact and its contributed renderers share one. The worker never caches
itself.

The page gets a small signal built from two sources, because either alone
lies: `navigator.onLine` is optimistic behind a captive portal, and only the
worker knows it actually served from cache. A display shows a quiet corner
indicator rather than blanking a lobby wall, and resolves its descriptor again
by itself when the network returns — nobody presses Retry on a lobby TV.

Every caching decision lives in a module that touches no global, so the
behaviour is exercised directly rather than through a worker a test runner
cannot host. Registration is an enhancement: no service workers, plain HTTP,
or a refusing policy all leave a display that works exactly as before.

Co-Authored-By: Claude Opus 5 <[email protected]>

apps/api/tests/module-display-routes.test.ts

@@ -28,6 +28,7 @@ import { createApiApp } from "../src/app.js";
 import { createDisplayHostAssets, type DisplayHostAssets } from "../src/display-host-assets.js";
 import {
   DISPLAY_HOST_SCRIPT_PATH,
+  DISPLAY_SERVICE_WORKER_PATH,
   ModuleDisplayConfigurationError,
   moduleDisplayDescriptorUrl,
   moduleDisplayPageUrl,
@@ -51,6 +52,7 @@ const displayPermission = "announcements.display.view";
 const displayHostAssets: DisplayHostAssets = createDisplayHostAssets(
   `<!doctype html><html><body><div id="ekklesia-display"></div><script type="module" src="${DISPLAY_HOST_SCRIPT_PATH}"></script></body></html>`,
   "globalThis.__ekklesiaDisplayHostLoaded = true;\n",
+  "globalThis.__ekklesiaDisplayWorkerLoaded = true;\n",
 );
 
 const temporaryDirectories: string[] = [];
@@ -477,7 +479,11 @@ describe("module display host asset publication", () => {
   });
 
   it("snapshots injected assets and rejects a validator that does not describe its script", async () => {
-    const source = createDisplayHostAssets("<html>display host</html>", "export const host = 1;\n");
+    const source = createDisplayHostAssets(
+      "<html>display host</html>",
+      "export const host = 1;\n",
+      "export const worker = 1;\n",
+    );
     const mutable = { ...source };
     const { app } = await install({ assets: mutable, display: true });
     mutable.html = "<html>mutated secret</html>";
@@ -498,4 +504,55 @@ describe("module display host asset publication", () => {
 
     expect((await app.request(DISPLAY_HOST_SCRIPT_PATH)).status).toBe(404);
   });
+
+  it("serves the offline service worker from the path whose directory is its scope", async () => {
+    const { app } = await install({ display: true });
+
+    const response = await app.request(DISPLAY_SERVICE_WORKER_PATH);
+
+    // `/display/sw.js`, so the worker's default scope is `/display/` — display pages and nothing
+    // else. The Dashboard is never controlled by it (ADR 0043).
+    expect(DISPLAY_SERVICE_WORKER_PATH).toBe("/display/sw.js");
+    expect(response.status).toBe(200);
+    expect(await response.text()).toBe(displayHostAssets.serviceWorker);
+    expect(response.headers.get("content-type")).toBe("text/javascript; charset=utf-8");
+    // `no-cache` with a strong validator, exactly like the host script: a browser re-checks the
+    // worker on its own schedule, which is how a screen avoids being trapped on an obsolete one.
+    expect(response.headers.get("cache-control")).toBe("no-cache");
+    expect(response.headers.get("etag")).toBe(displayHostAssets.serviceWorkerEtag);
+    expect(response.headers.get("x-content-type-options")).toBe("nosniff");
+  });
+
+  it("revalidates the service worker with its own validator, never the host script's", async () => {
+    const { app } = await install({ display: true });
+
+    const notModified = await app.request(DISPLAY_SERVICE_WORKER_PATH, {
+      headers: { "if-none-match": displayHostAssets.serviceWorkerEtag },
+    });
+    expect(notModified.status).toBe(304);
+
+    const wrongValidator = await app.request(DISPLAY_SERVICE_WORKER_PATH, {
+      headers: { "if-none-match": displayHostAssets.scriptEtag },
+    });
+    expect(wrongValidator.status).toBe(200);
+  });
+
+  it("does not publish the service worker when no eligible display uses it", async () => {
+    const { app } = await install({ ui: true, withAssets: false });
+
+    expect((await app.request(DISPLAY_SERVICE_WORKER_PATH)).status).toBe(404);
+  });
+
+  it("cannot be reached through a neighbouring or request-selected path", async () => {
+    const { app } = await install({ display: true });
+
+    for (const candidate of [
+      `${DISPLAY_SERVICE_WORKER_PATH}.map`,
+      "/display/sw.js/",
+      "/sw.js",
+      "/assets/ekklesia-display/sw.js",
+    ]) {
+      expect((await app.request(candidate)).status, candidate).toBe(404);
+    }
+  });
 });