v4: a deployment chooses which ways federated sign-in goes — every instance took users of the others and let its own sign in to them, with no way to turn either off. GERBEUR_FEDERATION_INCOMING=0 keeps users of other instances out: the login and register pages offer no other instance, /manage no list of instances, their sign-ins are refused, and those already in are signed out until it is on again, their homes' events still applied meanwhile. GERBEUR_FEDERATION_OUTGOING=0 keeps the instance's users from signing in elsewhere: its /authorize page lets no instance sign them in, its metadata says so (gerbeur_outgoing), which other instances check before sending anyone there, the settings page lists no instances, and at startup every grant is withdrawn, which signs them out there. Both are on by default and reach the client through index.html. The dev instance gerbeur (:3000) takes no users of the others, while its own still sign in to them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/main.ts

@@ -1,6 +1,8 @@
 /**
  * Builds the Oak application and, when run directly, starts the recurring
- * jobs and listens on {@link LISTEN_HOST}:{@link PORT}. A request goes
+ * jobs and listens on {@link LISTEN_HOST}:{@link PORT}, having first signed
+ * the users out of other instances if they may no longer sign in there
+ * (`FEDERATION.outgoing`). A request goes
  * through the error middleware, the OG tags and CORS, then the routers, one
  * per resource of the API (`API.md` lists them), and last the built frontend,
  * which answers every path no router did.
@@ -37,7 +39,13 @@ import usersRouter from "./routes/users.ts";
 import wellKnownRouter from "./routes/well-known.ts";
 import wsRouter from "./routes/ws.ts";
 
-import { ALLOWED_ORIGINS, LISTEN_HOST, PORT, PUBLIC_URL } from "./config.ts";
+import {
+  ALLOWED_ORIGINS,
+  FEDERATION,
+  LISTEN_HOST,
+  PORT,
+  PUBLIC_URL,
+} from "./config.ts";
 import { errorMiddleware } from "./middleware/error.ts";
 import { ogMiddleware } from "./middleware/og.ts";
 import { routeStaticFilesFrom } from "./lib/static.ts";
@@ -46,7 +54,10 @@ import { rateLimitSweepJob } from "./lib/rate-limit.ts";
 import { digestJob } from "./services/digest-service.ts";
 import { communityJob } from "./services/community-service.ts";
 import { pingJob, typingExpiryJob } from "./services/ws-service.ts";
-import { outboxJob } from "./services/identity-provider-service.ts";
+import {
+  outboxJob,
+  withdrawAllGrants,
+} from "./services/identity-provider-service.ts";
 
 /**
  * The whole server, exported so the tests' helpers can send it requests
@@ -115,6 +126,12 @@ app.addEventListener(
 );
 
 if (import.meta.main) {
+  if (!FEDERATION.outgoing) {
+    const withdrawn = await withdrawAllGrants();
+    if (withdrawn > 0) {
+      console.log(`Signed users out of other instances (${withdrawn} grants)`);
+    }
+  }
   startJobs([
     digestJob,
     communityJob,