v4: users sign in to one gerbeur instance with their account on another — topic instances, each with its own admins, domain and data, had no way to share accounts. Sign-in is federated over a subset of OpenID Connect, the authorization code flow with PKCE: each instance is an identity provider for its own users and a client for the others', and signs what it tells them with an Ed25519 key kept in federation_keys and published at /.well-known/jwks.json, beside its metadata at /.well-known/openid-configuration. Only sign-in is federated: an account from another instance is a local users row linked to (issuer, subject) in user_identities, with no password (password_hash ''), so sessions, roles, permissions, the socket and every foreign key work as before.

A remote user is name@host here, in users.username, in paths (/@alice@music.example) and in mentions, through one handle rule in shared/validation.ts that paths, MENTION_RE and Markdown all build on; Markdown undoes GFM's email autolink of a handle. Their name, email and avatar come from their home at each sign-in and by its signed events (federation_outbox, delivered by the federation job with backoff): a rename at home leaves the old handle an alias here, and a password change or a withdrawn grant ends their sessions here. Their password, email, avatar and name can't change here (assertLocalUser), nor can they sign in elsewhere from here.

An admin keeps the trusted and blocked instances on /manage (federation:manage): a trusted instance's users sign in from a button on the login page, a blocked one's can't and are signed out at once, and any other's join with an invite from the register page. The /authorize page asks the user once per instance, and /settings lists the instances their account signs in to, each disconnectable; /login?next= brings a signed-out visitor back to /authorize. Emails stay unique among local accounts only, and migration 0024 adds the tables, makes that index partial and reserves authorize and federation as category slugs. remote/gerbeur-instance.ts is the only way to other instances: https and public addresses only, no redirects, no certificate fallback, unless GERBEUR_FEDERATION_ALLOW_PRIVATE lets two instances on one machine talk.

298 backend tests and 702 frontend tests pass, and deno check, deno fmt, deno lint, tsc, eslint, doc:lint and i18n:compile --strict are clean; checked end to end with two built instances on one machine: sign-in through the other's login and consent pages, consent skipped once granted and asked again after disconnecting, a rename and a password change at home reaching the other, and blocking; screenshots of the login, consent, profile, settings and manage pages at 1280 and 390px.

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

api/routes/me.ts

@@ -7,7 +7,8 @@
  *
  * - `GET /api/me` — signed in. Answers the caller's own user.
  * - `PATCH /api/me` — signed in. Changes the caller's description and email,
- *   and answers their user. A username changes only by `user:manage`, through
+ *   and answers their user. The email of an account from another instance is
+ *   that instance's: 400. A username changes only by `user:manage`, through
  *   `PATCH /api/users/:key/username`.
  * - `GET /api/me/media-token` — signed in. Answers a media token, for a
  *   session that predates them.
@@ -15,9 +16,17 @@
  * - `PATCH /api/me/settings` — signed in. Changes some of the caller's
  *   settings, and answers them all.
  * - `PUT /api/me/password` — signed in, with the current password. Sets a new
- *   one, ending every other session, and answers fresh tokens.
+ *   one, ending every other session, and answers fresh tokens. 400 for an
+ *   account from another instance, which has no password here.
  * - `GET /api/me/playlists?contains=:dumpId` — signed in. Answers each of the
  *   caller's playlists and whether it holds the dump.
+ * - `GET /api/me/grants` — signed in. Answers the other instances the caller
+ *   let their account sign in to.
+ * - `DELETE /api/me/grants/:host` — signed in. Withdraws that grant, which
+ *   ends the caller's sessions there.
+ *
+ * A change to the caller's email or password is told to those instances
+ * (`notifyClients`).
  *
  * @module
  */
@@ -39,11 +48,21 @@ import { toPublicUser, toSelfUser } from "../model/user.ts";
 import { createMediaToken, issueTokens } from "../lib/jwt.ts";
 import { verifyPassword } from "../lib/password.ts";
 import { type AuthContext, authMiddleware } from "../middleware/auth.ts";
-import { getUserById, updateUser } from "../services/user-service.ts";
+import {
+  assertLocalUser,
+  getUserById,
+  updateUser,
+} from "../services/user-service.ts";
 import { getSettings, updateSettings } from "../services/settings-service.ts";
 import { getPlaylistMembershipsForDump } from "../services/playlist-service.ts";
 import { broadcastUserUpdated } from "../services/ws-service.ts";
 import { VALIDATION } from "../../shared/validation.ts";
+import type { FederationGrant } from "../../shared/federation.ts";
+import {
+  listGrants,
+  notifyClients,
+  revokeGrant,
+} from "../services/identity-provider-service.ts";
 
 /**
  * Mounted by `api/main.ts` behind the error, Open Graph and CORS middleware.
@@ -65,6 +84,7 @@ router.patch("/", authMiddleware, async (ctx: AuthContext) => {
     throw new APIException(400, "Invalid request");
   }
   const userId = ctx.state.user.userId;
+  if (body.email !== undefined) assertLocalUser(getUserById(userId));
   // Only these fields: a stolen token alone must not change the role or the
   // password, which needs the current one.
   const updated = await updateUser(userId, {
@@ -72,6 +92,7 @@ router.patch("/", authMiddleware, async (ctx: AuthContext) => {
     email: body.email,
   });
   broadcastUserUpdated(toPublicUser(updated));
+  if (body.email !== undefined) await notifyClients(userId, { type: "update" });
   const responseBody: APIResponse<SelfUser> = {
     success: true,
     data: toSelfUser(updated),
@@ -127,7 +148,8 @@ router.put("/password", authMiddleware, async (ctx: AuthContext) => {
   }
 
   const user = getUserById(ctx.state.user.userId);
-  const valid = await verifyPassword(currentPassword, user.passwordHash);
+  assertLocalUser(user);
+  const valid = await verifyPassword(currentPassword, user.passwordHash ?? "");
   if (!valid) {
     throw new APIException(403, "Current password is incorrect");
   }
@@ -137,6 +159,7 @@ router.put("/password", authMiddleware, async (ctx: AuthContext) => {
   const updated = await updateUser(ctx.state.user.userId, {
     password: newPassword,
   });
+  await notifyClients(updated.id, { type: "revoke" });
   const responseBody: APIResponse<AuthTokens> = {
     success: true,
     data: await issueTokens(updated),
@@ -154,4 +177,22 @@ router.get("/playlists", authMiddleware, (ctx: AuthContext) => {
   ctx.response.body = responseBody;
 });
 
+router.get("/grants", authMiddleware, (ctx: AuthContext) => {
+  const responseBody: APIResponse<FederationGrant[]> = {
+    success: true,
+    data: listGrants(ctx.state.user.userId),
+  };
+  ctx.response.body = responseBody;
+});
+
+router.delete(
+  "/grants/:host",
+  authMiddleware,
+  async (ctx: AuthContext<"/grants/:host">) => {
+    await revokeGrant(ctx.state.user.userId, ctx.params.host);
+    const responseBody: APIResponse<null> = { success: true, data: null };
+    ctx.response.body = responseBody;
+  },
+);
+
 export default router;