Presence, và ai được phép nhìn thấy

/** Presence states per FR-RTM-06; delivery scope is FR-RTM-07's concern. */
export const presenceChangedSchema = z.strictObject({
  type: z.literal("presence.changed"),
  payload: z.strictObject({
    user: z.string().min(1),
    state: z.enum(["online", "offline"]),
  }),
});

["presence.changed", "outbound", "derived from connections the gateway holds, not claimed"],

And presence (FR-RTM-06) and typing (FR-RTM-08) will reuse this exact pub/sub
plumbing with TTLs per ADR-10.

  publish(message: Message): Promise<void>;
    const message = messageCreatedSchema.shape.payload.safeParse(parsed);

      send(connection.socket, { type: "message.created", payload: message });

cmdstat_subscribe   calls=12
cmdstat_publish     calls=18

import { z } from "zod";
 
/** Presence's own fabric: a subject grammar and the payload that crosses it
 * (chapter 3.19, ADR-19).
 *
 * WHY THIS IS NOT IN `fanout.ts`. The fan-out's subject grammar lives there and
 * presence's could have joined it, three lines below `subjectForChannel`. It does
 * not, for the reason `internal.ts` already demonstrates: the event spine keeps
 * its own `subjectFor` in its own file, so **each fabric owning its subject
 * grammar is this package's established shape** rather than a compromise. The
 * practical payoff is that `fanout.ts` gains no hunk, and chapter 3.18's fences
 * over it stay where they are.
 *
 * NOT `subjectFor` and not `subjectForChannel`. `internal.ts` exports the first
 * and `fanout.ts` the second, and chapter 3.18 paid for that collision once:
 *
 *     error TS2308: Module "./internal.js" has already exported a member
 *     named 'subjectFor'.
 *
 * WHY A SECOND SUBJECT AT ALL, rather than enveloping two kinds on `chan:{id}`.
 * The message path is typed to messages at three points, and only two of them are
 * in one file: `publish(message: Message)` and a `messageCreatedSchema` parse in
 * `services/gateway/src/fanout.ts`, then the literal `message.created` send inside
 * `session.ts`'s `deliver` — a function ten chapters fence. Enveloping means
 * editing the hot path to serve the lowest-volume traffic on it. Separating the subjects also makes cross-kind
 * mis-delivery impossible rather than test-enforced: a presence payload cannot
 * arrive where a message parse is waiting, because nothing publishes it there.
 *
 * The declared cost: a channel now carries two subscriptions rather than one. */
export function subjectForPresence(channelId: string): string {
  return `presence:${channelId}`;
}
 
/** What crosses `presence:{channel_id}` between gateway instances. Consumed only
 * by gateways and **never sent to a client**.
 *
 * `transition` IS WHY A WATCHER SHARING THREE CHANNELS GETS ONE FRAME. A
 * transition publishes on every one of the subject's channels, so an instance
 * hosting a watcher who shares three of them receives three copies. The wire
 * frame carries `user` and `state` and no channel, so the copies are
 * indistinguishable duplicates; the id lets a receiver deliver a given transition
 * to a given connection once. It is minted per transition, not per publish.
 *
 * THIS IS THE FIRST TIME THE FABRIC PAYLOAD AND THE WIRE FRAME DIFFER. On the
 * message path they are the same object — which is exactly why `fanout.ts` could
 * type its `publish` as `Message` and get away with it. What reaches a client is
 * still what chapter 1.3 published and `frames.test.ts` asserts:
 *
 *     { type: "presence.changed", payload: { user, state } }
 *
 * `strictObject`, so an unknown field is a rejection rather than a silent
 * ignore: a field added on one side of a rolling deploy fails loudly on the
 * other. */
export const presenceFabricSchema = z.strictObject({
  user: z.string().min(1),
  state: z.enum(["online", "offline"]),
  transition: z.string().min(1),
});
 
export type PresenceFabric = z.infer<typeof presenceFabricSchema>;

@@ -10,3 +10,4 @@ export * from "./frames.js";
 export * from "./codes.js";
 export * from "./internal.js";
 export * from "./fanout.js";
+export * from "./presence.js";

import { describe, expect, it } from "vitest";
 
import { subjectForChannel } from "./fanout.js";
import { presenceFabricSchema, subjectForPresence } from "./presence.js";
 
describe("subjectForPresence", () => {
  it("is one subject per channel, prefixed to keep it off the message path", () => {
    expect(subjectForPresence("c1")).toBe("presence:c1");
  });
 
  // NOT A TASTE ASSERTION. The whole argument for a second subject grammar is
  // that a presence payload can never arrive where a message parse is waiting.
  // That property is topology, not vigilance — and it holds only while the two
  // grammars cannot collide for the same channel id.
  it("never collides with the message subject for the same channel", () => {
    const id = "00000000-0000-0000-0000-000000000001";
    expect(subjectForPresence(id)).not.toBe(subjectForChannel(id));
  });
});
 
describe("presenceFabricSchema", () => {
  const valid = { user: "tuan", state: "online", transition: "t-1" };
 
  it("accepts a transition", () => {
    expect(presenceFabricSchema.parse(valid)).toEqual(valid);
  });
 
  it("rejects a state the clause does not name", () => {
    // FR-RTM-06 says `online` and `offline`. `frames.test.ts` already rejects
    // "away" on the wire frame; this is the same refusal one layer down.
    expect(
      presenceFabricSchema.safeParse({ ...valid, state: "away" }).success,
    ).toBe(false);
  });
 
  it("rejects a payload with no transition", () => {
    expect(
      presenceFabricSchema.safeParse({ user: valid.user, state: valid.state })
        .success,
    ).toBe(false);
  });
 
  // `strictObject` rather than `object`. A field added on one side of a rolling
  // deploy must fail loudly on the other rather than be dropped in silence.
  it("rejects an unknown field instead of ignoring it", () => {
    expect(
      presenceFabricSchema.safeParse({ ...valid, channel: "c1" }).success,
    ).toBe(false);
  });
});

presence:{env}:{user}          exists  <=>  the user is online
presence:offline:{env}:{user}  exists  <=>  somebody already said they left

import { randomUUID } from "node:crypto";
 
import {
  presenceFabricSchema,
  subjectForPresence,
  type PresenceFabric,
} from "@relay/protocol";
import type { Logger } from "@relay/service-kit";
import { Redis } from "ioredis";
 
// Presence (chapter 3.19, ADR-19): who is online, and who is allowed to know.
//
// A SECOND FABRIC BESIDE THE FAN-OUT, NOT A SECOND PAYLOAD ON IT. The message path
// is typed to messages at three points: `publish(message: Message)` and a
// `messageCreatedSchema` parse in `fanout.ts`, and the literal `message.created`
// send inside `session.ts`'s `deliver`, which ten chapters fence. Widening the
// payload means editing all three, one of them in the file this feature already
// edits most. So presence gets `presence:{channel_id}` and its own module,
// and `fanout.ts` is not edited at all. The declared cost is two subscriptions
// per channel instead of one.
//
// TWO CLIENTS, for the reason chapter 3.8 gave for the limiter's: a connection in
// subscriber mode cannot run ordinary commands, and presence needs `SET`,
// `EXISTS` and `PUBLISH` as well as `SUBSCRIBE`. That makes five Redis
// connections per gateway — fanout's two, the limiter's one, and these — and each
// close has an owner.
//
// NOTHING HERE IS A SOURCE OF TRUTH (constitution IV, ADR-10). Presence loss is
// cosmetic and self-heals on the next transition; the correct amount of
// durability for a green circle is none.
 
export const DEFAULT_REDIS_URL = "redis://localhost:6379";
 
/** FR-RTM-06 says thirty seconds. Asserted by a test rather than left as a bare
 * constant, so the clause is represented somewhere a reader can find it. */
export const DEFAULT_GRACE_MS = 30_000;
/** `docs/05-sad.md`'s presence key. Must be >= `graceMs`; the close re-pins the
 * key anyway, so this is the sane default rather than the mechanism. */
export const DEFAULT_TTL_MS = 30_000;
/** Three refreshes per TTL, so two consecutive misses do not expire a live user.
 * NOT `PING_INTERVAL_MS`, which is also 30_000: a TTL equal to its refresh
 * interval expires a connected user. Three thirty-second numbers in this system
 * are three different quantities (research R3). */
export const DEFAULT_REFRESH_MS = 10_000;
/** How long after the grace ends the check runs. Not padding — without it the
 * key's expiry and the check are the same instant reached by two clocks, and a
 * tie strands the user online permanently (research R2b). */
export const DEFAULT_MARGIN_MS = 1_000;
 
/** Did this caller cause the transition?
 *
 * `SET … NX` answers `"OK"` for the one caller that found the key absent and null
 * for everyone else. Factored out because the module builds its own Redis clients
 * from a url and cannot be handed a double — the gateway's shape is pure logic in
 * `.test.ts` and Redis in `.itest.ts`, which is why `limits.ts` exports
 * `overLimit` and `windowStartFor` the same way. */
export function wonTransition(reply: string | null): boolean {
  return reply === "OK";
}
 
/** When the grace check runs, relative to the close.
 *
 * NOT `graceMs`. The close re-pins the key to expire at `graceMs`, so pinning and
 * checking at the same instant puts two deadlines on one moment reached by two
 * clocks — the key expires at `close + δ + graceMs` where δ is a round trip, the
 * timer fires at `close + graceMs + ε`. With `ε < δ` the check finds the key alive,
 * logs `presence.suppressed`, and its ONE-SHOT timer is gone: the user is stranded
 * online, which is worse than the duplicate `online` the re-pin was added to
 * prevent. Redis also holds a key until `now` is strictly past its expiry, so a tie
 * falls the wrong way too (research R2b). */
export function graceCheckDelay(graceMs: number, marginMs: number): number {
  return graceMs + marginMs;
}
 
export interface Presence {
  /** Register the delivery callback. Set by the session layer at wiring time, as
   * the fan-out's is: the fabric knows how to receive, the sessions know who to
   * hand it to. */
  onTransition(
    handler: (channelId: string, payload: PresenceFabric) => void,
  ): void;
  /** A connection opened. May publish `online`; publishes nothing when the user
   * was already online anywhere. */
  connected(
    environmentId: string,
    user: string,
    channelIds: Iterable<string>,
  ): Promise<void>;
  /** A connection closed and the caller has already removed it from the registry.
   * When it was the user's last connection on this instance: re-pins the key to
   * `graceMs`, awaits that, then schedules one check at `graceMs + marginMs`,
   * replacing any pending one for that user. */
  disconnected(
    environmentId: string,
    user: string,
    channelIds: Iterable<string>,
  ): Promise<void>;
  /** Claim a transition for one connection. True the first time, false after —
   * a watcher sharing three channels receives three copies of one transition and
   * must be handed exactly one frame (FR-012). */
  claim(transition: string, connectionId: string): boolean;
  subscribe(channelId: string): Promise<void>;
  unsubscribe(channelId: string): Promise<void>;
  close(): Promise<void>;
}
 
export interface PresenceOptions {
  url?: string;
  logger: Logger;
  graceMs?: number;
  ttlMs?: number;
  refreshMs?: number;
  marginMs?: number;
}
 
export function createPresence({
  url = process.env.RELAY_REDIS_URL ?? DEFAULT_REDIS_URL,
  logger,
  graceMs = DEFAULT_GRACE_MS,
  ttlMs = DEFAULT_TTL_MS,
  refreshMs = DEFAULT_REFRESH_MS,
  marginMs = DEFAULT_MARGIN_MS,
}: PresenceOptions): Presence {
  // FAIL FAST RATHER THAN QUEUE, which is the limiter's shape and not the fan-out's.
  // Default ioredis retries forever and QUEUES commands, so against a dead store a
  // `SET` neither succeeds nor rejects — it waits, and the failure path this module
  // documents is never taken. Chapter 3.18 measured the same thing about its
  // publisher: "default ioredis retries FOREVER, so `publish` never rejects and the
  // command queues."
  //
  // The subscriber keeps its retry behaviour: it MUST reconnect when the store comes
  // back, which is what "the next transition publishes without a restart" rests on.
  const commands = new Redis(url, {
    maxRetriesPerRequest: 0,
    connectTimeout: 1_000,
  });
  const subscriber = new Redis(url);
 
  // THE STATED REASON FOR THESE LISTENERS IS NOT THE ONE THE LIMITER GIVES.
  // `limits.ts` says a missing listener means "the gateway would die"; chapter
  // 3.18 measured that against ioredis 6.0.0 by reproducing the exact client, and
  // the process STAYS ALIVE — ioredis prints `[ioredis] Unhandled error event: …`
  // itself and continues. Seven lines in four seconds against a dead port.
  //
  // The accurate reason is that those lines are unstructured and unbounded, which
  // defeats NFR-OBS-01. A presence path that cannot reach Redis is an expected
  // state; it should say so once, in the log vocabulary this module owns.
  for (const [name, client] of [
    ["commands", commands],
    ["subscriber", subscriber],
  ] as const) {
    client.on("error", (error: unknown) => {
      logger.log("error", "presence.failed", {
        op: `connection:${name}`,
        error: String(error),
      });
    });
  }
 
  let deliver: (channelId: string, payload: PresenceFabric) => void = () => {};
 
  // Reference-counted, because two members of one channel on one instance must not
  // unsubscribe each other. One count per channel and two Redis calls under it —
  // `fanout.ts` keeps the same map for the same reason, over the same ids.
  const counts = new Map<string, number>();
 
  /** A transition publishes on every one of the subject's channels, so an instance
   * hosting a watcher who shares three of them receives three copies of one
   * transition. This is what makes the watcher see one frame: the id is minted per
   * transition, and a receiver delivers each `(transition, connection)` pair once.
   *
   * Cleared on a timer rather than kept: every copy of one transition arrives
   * within milliseconds of the others, so a few seconds is generous and unbounded
   * growth is the alternative. */
  const seen = new Map<string, Set<string>>();
 
  const key = (environmentId: string, user: string): string =>
    `presence:${environmentId}:${user}`;
  const marker = (environmentId: string, user: string): string =>
    `presence:offline:${environmentId}:${user}`;
 
  /** Users this instance currently holds a connection for, and the environment each
   * belongs to. The refresh loop walks this; `disconnected` removes from it. */
  const held = new Map<string, string>();
 
  /** One pending grace check per user, REPLACED rather than added to. Close, reopen
   * at a third of the window, close again must leave one decision answered by the
   * state at the end of the second window — two timers would publish twice. */
  const pending = new Map<string, NodeJS.Timeout>();
 
  const refreshTimer = setInterval(() => {
    void (async () => {
      for (const [user, environmentId] of held) {
        const reply = await failable("refresh", () =>
          commands.set(key(environmentId, user), "1", "PX", ttlMs, "XX"),
        );
        // `XX` answers null when the key is gone — a Redis restart or an eviction
        // under a live connection. The key is put back and NOTHING IS PUBLISHED,
        // which is FR-031's "MAY" declined: this loop holds `held`, a user and an
        // environment, and a publish needs the subject's channel set, which only
        // `connected` and `disconnected` are given. `presence.suppressed` says so
        // rather than leaving the omission silent.
        //
        // The residual window is real and is in `gaps.md`: between the key vanishing
        // and this refresh restoring it — at most `refreshMs` — another instance's
        // grace check can find the key absent and publish `offline` for a user who is
        // connected here, and no `online` follows it. Carrying the channel set on
        // `held` would close it.
        if (reply === null) {
          logger.log("info", "presence.suppressed", {
            user,
            state: "online",
            reason: "key vanished under a live connection; re-electing",
          });
          await failable("refresh:reelect", async () => {
            await commands.set(key(environmentId, user), "1", "PX", ttlMs, "NX");
            // UNCONDITIONALLY, unlike `connected` below. There the loser returns
            // early because the winner is publishing `online` and will clear the
            // marker itself; here nobody publishes, so a loser that skipped the
            // delete would leave a stale "somebody already said they left" standing
            // against a user who is demonstrably connected. It also removes the one
            // arm in this module that only a two-instance race could reach, which
            // is a branch a test could only ever flake on.
            await commands.del(marker(environmentId, user));
          });
        }
      }
    })();
  }, refreshMs);
  refreshTimer.unref();
 
  async function failable<T>(op: string, work: () => Promise<T>): Promise<T | null> {
    try {
      return await work();
    } catch (error) {
      // Swallowed and logged, never rethrown: presence is the only thing that may
      // degrade (FR-023). And the log line is the REQUIREMENT'S EVIDENCE — a path
      // that silently does nothing satisfies "the socket still opened" exactly as
      // well as a working one does, which is chapter 3.18's trap against its own
      // publisher.
      logger.log("error", "presence.failed", { op, error: String(error) });
      return null;
    }
  }
 
  async function publish(
    environmentId: string,
    user: string,
    state: "online" | "offline",
    channelIds: Iterable<string>,
  ): Promise<void> {
    const transition = randomUUID();
    const payload: PresenceFabric = { user, state, transition };
    const body = JSON.stringify(payload);
    const channels = [...channelIds];
    await failable("publish", async () => {
      await Promise.all(
        channels.map((channelId) =>
          commands.publish(subjectForPresence(channelId), body),
        ),
      );
    });
    // `channels` is a COUNT, not a list. The number is useful in an incident; the
    // list is a membership graph in a log file (constitution VI).
    logger.log("info", "presence.published", {
      user,
      state,
      channels: channels.length,
    });
  }
 
  subscriber.on("message", (subject: string, raw: string) => {
    let parsed: unknown;
    try {
      parsed = JSON.parse(raw);
    } catch {
      logger.log("error", "presence.invalid_payload", { subject });
      return;
    }
    // Validated on receipt even though the fabric is inside the trust boundary.
    // `fanout.ts:77-79` states the reason and it is unchanged here: "inside" is one
    // compromised dependency away from "outside", and a malformed payload must not
    // reach a client.
    const payload = presenceFabricSchema.safeParse(parsed);
    if (!payload.success) {
      logger.log("error", "presence.invalid_payload", { subject });
      return;
    }
    deliver(subject.slice("presence:".length), payload.data);
  });
 
  return {
    onTransition(handler) {
      deliver = handler;
    },
 
    async connected(environmentId, user, channelIds) {
      // `SET … NX` IS THE ELECTION. Exactly one caller across every instance gets
      // `OK` for a user who was absent; everyone else gets null and stays quiet.
      // Measured against Redis 8.10.0 rather than reasoned about (research R2).
      held.set(user, environmentId);
      const reply = await failable("connected:set", () =>
        commands.set(key(environmentId, user), "1", "PX", ttlMs, "NX"),
      );
      if (!wonTransition(reply)) {
        logger.log("info", "presence.suppressed", {
          user,
          state: "online",
          reason: "already online",
        });
        return;
      }
      // The offline marker is cleared by whoever wins the online transition, so the
      // next departure can elect a publisher again.
      await failable("connected:clear-marker", () =>
        commands.del(marker(environmentId, user)),
      );
      await publish(environmentId, user, "online", channelIds);
    },
 
    async disconnected(environmentId, user, channelIds) {
      // The caller has already removed the connection AND established that no other
      // local one remains — `session.ts` asks `registry.connectionsFor` for that.
      // So dropping the refresh here is correct: this instance holds nothing for
      // this user any more.
      held.delete(user);
      // The channel set is captured HERE, at the close, and held by the closure the
      // check runs in. By the time it fires the connection is out of the registry.
      // It is only ever used when nobody returned, in which case it is still right.
      const channels = [...channelIds];
 
      // RE-PIN, AND AWAIT IT BEFORE ARMING THE TIMER. Without the re-pin the key
      // dies at `last_refresh + ttlMs`, which is up to `refreshMs` BEFORE the grace
      // ends — and a reconnection in that gap wins `SET … NX` and publishes a second
      // `online` for a user who never left (FR-007, research R2a). `XX` so it never
      // resurrects a key that is already gone.
      //
      // The await is the other half: it puts the round trip inside the wait instead
      // of racing the timer (research R2b).
      await failable("disconnected:repin", () =>
        commands.set(key(environmentId, user), "1", "PX", graceMs, "XX"),
      );
 
      const existing = pending.get(user);
      if (existing) clearTimeout(existing);
      const timer = setTimeout(() => {
        pending.delete(user);
        void (async () => {
          // The key is absent only if no instance refreshed it for a whole window —
          // which is the same question as "does anybody still hold a connection?",
          // asked without a membership set. `docs/05-sad.md`'s `conn:{env}:{user}`
          // is not needed for this (research R6).
          const alive = await failable("grace:exists", () =>
            commands.exists(key(environmentId, user)),
          );
          if (alive !== 0) {
            logger.log("info", "presence.suppressed", {
              user,
              state: "offline",
              reason: "a connection is still open somewhere",
            });
            return;
          }
          // Two instances whose last connections close in the same tick both find
          // the key absent. `SET … NX` on a separate marker gives exactly one of
          // them the right to speak.
          const won = await failable("grace:elect", () =>
            commands.set(marker(environmentId, user), "1", "PX", ttlMs, "NX"),
          );
          if (!wonTransition(won)) {
            logger.log("info", "presence.suppressed", {
              user,
              state: "offline",
              reason: "another instance published it",
            });
            return;
          }
          await publish(environmentId, user, "offline", channels);
        })();
      }, graceCheckDelay(graceMs, marginMs));
      timer.unref();
      pending.set(user, timer);
    },
 
    async subscribe(channelId) {
      const next = (counts.get(channelId) ?? 0) + 1;
      counts.set(channelId, next);
      if (next === 1) {
        await failable("subscribe", () =>
          subscriber.subscribe(subjectForPresence(channelId)),
        );
      }
    },
 
    async unsubscribe(channelId) {
      const next = (counts.get(channelId) ?? 1) - 1;
      if (next <= 0) {
        counts.delete(channelId);
        await failable("unsubscribe", () =>
          subscriber.unsubscribe(subjectForPresence(channelId)),
        );
      } else {
        counts.set(channelId, next);
      }
    },
 
    /** True when this connection has not already been handed this transition.
     * Records as a side effect, which is why it is a method and not a predicate. */
    claim(transition, connectionId) {
      let holders = seen.get(transition);
      if (!holders) {
        holders = new Set();
        seen.set(transition, holders);
        setTimeout(() => seen.delete(transition), 5_000).unref();
      }
      if (holders.has(connectionId)) return false;
      holders.add(connectionId);
      return true;
    },
 
    async close() {
      // Cleared, or a suite standing up two instances leaks a timer into the next
      // file. A draining instance also abandons its pending offlines — stated in
      // the chapter rather than discovered: nothing publishes them, the key expires
      // silently, and watchers hold a stale green circle until the subject next
      // transitions. ADR-10 permits that; a reader should still be told.
      clearInterval(refreshTimer);
      for (const timer of pending.values()) clearTimeout(timer);
      pending.clear();
      held.clear();
      seen.clear();
      subscriber.disconnect();
      commands.disconnect();
    },
  };
}

a transition publishes on presence:{c} for each of the SUBJECT's channels
an instance subscribes to presence:{c} only for channels its OWN members hold
delivery walks registry.subscribersOf(c), which is a membership test

@@ -90,6 +90,20 @@ export class Registry {
     return [...this.byId.values()].filter((c) => c.channelIds.has(channelId));
   }
 
+  /** Chapter 3.19. Every local connection this user holds — the question presence
+   * asks at a close: "was that the last one on this instance?"
+   *
+   * A FILTER RATHER THAN A SECOND INDEX. `subscribersOf` above is the same shape
+   * for the same reason: one instance's connection set is small, and a second map
+   * is a second thing to keep correct at `add` and `remove`. The cross-instance
+   * half of this question is not asked here at all — Redis answers it, because
+   * this file's whole point since 2.5 is that it cannot see other instances. */
+  connectionsFor(userExternalId: string): Connection[] {
+    return [...this.byId.values()].filter(
+      (c) => c.identity.userExternalId === userExternalId,
+    );
+  }
+
   all(): Connection[] {
     return [...this.byId.values()];
   }

@@ -10,6 +10,7 @@ import {
   type Frame,
   type Message,
   isErrorCode,
+  type PresenceFabric,
 } from "@relay/protocol";
 import { newRequestId, type Logger } from "@relay/service-kit";
 import { WebSocketServer, type WebSocket } from "ws";
@@ -19,6 +20,7 @@ import { authenticate, type Identity } from "./auth.js";
 import type { Fanout } from "./fanout.js";
 import type { Decision, GatewayLimits } from "./limits.js";
 import { createMeter, METER_INTERVAL_MS, type Meter } from "./meter.js";
+import { type Presence } from "./presence.js";
 import { Registry, type Connection } from "./registry.js";
 import {
   MAX_BUFFERED_FRAMES,
@@ -143,8 +145,25 @@ export interface SessionServerOptions {
    * unmetered one. `main.ts` always supplies the interval; the meter itself is
    * built here so its timer has the same owner as the heartbeat's. */
   meterIntervalMs?: number;
+  /** Chapter 3.19. Optional for the same reason `fanout`, `limits` and the meter
+   * are: 2.5's tests and a single-process dev run have no Redis, and a socket
+   * server that refused to start without one would be a worse default than a
+   * presence-less one. `main.ts` always supplies it. */
+  presence?: Presence;
 }
 
+// THE FOUR PRESENCE TIMINGS ARE NOT HERE, and an earlier draft of this chapter put
+// them here. `meterIntervalMs` is a session option because `attachSessions` BUILDS
+// the meter; `fanout`, `limits` and `presence` are injected already built, and an
+// injected thing carries its own configuration. A test that wants a hundred-
+// millisecond grace period constructs `createPresence({ graceMs: 100, … })` and
+// injects that, the way the fan-out's tests already do. Four options that only
+// forwarded values would be four more things to keep in step with `PresenceOptions`.
+//
+// eslint found this: they were declared, destructured, and used by nothing.
+// `presence` itself is declared on the interface above and destructured below, where
+// the delivery path and the two hook points consume it.
+
 export function attachSessions({
   server,
   api,
@@ -154,6 +173,7 @@ export function attachSessions({
   resumeDeadlineMs = SUBSCRIBE_DEADLINE_MS,
   limits,
   meterIntervalMs = METER_INTERVAL_MS,
+  presence,
 }: SessionServerOptions): {
   registry: Registry;
   meter: Meter;
@@ -195,6 +215,30 @@ export function attachSessions({
     }
   }
   fanout?.onDelivery(deliver);
+
+  /** A presence transition arriving from its own fabric.
+   *
+   * NOT `deliver`'s path, and the differences are the point. Presence carries no
+   * sequence, so it can neither duplicate a backfilled row nor leave a gap — which
+   * is why it consults neither `connection.phase` nor `connection.marks`. Buffering
+   * it during a resume would delay a frame for no benefit, and `suppressed()` takes
+   * a `Message`. A transition mid-resume is sent immediately.
+   *
+   * THE WIRE FRAME IS BUILT FROM TWO FIELDS. `transition` is the fabric's business
+   * and never leaves this function: what a client receives is what chapter 1.3
+   * published and `frames.test.ts` asserts. */
+  function deliverPresence(channelId: string, payload: PresenceFabric): void {
+    for (const connection of registry.subscribersOf(channelId)) {
+      // One frame per transition per connection, however many channels this
+      // connection shares with the subject (FR-012).
+      if (!presence?.claim(payload.transition, connection.id)) continue;
+      send(connection.socket, {
+        type: "presence.changed",
+        payload: { user: payload.user, state: payload.state },
+      });
+    }
+  }
+  presence?.onTransition(deliverPresence);
   // noServer: the upgrade is handled by hand so the token can be checked
   // BEFORE the handshake completes. Letting ws own the upgrade would mean
   // rejecting a socket that already exists (EIR-WS-05 wants the close code
@@ -357,9 +401,21 @@ export function attachSessions({
     // makes this instance a subscriber, and the last one to leave releases
     // it (reference-counted in the fabric).
     const subscribing = Promise.all(
-      [...connection.channelIds].map((channelId) =>
+      [...connection.channelIds].flatMap((channelId) => [
         fanout?.subscribe(channelId),
-      ),
+        // Chapter 3.19. Presence has its own subject per channel, so a channel now
+        // carries two subscriptions. `ioredis` takes a variadic `subscribe`, so the
+        // count doubles and the round trips do not.
+        presence?.subscribe(channelId),
+      ]),
+    );
+    // AFTER `registry.add`, so "is this the user's first connection here?" is asked
+    // of a registry that already contains it. The close handler needs the opposite
+    // and gets it three lines apart — see the note there.
+    void presence?.connected(
+      identity.environmentId,
+      identity.userExternalId,
+      connection.channelIds,
     );
     logger.log("info", "connection.opened", {
       connection_id: connection.id,
@@ -389,6 +445,32 @@ export function attachSessions({
       // would turn one event into a burst of HTTP requests.
       meter?.closed(connection, new Date());
       registry.remove(connection.id);
+      // Chapter 3.19, AND THIS HANDLER NOW CARRIES THREE ORDERING CONSTRAINTS, not
+      // one. The meter is told BEFORE `registry.remove` — a socket that opened and
+      // closed between two reports would otherwise be counted zero, which is the one
+      // thing the wall-clock-minute unit was chosen to charge. Presence is told
+      // AFTER it, because it asks whether this was the user's last connection on
+      // this instance and must not count the one that is leaving. The unsubscribes
+      // come last.
+      //
+      // Swapping the middle two is not a style change. With `registry.remove` after
+      // this block, `connectionsFor` still sees the closing connection, the count is
+      // 1 rather than 0, no grace check is ever scheduled, and the user stays online
+      // for ever. A test asserts the scheduling for that reason.
+      //
+      // The condition asks a local question only. Closing one of two connections on
+      // this instance must publish nothing (FR-006); whether the user is still
+      // connected on some OTHER instance is Redis's to answer, and this registry has
+      // been unable to see other instances since 2.5.
+      if (
+        registry.connectionsFor(connection.identity.userExternalId).length === 0
+      ) {
+        void presence?.disconnected(
+          connection.identity.environmentId,
+          connection.identity.userExternalId,
+          connection.channelIds,
+        );
+      }
       // Releasing a subscription can fail — a broker that went away, or a
       // fabric already closed while sockets were still draining — and a
       // close handler is the last place that should throw. The subscribe
@@ -396,14 +478,24 @@ export function attachSessions({
       // unhandled rejection during teardown is how chapter 2.8's lane found
       // out. Nothing to recover: the connection is gone either way.
       void Promise.all(
-        [...connection.channelIds].map((channelId) =>
+        [...connection.channelIds].flatMap((channelId) => [
           fanout?.unsubscribe(channelId).catch((error: unknown) => {
             logger.log("error", "fanout.unsubscribe_failed", {
               channel: channelId,
               error: String(error),
             });
           }),
-        ),
+          // Inside the same swallowing wrapper, because a close handler is the last
+          // place that should throw — chapter 2.8's lane found the unhandled
+          // rejection on the fan-out's release path for exactly this reason.
+          presence?.unsubscribe(channelId).catch((error: unknown) => {
+            logger.log("error", "presence.failed", {
+              op: "unsubscribe",
+              channel: channelId,
+              error: String(error),
+            });
+          }),
+        ]),
       );
       logger.log("info", "connection.closed", {
         connection_id: connection.id,

logger.log("error", "presence.failed", { op, error: String(error) });

// Chapter 3.19. THIS IS `limits.ts`'s CASE, NOT `fanout.ts`'s, and the
// distinction is the rule's own reason. The entry above is justified by
// "this client touches no keys" — a publish onto a channel UUID, and a
// subject is not readable at all. Presence's client touches keys and they
// are environment-scoped: `presence:{env}:{user}`, exactly the shape the
// restriction exists to guard.
"services/gateway/src/presence.ts",

@@ -4,6 +4,7 @@ import { createLogger, serve, type Logger } from "@relay/service-kit";
 import { createApiClient } from "./api-client.js";
 import { createFanout } from "./fanout.js";
 import { createGatewayLimits } from "./limits.js";
+import { createPresence } from "./presence.js";
 import { attachSessions } from "./session.js";
 
 // The gateway — SAD §4.1: terminates WebSockets and never writes to the
@@ -42,6 +43,12 @@ export function createServer(logger?: Logger) {
   // created here rather than inside `attachSessions` so the tests that call
   // that function directly stay Redis-free, and so its close has an owner.
   const limits = createGatewayLimits();
+  // Chapter 3.19. The FOURTH and FIFTH Redis clients, and the reason is chapter
+  // 3.8's verbatim: a connection in subscribe mode cannot run `SET` or `EXISTS`,
+  // so presence needs a subscriber and a command client of its own. Created here
+  // rather than inside `attachSessions` so the tests that call that function
+  // directly stay Redis-free, and so its close has an owner.
+  const presence = createPresence({ logger: log });
   // Chapter 3.11. THE FIRST SECRET THIS SERVICE HAS EVER HELD, and it is not a
   // signing secret: chapter 3.2's claim that "the gateway holds no signing
   // secret" is untouched, because this one verifies nothing and signs nothing.
@@ -67,6 +74,7 @@ export function createServer(logger?: Logger) {
     logger: log,
     fanout,
     limits,
+    presence,
     // Overridable so `meter.itest.ts` can drive a spawned gateway without
     // waiting a real minute per assertion. The two tests there are the ones an
     // in-process gateway cannot run — a signal has to arrive at a process — and
@@ -89,6 +97,7 @@ export function createServer(logger?: Logger) {
     await sessions.close();
     await fanout.close();
     await limits.close();
+    await presence.close();
   }
   return Object.assign(server, { shutdown });
 }

import { describe, expect, it } from "vitest";
 
import {
  DEFAULT_GRACE_MS,
  DEFAULT_MARGIN_MS,
  DEFAULT_REFRESH_MS,
  DEFAULT_TTL_MS,
  graceCheckDelay,
  wonTransition,
} from "./presence.js";
 
// PURE LOGIC ONLY, and that is the file's whole shape. `createPresence` builds its
// own Redis clients from a url and cannot be handed a double, so anything
// reply-dependent lives in `presence.itest.ts`. `limits.test.ts` is the precedent:
// it covers `windowStartFor` and `overLimit` and nothing that touches a client.
 
describe("wonTransition", () => {
  it("is true only for the caller that SET … NX answered", () => {
    expect(wonTransition("OK")).toBe(true);
    expect(wonTransition(null)).toBe(false);
  });
});
 
describe("the timings", () => {
  it("keeps FR-RTM-06's thirty seconds somewhere a reader can find it", () => {
    // Without this the clause's number lives only in a constant somebody edits.
    expect(DEFAULT_GRACE_MS).toBe(30_000);
  });
 
  it("refreshes well inside the TTL", () => {
    // A TTL equal to its refresh interval expires a connected user. Three
    // refreshes per TTL survive two consecutive misses.
    expect(DEFAULT_REFRESH_MS).toBeLessThan(DEFAULT_TTL_MS / 2);
  });
 
  it("checks after the grace ends, never exactly on it", () => {
    // Two deadlines on one instant, reached by two clocks, strand the user online
    // permanently — the check wins the race, finds the key alive, and its one-shot
    // timer is gone (research R2b).
    expect(DEFAULT_MARGIN_MS).toBeGreaterThan(0);
  });
 
  it("defaults the margin to a second", () => {
    expect(DEFAULT_MARGIN_MS).toBe(1_000);
  });
 
  it("defaults the TTL to the SAD's thirty seconds", () => {
    expect(DEFAULT_TTL_MS).toBe(30_000);
  });
 
  // NOT ASSERTED: `ttlMs >= graceMs`. The close re-pins the key, which is what
  // makes the grace correct — the numeric relation is the sane default, not the
  // mechanism, and a test may set the TTL below the grace deliberately to open the
  // gap the reconnect-late case needs.
});
 
describe("graceCheckDelay", () => {
  it("is the grace plus the margin, never the grace alone", () => {
    expect(graceCheckDelay(30_000, 1_000)).toBe(31_000);
    expect(graceCheckDelay(300, 50)).toBe(350);
  });
 
  // The whole point of the margin: the check must never land on the instant the
  // re-pinned key expires, because Redis holds a key until `now` is strictly past
  // its expiry and a tie leaves the user online with no timer left to try again.
  it("never returns the grace unchanged", () => {
    expect(graceCheckDelay(30_000, DEFAULT_MARGIN_MS)).toBeGreaterThan(30_000);
  });
});

import { spawn, type ChildProcess } from "node:child_process";
import { randomUUID } from "node:crypto";
import { existsSync } from "node:fs";
import { createRequire } from "node:module";
import { dirname, join } from "node:path";
import { fileURLToPath } from "node:url";
import {
  createServer as createNetServer,
  connect as connectSocket,
  type AddressInfo,
  type Server as NetServer,
  type Socket,
} from "node:net";
 
import { createLogger, serve, type Logger } from "@relay/service-kit";
import { docsUrl, subjectForPresence } from "@relay/protocol";
// A CLIENT BELONGING TO NEITHER MODULE, for one reason: the two rejection paths on
// the receive half cannot be reached through `createPresence`, which only ever
// publishes what its own schema produced. `eslint.config.mjs` carries the exemption
// and the argument; this client publishes and reads nothing.
import { Redis } from "ioredis";
import { WebSocket } from "ws";
import { afterAll, beforeAll, describe, expect, it } from "vitest";
 
import { createApiClient } from "./api-client.js";
import { createFanout } from "./fanout.js";
import {
  createPresence,
  DEFAULT_REDIS_URL,
  type PresenceOptions,
} from "./presence.js";
import { attachSessions } from "./session.js";
 
// CHAPTER 3.19, PHASE 1 — THE FAILING STATE, OBSERVED.
//
// `presence.changed` has been in the protocol union since chapter 1.3. Its states
// are `online` and `offline`, `frames.test.ts` asserts its shape and rejects
// `state: "away"`, and chapter 3.12's gauntlet proves a client cannot forge one.
// Nothing has ever produced one — chapter 3.17's `gaps.md` item 2 recorded it as
// "a declared frame with no sender" and assigned it to this chapter by name.
//
// THIS TEST IS RED ON PURPOSE UNTIL PHASE 3. The phase 1 and phase 2 commits both
// carry it failing, and their commit bodies say so: a red lane nobody explained is
// indistinguishable from a red lane nobody noticed, and CI cannot tell them apart.
//
// It must be red for the RIGHT reason — no producer exists — rather than because
// the fixture is wrong. That is why it asserts the positive (a frame arrives) and
// not the negative: "nothing arrived" is true of a broken harness too.
 
const silent: Logger = createLogger("gateway", () => {});
const HERE = dirname(fileURLToPath(import.meta.url));
const REPO = join(HERE, "..", "..", "..");
const require_ = createRequire(import.meta.url);
 
interface Seeder {
  createEnvironment: (db: unknown, input: { name: string }) => Promise<{ id: string }>;
  createApiKey: (
    db: unknown,
    input: { environmentId: string },
  ) => Promise<{ credential: string }>;
  Repository: new (
    db: unknown,
    environmentId: string,
  ) => {
    createUser: (externalId: string, displayName: string) => Promise<{ id: string }>;
    createChannel: (
      externalId: string,
      type: "public" | "private",
    ) => Promise<{ id: string }>;
    addMember: (channelId: string, userId: string) => Promise<boolean>;
  };
}
 
interface ApiUnderTest {
  url: string;
  credential: string;
  /** One per test — see the note in the seed. */
  subjects: string[];
  /** The three shared channels, by the id a `message.send` frame names. */
  channelIds: string[];
  /** Counts rows in the outbox. Reached through the api's BUILT `dist`, which is
   * premise 2 — the gateway package has no `pg` dependency of its own. */
  outboxCount: () => Promise<number>;
  /** A SECOND TENANT, with its own credential and its own user. Presence keys are
   * `{env}`-scoped and channel ids are unguessable, so the isolation is structural
   * — which is exactly why it needs a test that could see it fail. */
  otherCredential: string;
  stop: () => void;
}
 
/** Two members of ONE channel, which no existing gateway fixture provides:
 * `seedSocketTenants` gives one user per tenant, and presence needs a watcher and
 * a subject who share a channel. */
async function startApi(): Promise<ApiUnderTest> {
  const port = 4700 + Math.floor(Math.random() * 200);
  const dist = join(REPO, "services", "api", "dist");
  if (!existsSync(join(dist, "main.js"))) {
    throw new Error(
      "the api is not built — run `pnpm build` before this lane " +
        "(the suite talks to the real service, not a stub)",
    );
  }
  const client = require_(join(dist, "db", "client.js")) as {
    createDb: (pool: unknown) => unknown;
    createPool: () => { query: (sql: string) => Promise<unknown> };
  };
  const seeder = require_(join(dist, "db", "repository.js")) as Seeder;
  const pool = client.createPool();
  const db = client.createDb(pool);
 
  const environment = await seeder.createEnvironment(db, {
    name: `presence-itest-${randomUUID().slice(0, 8)}`,
  });
  const repo = new seeder.Repository(db, environment.id);
  const watcher = await repo.createUser("linh", "Linh");
  const subject = await repo.createUser("tuan", "Tuan");
  // A user who belongs to nothing: FR-RTM-07's degenerate case, where a transition
  // publishes on no subject at all.
  await repo.createUser("hermit", "Hermit");
 
  // ONE SUBJECT PER TEST, and this is not tidiness. Presence state lives in Redis
  // under `presence:{env}:{user}` with a thirty-second TTL, so a subject who came
  // online in one test is STILL ONLINE in the next — `SET … NX` correctly refuses,
  // nothing publishes, and every later test sees an empty frame list. The first run
  // of this suite failed exactly that way: test 1 passed and tests 2 to 4 reported
  // "expected [] to have a length of 1", which reads like a broken fabric and is a
  // shared fixture.
  //
  // T016 asked for fresh channel AND user ids per run for this reason and the first
  // implementation did the channels only. Chapter 3.18's `isolation-fixtures.ts`
  // learned the same lesson: a fixture nobody else depends on beats a rule nobody
  // remembers.
  const channels = await Promise.all(
    // THREE shared channels, so the dedup can be asserted by count. Without the
    // transition id a watcher sharing three sees three frames for one arrival.
    ["fleet", "ops", "night-shift"].map((name) =>
      repo.createChannel(name, "public"),
    ),
  );
  // CONCURRENTLY. Seeded one at a time this was 40 users x 3 memberships of
  // sequential round trips, and the file spent more time in its fixture than in its
  // assertions.
  const subjects = Array.from({ length: 60 }, (_, i) => `subject-${i}`);
  const seeded = await Promise.all(
    subjects.map((name) => repo.createUser(name, name)),
  );
  await Promise.all(
    channels.flatMap((channel) =>
      [watcher, subject, ...seeded].map((user) =>
        repo.addMember(channel.id, user.id),
      ),
    ),
  );
  // A user who is a member of a PRIVATE channel that nobody else joins. The watcher
  // is not in it, so it shares no channel with this user at all.
  const recluse = await repo.createUser("recluse", "Recluse");
  const vault = await repo.createChannel("vault", "private");
  await repo.addMember(vault.id, recluse.id);
 
  const key = await seeder.createApiKey(db, {
    environmentId: environment.id,
  });
 
  // THE SECOND TENANT. Its own environment, its own user, its own channel, its own
  // key — nothing shared with the first but a Redis instance and a gateway.
  const other = await seeder.createEnvironment(db, {
    name: `presence-itest-other-${randomUUID().slice(0, 8)}`,
  });
  const otherRepo = new seeder.Repository(db, other.id);
  const stranger = await otherRepo.createUser("stranger", "Stranger");
  const elsewhere = await otherRepo.createChannel("elsewhere", "public");
  await otherRepo.addMember(elsewhere.id, stranger.id);
  const otherKey = await seeder.createApiKey(db, { environmentId: other.id });
 
  const child: ChildProcess = spawn("node", [join(dist, "main.js")], {
    env: {
      ...process.env,
      PORT: String(port),
      RELAY_OUTBOX_RELAY: "off",
      RELAY_NOTIFICATION_RELAY: "off",
      RELAY_EVENT_CONSUMER: "off",
    },
    stdio: "ignore",
  });
  const url = `http://127.0.0.1:${port}`;
  for (let i = 0; i < 100; i += 1) {
    try {
      const res = await fetch(`${url}/health`);
      if (res.ok) break;
    } catch {
      /* not up yet */
    }
    await new Promise((r) => setTimeout(r, 100));
  }
  return {
    url,
    credential: key.credential,
    subjects,
    outboxCount: async () => {
      const result = (await pool.query("select count(*)::int as n from outbox")) as {
        rows: { n: number }[];
      };
      return result.rows[0]?.n ?? 0;
    },
    channelIds: channels.map((c) => c.id),
    otherCredential: otherKey.credential,
    stop: () => child.kill(),
  };
}
 
/** Every frame of a type this socket has received. **Assertions here are by COUNT,
 * not by arrival**: "a frame showed up" is equally true of a producer that publishes
 * three, and three is exactly what the dedup exists to prevent. */
function collect(
  socket: WebSocket,
  type: string,
  about?: string,
): { frames: { payload: { user: string; state: string } }[] } {
  const frames: { payload: { user: string; state: string } }[] = [];
  socket.on("message", (raw: unknown) => {
    const frame = JSON.parse(String(raw)) as {
      type: string;
      payload: { user: string; state: string };
    };
    if (frame.type !== type) return;
    // FILTERED BY SUBJECT, and the first version of this helper was not — which is
    // how FR-011 announced itself. A watcher's own arrival is delivered to the
    // watcher, because a subject shares every one of their channels with
    // themselves, so an unfiltered collector sees two frames where the test means
    // one. The behaviour is correct and the assertion was wrong.
    if (about !== undefined && frame.payload.user !== about) return;
    frames.push(frame);
  });
  return { frames };
}
 
/** Let the fabric settle. Redis pub/sub is fire-and-forget, so a test cannot poll a
 * queue — a negative assertion has to wait out a window instead. */
const quiet = (ms = 700) => new Promise((r) => setTimeout(r, ms));
 
/** Wait for a frame of a given type, or fail loudly. Redis pub/sub is
 * fire-and-forget, so a test cannot poll a queue — it waits with a deadline. */
function waitForFrame(
  socket: WebSocket,
  type: string,
  timeoutMs = 5_000,
): Promise<Record<string, unknown>> {
  return new Promise((resolve, reject) => {
    const onMessage = (raw: unknown): void => {
      const frame = JSON.parse(String(raw)) as { type: string };
      if (frame.type === type) {
        socket.off("message", onMessage);
        resolve(frame as unknown as Record<string, unknown>);
      }
    };
    socket.on("message", onMessage);
    setTimeout(
      () => reject(new Error(`no ${type} frame within ${timeoutMs}ms`)),
      timeoutMs,
    );
  });
}
 
/** One gateway instance: its own server, its own fabric clients, its own
 * `Presence`. Two of these on one Redis is what the cross-instance cases need,
 * and **no existing gateway suite stands up two at once** — `fanout.itest.ts`
 * does it at the fabric level with two `createFanout` clients and no sessions,
 * and every other file builds exactly one `attachSessions` inside its own
 * describe. So this is new harness rather than a pattern to copy.
 *
 * `presence` is INJECTED already built, the way `fanout` and `limits` are, which
 * is why a test that wants a hundred-millisecond grace period passes those
 * timings to `createPresence` here rather than to `attachSessions`. */
interface LogLine {
  level: string;
  msg: string;
  fields: Record<string, unknown>;
}
 
interface Instance {
  url: string;
  /** Everything this instance logged. **The log line is the requirement's evidence**
   * on every failure path: a presence module that does nothing satisfies "the socket
   * still opened" exactly as well as a working one, which is chapter 3.18's trap
   * against its own publisher. */
  logs: LogLine[];
  close: () => Promise<void>;
}
 
/** A TCP proxy in front of the real Redis, so a test can sever and restore a
 * connection without touching anything shared.
 *
 * **NEVER `docker compose stop redis`.** The gateway's integration files run in
 * PARALLEL — `services/api/src/limits/limits.itest.ts:484` already writes the rule
 * down: "a dead port rather than stopping the container, because the lane runs files
 * in PARALLEL and stopping Redis would break every other suite mid-run". A dead port
 * covers "down" and cannot cover "restored", and `redis-server` is not installed on
 * the lane machine, so the proxy is what is left. */
async function startRedisProxy(): Promise<{
  url: string;
  cut: () => Promise<void>;
  restore: () => Promise<void>;
  close: () => Promise<void>;
}> {
  const target = new URL(process.env.RELAY_REDIS_URL ?? "redis://localhost:6379");
  const live = new Set<Socket>();
  let server: NetServer | null = null;
  let port = 0;
 
  const listen = (onPort: number) =>
    new Promise<number>((resolve) => {
      const next = createNetServer((client) => {
        const upstream = connectSocket(
          Number(target.port || 6379),
          target.hostname,
        );
        client.pipe(upstream);
        upstream.pipe(client);
        for (const socket of [client, upstream]) {
          live.add(socket);
          socket.on("error", () => socket.destroy());
          socket.on("close", () => live.delete(socket));
        }
      });
      next.listen(onPort, "127.0.0.1", () => {
        server = next;
        resolve((next.address() as AddressInfo).port);
      });
    });
 
  port = await listen(0);
  return {
    url: `redis://127.0.0.1:${port}`,
    cut: async () => {
      for (const socket of live) socket.destroy();
      live.clear();
      await new Promise<void>((resolve) =>
        server ? server.close(() => resolve()) : resolve(),
      );
      server = null;
    },
    // Re-listening on the SAME port is what "without a restart" means: ioredis
    // reconnects on its own and the module is never rebuilt.
    restore: async () => {
      await listen(port);
    },
    close: async () => {
      for (const socket of live) socket.destroy();
      await new Promise<void>((resolve) =>
        server ? server.close(() => resolve()) : resolve(),
      );
    },
  };
}
 
async function startInstance(
  apiUrl: string,
  presenceOptions: Partial<PresenceOptions> = {},
): Promise<Instance> {
  const logs: LogLine[] = [];
  // THE SINK RECEIVES A JSON STRING, not an object, and its fields are spread at the
  // top level rather than nested. The first version of this pushed the raw line, so
  // every `l.msg` was undefined and the log assertions silently matched nothing —
  // which is the failure mode "observability you can't test rots" warns about, in
  // the test rather than the code.
  const recording = createLogger("gateway", (line) => {
    const parsed = JSON.parse(line) as Record<string, unknown>;
    logs.push({
      level: String(parsed.level),
      msg: String(parsed.msg),
      fields: parsed,
    });
  });
  const fanout = createFanout({ logger: silent });
  const presence = createPresence({ logger: recording, ...presenceOptions });
  const server = serve({
    service: "gateway",
    health: () => ({}),
    logger: silent,
    notFoundDocsUrl: docsUrl("not_found"),
  });
  const sessions = attachSessions({
    server,
    api: createApiClient(apiUrl),
    logger: silent,
    fanout,
    presence,
  });
  await new Promise<void>((resolve) => server.listen(0, resolve));
  return {
    url: `ws://127.0.0.1:${(server.address() as AddressInfo).port}`,
    logs,
    close: async () => {
      await sessions.close();
      await fanout.close();
      await presence.close();
      await new Promise<void>((resolve) => server.close(() => resolve()));
    },
  };
}
 
// ONE API FOR THE WHOLE FILE. Three describes each spawning their own was three
// process launches and three seeds, and it dominated the file's 54 s. The instances
// stay per-describe because their timings differ; those are cheap.
// Assigned in the file-level `beforeAll`; the non-null assertion is the same one
// every suite here makes about its fixture.
let api!: ApiUnderTest;
let nextSubject = 0;
const takeSubject = (): string => api.subjects[nextSubject++] as string;
 
beforeAll(async () => {
  api = await startApi();
}, 60_000);
 
afterAll(() => {
  api?.stop();
});
 
describe("presence: a member sees a co-member arrive (FR-RTM-05, FR-RTM-06)", () => {
  let a: Instance;
  let b: Instance;
  const sockets: WebSocket[] = [];
 
  const mintToken = async (user: string) => {
    const res = await fetch(`${api.url}/auth/dev-token`, {
      method: "POST",
      headers: {
        "content-type": "application/json",
        authorization: `Bearer ${api.credential}`,
      },
      body: JSON.stringify({ user, ttl_seconds: 3600 }),
    });
    if (!res.ok) throw new Error(`dev-token: ${res.status}`);
    return ((await res.json()) as { token: string }).token;
  };
 
  let nextSubject = 0;
  /** The next unused subject. Every test that brings someone online takes one. */
  const takeSubject = () => api.subjects[nextSubject++] as string;
 
  const connect = (token: string, instance: Instance = a) => {
    const socket = new WebSocket(`${instance.url}/v1/ws?token=${token}`);
    sockets.push(socket);
    return socket;
  };
 
  beforeAll(async () => {
    // TWO INSTANCES ON ONE REDIS — same code, same fabric, no knowledge of each
    // other, which is chapter 2.6's phrase for the only property a single-process
    // test cannot show. Instance A hosts the watcher; B hosts the subject.
    a = await startInstance(api.url);
    b = await startInstance(api.url);
  }, 60_000);
 
  afterAll(async () => {
    for (const socket of sockets) socket.close();
    await a?.close();
    await b?.close();
  });
 
  // T021. The clause, on one instance.
  it("delivers presence.changed online to a connected co-member", async () => {
    const watcher = connect(await mintToken("linh"));
    await waitForFrame(watcher, "connection.ack");
    const who = takeSubject();
    const seen = collect(watcher, "presence.changed", who);
 
    const subject = connect(await mintToken(who));
    await waitForFrame(subject, "connection.ack");
    await quiet();
 
    expect(seen.frames).toEqual([
      { type: "presence.changed", payload: { user: who, state: "online" } },
    ]);
  }, 30_000);
 
  // T022. The property a single-process test cannot show (chapter 2.6's phrase).
  it("delivers it when the subject is on another instance", async () => {
    const watcher = connect(await mintToken("linh"), a);
    await waitForFrame(watcher, "connection.ack");
    const who = takeSubject();
    const seen = collect(watcher, "presence.changed", who);
 
    const subject = connect(await mintToken(who), b);
    await waitForFrame(subject, "connection.ack");
    await quiet();
 
    expect(seen.frames).toHaveLength(1);
  }, 30_000);
 
  // T023, FR-012. The watcher shares THREE channels with the subject and a
  // transition publishes on all three, so this instance receives three copies of
  // one transition. Without the transition id this is three frames.
  it("delivers ONE frame to a watcher sharing three channels", async () => {
    const watcher = connect(await mintToken("linh"));
    await waitForFrame(watcher, "connection.ack");
    const who = takeSubject();
    const seen = collect(watcher, "presence.changed", who);
 
    const subject = connect(await mintToken(who), b);
    await waitForFrame(subject, "connection.ack");
    await quiet();
 
    expect(seen.frames).toHaveLength(1);
  }, 30_000);
 
  // T024, FR-006. The state did not change, so nothing is published — asserted in
  // a run where the FIRST connection did produce a frame, so a dead producer
  // cannot satisfy it.
  it("publishes nothing for a second connection of a user already online", async () => {
    const watcher = connect(await mintToken("linh"));
    await waitForFrame(watcher, "connection.ack");
    const who = takeSubject();
    const seen = collect(watcher, "presence.changed", who);
 
    const first = connect(await mintToken(who), b);
    await waitForFrame(first, "connection.ack");
    await quiet();
    expect(seen.frames).toHaveLength(1);
 
    const second = connect(await mintToken(who), a);
    await waitForFrame(second, "connection.ack");
    await quiet();
    expect(seen.frames).toHaveLength(1);
  }, 30_000);
 
  // T025, FR-011. A subject shares every one of their channels with themselves, so
  // the scoping rule includes them. Both readings satisfy FR-RTM-07 — "only users
  // sharing a channel" is an upper bound — and one of them has to be the one that
  // ships.
  it("delivers the subject's own transition to the subject's own socket", async () => {
    // Collected BEFORE the ack, because the subject's own transition is published
    // as soon as the registry has the connection and can arrive immediately after.
    const who = takeSubject();
    const subject = connect(await mintToken(who));
    const seen = collect(subject, "presence.changed", who);
    await waitForFrame(subject, "connection.ack");
    await quiet();
 
    // The subject's own connect elects the transition and its socket is subscribed
    // to the same channels, so it hears itself arrive. Exactly one frame, not three
    // — the dedup applies to the subject like anyone else.
    expect(seen.frames).toEqual([
      { type: "presence.changed", payload: { user: who, state: "online" } },
    ]);
  }, 30_000);
 
  // T026, FR-RTM-07's degenerate case. A member of no channel publishes on no
  // subject, and the connect still succeeds.
  it("publishes to nobody for a subject who is a member of no channel", async () => {
    const watcher = connect(await mintToken("linh"));
    await waitForFrame(watcher, "connection.ack");
    const seen = collect(watcher, "presence.changed", "hermit");
 
    const hermit = connect(await mintToken("hermit"), b);
    await waitForFrame(hermit, "connection.ack");
    await quiet();
 
    expect(seen.frames).toEqual([]);
  }, 30_000);
});
 
// ── THE GRACE PERIOD (FR-RTM-06) ─────────────────────────────────────────────
//
// MILLISECONDS, NOT HALF-MINUTES. The clause's thirty seconds is asserted once, in
// `presence.test.ts`, against the production default; every case here runs on a
// scaled window because six real grace periods would cost 180 s against 44 s of
// lane headroom. The timings are injected into `createPresence`, not into
// `attachSessions` — presence is built and handed over, so it carries its own
// configuration.
// SCALED HARD, AND THE NUMBER IS A BUDGET DECISION. At 500/100 this file cost 65.4 s
// of a 240 s lane budget with 32.7 s of headroom — R18's concern, arriving. At
// 250/60 it costs a third of that. The margin is still ~20x a local Redis round
// trip, which is what it has to clear.
const GRACE = 250;
const MARGIN = 60;
const SETTLE = GRACE + MARGIN + 200; // past the check, with room for a round trip
 
describe("presence: the grace period (FR-RTM-06)", () => {
  let a: Instance;
  let b: Instance;
  const sockets: WebSocket[] = [];
 
  const mintToken = async (user: string) => {
    const res = await fetch(`${api.url}/auth/dev-token`, {
      method: "POST",
      headers: {
        "content-type": "application/json",
        authorization: `Bearer ${api.credential}`,
      },
      body: JSON.stringify({ user, ttl_seconds: 3600 }),
    });
    if (!res.ok) throw new Error(`dev-token: ${res.status}`);
    return ((await res.json()) as { token: string }).token;
  };
 
  const connect = (token: string, instance: Instance = a) => {
    const socket = new WebSocket(`${instance.url}/v1/ws?token=${token}`);
    sockets.push(socket);
    return socket;
  };
 
  /** Connect, wait for the ack, and let the arrival settle so the `online` frame is
   * out of the way before a test starts watching for `offline`. */
  const arrive = async (who: string, instance: Instance = a) => {
    const socket = connect(await mintToken(who), instance);
    await waitForFrame(socket, "connection.ack");
    await quiet(150);
    return socket;
  };
 
  beforeAll(async () => {
    const timings = {
      graceMs: GRACE,
      ttlMs: GRACE,
      refreshMs: 150,
      marginMs: MARGIN,
    };
    a = await startInstance(api.url, timings);
    b = await startInstance(api.url, timings);
  }, 60_000);
 
  afterAll(async () => {
    for (const socket of sockets) socket.close();
    await a?.close();
    await b?.close();
  });
 
  // T036. The clause: one offline, and not before the window.
  it("publishes one offline after the window and nothing before it", async () => {
    const who = takeSubject();
    const watcher = await arrive("linh");
    const seen = collect(watcher, "presence.changed", who);
    const subject = await arrive(who, b);
 
    subject.close();
    await quiet(GRACE - 150);
    expect(seen.frames.filter((f) => f.payload.state === "offline")).toEqual([]);
 
    await quiet(SETTLE);
    expect(seen.frames.filter((f) => f.payload.state === "offline")).toHaveLength(1);
  }, 30_000);
 
  // T038. A reconnection inside the window is invisible: no offline, and no second
  // online either, because the state never changed.
  it("publishes nothing at all for a reconnection inside the window", async () => {
    const who = takeSubject();
    const watcher = await arrive("linh");
    const subject = await arrive(who, b);
    // COLLECTED AFTER THE ARRIVAL. The subject's own `online` is a real frame and
    // this test is about what happens from the close onward; watching from before
    // it makes the arrival look like a violation.
    const seen = collect(watcher, "presence.changed", who);
 
    subject.close();
    await quiet(GRACE / 2);
    await arrive(who, b);
    await quiet(SETTLE);
 
    expect(seen.frames).toEqual([]);
  }, 30_000);
 
  // T039. The same, landing on the OTHER instance — the case the TTL-as-liveness
  // signal exists for. Nothing coordinates the two gateways.
  it("publishes nothing when the reconnection lands on another instance", async () => {
    const who = takeSubject();
    const watcher = await arrive("linh");
    const subject = await arrive(who, b);
    const seen = collect(watcher, "presence.changed", who);
 
    subject.close();
    await quiet(GRACE / 2);
    await arrive(who, a);
    await quiet(SETTLE);
 
    expect(seen.frames).toEqual([]);
  }, 30_000);
 
  // T041, FR-006. Two connections here, one closes: nothing. Then the last one.
  it("publishes nothing while another connection remains open", async () => {
    const who = takeSubject();
    const watcher = await arrive("linh");
    const first = await arrive(who, b);
    const second = await arrive(who, b);
    const seen = collect(watcher, "presence.changed", who);
 
    first.close();
    await quiet(SETTLE);
    expect(seen.frames).toEqual([]);
 
    second.close();
    await quiet(SETTLE);
    expect(seen.frames.filter((f) => f.payload.state === "offline")).toHaveLength(1);
  }, 30_000);
 
  // T042. The two connections on two DIFFERENT instances, which no local registry
  // can see — the key's TTL is what answers it.
  it("publishes nothing while a connection remains on another instance", async () => {
    const who = takeSubject();
    const watcher = await arrive("linh");
    const onA = await arrive(who, a);
    await arrive(who, b);
    const seen = collect(watcher, "presence.changed", who);
 
    onA.close();
    await quiet(SETTLE);
    expect(seen.frames).toEqual([]);
  }, 30_000);
 
  // T043, FR-028. Close, reopen inside the window, close again: ONE decision,
  // answered by the state at the end of the SECOND window. Two pending timers
  // would publish twice.
  it("leaves one decision for two closes inside one window", async () => {
    const who = takeSubject();
    const watcher = await arrive("linh");
    const seen = collect(watcher, "presence.changed", who);
 
    const first = await arrive(who, b);
    first.close();
    await quiet(GRACE / 3);
    const second = await arrive(who, b);
    second.close();
    await quiet(SETTLE + GRACE);
 
    expect(seen.frames.filter((f) => f.payload.state === "offline")).toHaveLength(1);
  }, 30_000);
 
  // T045, and FR-RTM-09's five is enforced NOWHERE — `policy.ts:13` mentions it in a
  // comment and nothing counts — so the reference count is unbounded and two is the
  // easy case. Five connections, closed one at a time: nothing until the last.
  it("publishes nothing until the fifth of five connections closes", async () => {
    const who = takeSubject();
    const watcher = await arrive("linh");
    const open = [];
    for (let i = 0; i < 5; i += 1) open.push(await arrive(who, i % 2 ? a : b));
    const seen = collect(watcher, "presence.changed", who);
 
    // Closed together and asserted once, rather than a settle per close. Four
    // separate windows cost four times as much and test the same property: while
    // ANY connection remains, nothing is published.
    for (const socket of open.slice(0, 4)) socket.close();
    await quiet(SETTLE);
    expect(seen.frames).toEqual([]);
 
    open[4]?.close();
    await quiet(SETTLE);
    expect(seen.frames.filter((f) => f.payload.state === "offline")).toHaveLength(1);
  }, 60_000);
 
  // T044. A DEPLOY DRAIN. `docs/05-sad.md:634` stops the gateway accepting on
  // SIGTERM and clients reconnect elsewhere, so a mass disconnect is the real path
  // rather than a hypothetical — and it is the worst case for whatever schedules
  // the grace check: twelve pending timers and twelve round trips at once.
  it("publishes one offline per user when six close in the same tick", async () => {
    const watcher = await arrive("linh");
    const crowd: string[] = [];
    for (let i = 0; i < 6; i += 1) crowd.push(takeSubject());
    const sockets_ = await Promise.all(
      crowd.map((who, i) => arrive(who, i % 2 ? a : b)),
    );
    const seen = crowd.map((who) => collect(watcher, "presence.changed", who));
 
    for (const socket of sockets_) socket.close();
    await quiet(SETTLE + 400);
 
    const offlines = seen.map(
      (s) => s.frames.filter((f) => f.payload.state === "offline").length,
    );
    // EVERY user, EXACTLY once — not "at least one somewhere", which a partial
    // drain would also satisfy.
    expect(offlines).toEqual(crowd.map(() => 1));
  }, 90_000);
 
  // T046. Two instances whose last connections close in the same tick both find the
  // key absent at the check. The election is the only thing between that and two
  // frames at the watcher.
  it("publishes one offline when two instances close in the same tick", async () => {
    const who = takeSubject();
    const watcher = await arrive("linh");
    const seen = collect(watcher, "presence.changed", who);
    const onA = await arrive(who, a);
    const onB = await arrive(who, b);
 
    onA.close();
    onB.close();
    await quiet(SETTLE);
 
    expect(seen.frames.filter((f) => f.payload.state === "offline")).toHaveLength(1);
  }, 30_000);
});
 
// ── THE GAP BETWEEN THE KEY'S DEATH AND THE GRACE'S END ──────────────────────
//
// `ttlMs` DELIBERATELY BELOW `graceMs`, which nothing forbids and one thing needs.
// The key's expiry counts from the last refresh; the grace counts from the close.
// Without the close re-pinning the key those are different instants, and every
// reconnect case above lands in the FIRST part of the window where the key is still
// alive and the bug is invisible. This describe opens the gap on purpose.
describe("presence: a reconnection after the TTL would have lapsed (FR-007)", () => {
  const TTL = 150;
  const LATE_GRACE = 600;
  let a: Instance;
  let b: Instance;
  const sockets: WebSocket[] = [];
 
  const mintToken = async (user: string) => {
    const res = await fetch(`${api.url}/auth/dev-token`, {
      method: "POST",
      headers: {
        "content-type": "application/json",
        authorization: `Bearer ${api.credential}`,
      },
      body: JSON.stringify({ user, ttl_seconds: 3600 }),
    });
    if (!res.ok) throw new Error(`dev-token: ${res.status}`);
    return ((await res.json()) as { token: string }).token;
  };
  const arrive = async (who: string, instance: Instance = a) => {
    const socket = new WebSocket(`${instance.url}/v1/ws?token=${await mintToken(who)}`);
    sockets.push(socket);
    await waitForFrame(socket, "connection.ack");
    await quiet(150);
    return socket;
  };
 
  beforeAll(async () => {
    const timings = {
      graceMs: LATE_GRACE,
      ttlMs: TTL,
      refreshMs: 60,
      marginMs: 80,
    };
    a = await startInstance(api.url, timings);
    b = await startInstance(api.url, timings);
  }, 60_000);
 
  afterAll(async () => {
    for (const socket of sockets) socket.close();
    await a?.close();
    await b?.close();
  });
 
  // T040. The reconnection lands AFTER `ttlMs` has lapsed and BEFORE the grace ends
  // — the window every other reconnect test misses. Without the re-pin the key is
  // gone by now, `SET … NX` succeeds, and the watcher sees a second `online` for a
  // user who never left.
  it("publishes no second online when the reconnect lands past the TTL", async () => {
    const who = takeSubject();
    const watcher = await arrive("linh");
    const subject = await arrive(who, b);
    const seen = collect(watcher, "presence.changed", who);
 
    subject.close();
    // Past `ttlMs` (150 ms) and well inside the grace (600 ms).
    await quiet(350);
    await arrive(who, b);
    await quiet(LATE_GRACE + 350);
 
    expect(seen.frames).toEqual([]);
  }, 30_000);
 
  // T049. The re-pin is AWAITED before the timer is armed, asserted by outcome
  // rather than by reading `PTTL`: with a 200 ms TTL and a 900 ms grace, an
  // `offline` that arrives on the grace's schedule can only mean the key was
  // re-pinned. If the close left the key on its refresh TTL it would have expired
  // at ~200 ms and the check would still have found it absent — so the tell is
  // that nothing arrives EARLY, and the frame lands after the grace.
  //
  // Reading `PTTL` directly would need a raw ioredis client, which
  // `eslint.config.mjs` restricts and this file is not exempted from. Asserting the
  // behaviour is the stronger test anyway.
  it("holds the key for the grace, not for the TTL", async () => {
    const who = takeSubject();
    const watcher = await arrive("linh");
    const subject = await arrive(who, b);
    const seen = collect(watcher, "presence.changed", who);
 
    const closedAt = Date.now();
    subject.close();
    await quiet(TTL + 120);
    expect(seen.frames.filter((f) => f.payload.state === "offline")).toEqual([]);
 
    await quiet(LATE_GRACE);
    const offline = seen.frames.filter((f) => f.payload.state === "offline");
    expect(offline).toHaveLength(1);
    expect(Date.now() - closedAt).toBeGreaterThan(LATE_GRACE);
  }, 30_000);
});
 
// ── WHO IS ALLOWED TO SEE IT (FR-RTM-07, FR-CHN-05, constitution I) ──────────
//
// EVERY NEGATIVE HERE IS ASSERTED BESIDE A POSITIVE IN THE SAME RUN. "Nothing
// arrived" is equally true of correct scoping and of a producer that stopped
// working, and only one of those is the property under test. Each case connects a
// watcher who MUST receive alongside one who must not, and asserts both.
describe("presence: who is allowed to see it (FR-RTM-07, FR-CHN-05)", () => {
  let a: Instance;
  let b: Instance;
  const sockets: WebSocket[] = [];
 
  const mint = async (user: string, credential: string) => {
    const res = await fetch(`${api.url}/auth/dev-token`, {
      method: "POST",
      headers: {
        "content-type": "application/json",
        authorization: `Bearer ${credential}`,
      },
      body: JSON.stringify({ user, ttl_seconds: 3600 }),
    });
    if (!res.ok) throw new Error(`dev-token ${user}: ${res.status}`);
    return ((await res.json()) as { token: string }).token;
  };
 
  const open = async (token: string, instance: Instance = a) => {
    const socket = new WebSocket(`${instance.url}/v1/ws?token=${token}`);
    sockets.push(socket);
    await waitForFrame(socket, "connection.ack");
    return socket;
  };
 
  beforeAll(async () => {
    a = await startInstance(api.url);
    b = await startInstance(api.url);
  }, 60_000);
 
  afterAll(async () => {
    for (const socket of sockets) socket.close();
    await a?.close();
    await b?.close();
  });
 
  // T057. The subject shares three channels with the co-member and none with the
  // recluse, whose only channel is private and unshared.
  it("delivers to a co-member and not to a user sharing no channel", async () => {
    const who = takeSubject();
    const member = await open(await mint("linh", api.credential));
    const outsider = await open(await mint("recluse", api.credential));
    const heard = collect(member, "presence.changed", who);
    const overheard = collect(outsider, "presence.changed", who);
 
    await open(await mint(who, api.credential), b);
    await quiet(450);
 
    expect(heard.frames).toHaveLength(1);
    expect(overheard.frames).toEqual([]);
  }, 30_000);
 
  // T058, FR-CHN-05's third verb. The recluse's only channel is PRIVATE and the
  // co-member is not in it, so a transition of the recluse's reaches nobody but the
  // recluse — while the same run shows the producer working for someone else.
  it("does not let a non-member observe presence in a private channel", async () => {
    const control = takeSubject();
    const member = await open(await mint("linh", api.credential));
    const aboutRecluse = collect(member, "presence.changed", "recluse");
    const aboutControl = collect(member, "presence.changed", control);
 
    await open(await mint("recluse", api.credential), b);
    await open(await mint(control, api.credential), b);
    await quiet(450);
 
    expect(aboutRecluse.frames).toEqual([]);
    // The control proves the path was alive for the length of the negative.
    expect(aboutControl.frames).toHaveLength(1);
  }, 30_000);
 
  // T059, constitution I. A different environment entirely: different key, different
  // user, different channel. Presence keys are `{env}`-scoped and channel ids are
  // unguessable UUIDs, so nothing about this should cross — asserted rather than
  // assumed, because "a leak here is a correctness defect, not a cosmetic one".
  it("delivers nothing to a user of another tenant", async () => {
    const who = takeSubject();
    const member = await open(await mint("linh", api.credential));
    const stranger = await open(
      await mint("stranger", api.otherCredential),
      b,
    );
    const heard = collect(member, "presence.changed", who);
    // UNFILTERED ON PURPOSE, and asserted as "everything it heard was about
    // itself". Filtering to the other tenant's subject would only prove that ONE
    // user did not leak; this catches any of them. The stranger does hear its own
    // arrival — a subject shares every channel with themselves (FR-011) — and an
    // earlier version of this test read that as a cross-tenant leak.
    const acrossTheBoundary = collect(stranger, "presence.changed");
 
    await open(await mint(who, api.credential), b);
    await quiet(450);
 
    expect(heard.frames).toHaveLength(1);
    expect(acrossTheBoundary.frames.map((f) => f.payload.user)).toEqual([
      "stranger",
    ]);
  }, 30_000);
 
  // T060, FR-029. A message and a transition on the same channel, at the same time.
  // Each must arrive as ITSELF. This is a property of the topology rather than of a
  // filter — a presence payload is published on a subject no message subscriber
  // subscribes to — so the test is checking that the topology is what it claims.
  it("never delivers a message as presence, or presence as a message", async () => {
    const who = takeSubject();
    const member = await open(await mint("linh", api.credential));
    const presenceFrames = collect(member, "presence.changed", who);
    const messages = collect(member, "message.created");
 
    await open(await mint(who, api.credential), b);
    await quiet(450);
 
    expect(presenceFrames.frames).toHaveLength(1);
    // No message was sent, and a presence payload must not be mistaken for one.
    expect(messages.frames).toEqual([]);
  }, 30_000);
 
  // T061, FR-027. A transition arriving while a connection is mid-resume is sent
  // immediately: presence carries no sequence, so it can neither duplicate a
  // backfilled row nor leave a gap, and `suppressed()` takes a `Message`. The
  // resuming socket presents a cursor, which puts it through the buffering phase.
  it("delivers a transition to a connection that is resuming", async () => {
    const who = takeSubject();
    const token = await mint("linh", api.credential);
    const resuming = new WebSocket(
      `${a.url}/v1/ws?token=${token}&cursor=${encodeURIComponent("{}")}`,
    );
    sockets.push(resuming);
    const heard = collect(resuming, "presence.changed", who);
    await waitForFrame(resuming, "connection.ack");
 
    await open(await mint(who, api.credential), b);
    await quiet(450);
 
    expect(heard.frames).toHaveLength(1);
  }, 30_000);
});
 
// ── WHEN REDIS IS GONE (FR-023, FR-024, FR-030) ──────────────────────────────
//
// EVERY TEST HERE COULD PASS AGAINST A MODULE THAT DOES NOTHING. "The socket still
// opened" is true of a working presence path and of an empty function, and chapter
// 3.18 recorded that trap against its own publisher: its `publish` swallows errors
// and resolves, so a 201 with Redis down proves nothing. What separates the two is
// the LOG LINE, and the restore case — a path that was never alive cannot come back.
describe("presence: when Redis is gone (FR-023, FR-024)", () => {
  const DEAD = "redis://127.0.0.1:1"; // the address the api's fan-out suite uses
  let broken: Instance;
  const sockets: WebSocket[] = [];
 
  const mint = async (user: string) => {
    const res = await fetch(`${api.url}/auth/dev-token`, {
      method: "POST",
      headers: {
        "content-type": "application/json",
        authorization: `Bearer ${api.credential}`,
      },
      body: JSON.stringify({ user, ttl_seconds: 3600 }),
    });
    if (!res.ok) throw new Error(`dev-token: ${res.status}`);
    return ((await res.json()) as { token: string }).token;
  };
 
  beforeAll(async () => {
    broken = await startInstance(api.url, { url: DEAD });
  }, 60_000);
 
  afterAll(async () => {
    for (const socket of sockets) socket.close();
    await broken?.close();
  });
 
  // T067, FR-023. A REAL ioredis client against a dead port, not a stub that
  // rejects: a stub skips connection handling, which is where a first draft of
  // `store.ts` got it wrong.
  it("opens the socket and completes the handshake anyway", async () => {
    const socket = new WebSocket(`${broken.url}/v1/ws?token=${await mint("linh")}`);
    sockets.push(socket);
    const ack = await waitForFrame(socket, "connection.ack");
    expect(ack).toHaveProperty("type", "connection.ack");
  }, 30_000);
 
  // T069, FR-024. THE ASSERTION THAT CARRIES FR-023. Without this, every other test
  // in this describe is satisfied by an empty function.
  it("logs presence.failed with an op and an error", async () => {
    const socket = new WebSocket(`${broken.url}/v1/ws?token=${await mint("linh")}`);
    sockets.push(socket);
    await waitForFrame(socket, "connection.ack");
    await quiet(600);
 
    const failures = broken.logs.filter((l) => l.msg === "presence.failed");
    expect(failures.length).toBeGreaterThan(0);
    for (const line of failures) {
      expect(line.level).toBe("error");
      expect(typeof line.fields.op).toBe("string");
      expect(typeof line.fields.error).toBe("string");
    }
  }, 30_000);
 
  // T068, FR-023. Presence must not be load-bearing: messages still reach the socket
  // with the presence path unreachable. The fan-out on this instance points at the
  // real Redis; only presence is broken.
  it("does not stop messages reaching a connected member", async () => {
    const watcher = new WebSocket(`${broken.url}/v1/ws?token=${await mint("linh")}`);
    sockets.push(watcher);
    await waitForFrame(watcher, "connection.ack");
    const messages = collect(watcher, "message.created");
 
    const sender = new WebSocket(`${broken.url}/v1/ws?token=${await mint("linh")}`);
    sockets.push(sender);
    await waitForFrame(sender, "connection.ack");
    sender.send(
      JSON.stringify({
        type: "message.send",
        payload: {
          idem_key: randomUUID(),
          channel: api.channelIds[0],
          text: "the presence path is down and this still arrives",
        },
      }),
    );
    await waitForFrame(sender, "message.ack");
    await quiet(400);
 
    expect(messages.frames.length).toBeGreaterThan(0);
  }, 30_000);
 
  // T071. A close handler is the last place that should throw, and chapter 2.8's
  // lane found the unhandled rejection on the fan-out's release path for exactly
  // this reason. An unhandled rejection fails the run, so this test asserts by
  // completing.
  it("closes a socket without throwing or leaving a rejection", async () => {
    const socket = new WebSocket(`${broken.url}/v1/ws?token=${await mint("linh")}`);
    sockets.push(socket);
    await waitForFrame(socket, "connection.ack");
    socket.close();
    await quiet(500);
    expect(true).toBe(true);
  }, 30_000);
});
 
// ── AND WHEN IT COMES BACK (FR-024's other half) ─────────────────────────────
describe("presence: when Redis comes back (FR-024)", () => {
  let proxy: Awaited<ReturnType<typeof startRedisProxy>>;
  let a: Instance;
  let b: Instance;
  const sockets: WebSocket[] = [];
 
  const mint = async (user: string) => {
    const res = await fetch(`${api.url}/auth/dev-token`, {
      method: "POST",
      headers: {
        "content-type": "application/json",
        authorization: `Bearer ${api.credential}`,
      },
      body: JSON.stringify({ user, ttl_seconds: 3600 }),
    });
    if (!res.ok) throw new Error(`dev-token: ${res.status}`);
    return ((await res.json()) as { token: string }).token;
  };
  const open = async (user: string, instance: Instance) => {
    const socket = new WebSocket(`${instance.url}/v1/ws?token=${await mint(user)}`);
    sockets.push(socket);
    await waitForFrame(socket, "connection.ack");
    return socket;
  };
 
  beforeAll(async () => {
    proxy = await startRedisProxy();
    a = await startInstance(api.url, { url: proxy.url });
    b = await startInstance(api.url, { url: proxy.url });
  }, 60_000);
 
  afterAll(async () => {
    for (const socket of sockets) socket.close();
    await a?.close();
    await b?.close();
    await proxy?.close();
  });
 
  // T070. **THE HALF THAT PROVES THE PATH WAS ALIVE.** Without it the whole failure
  // story above is satisfied by a module that never does anything: it would open
  // sockets, deliver messages, and log nothing but failures, forever.
  //
  // "Without a restart" is the load-bearing phrase — the module is never rebuilt,
  // ioredis reconnects on its own, and the proxy re-listens on the same port.
  it("publishes the next transition after the connection is restored", async () => {
    const watcher = await open("linh", a);
 
    await proxy.cut();
    await quiet(400);
    await proxy.restore();
    // ioredis backs off before retrying; give it room to notice.
    await quiet(2_500);
 
    const who = takeSubject();
    const heard = collect(watcher, "presence.changed", who);
    await open(who, b);
    await quiet(900);
 
    expect(heard.frames).toHaveLength(1);
  }, 60_000);
});
 
// ── DURABILITY IT MUST NOT ACQUIRE, AND THE REST OF THE VOCABULARY ───────────
describe("presence: no durability, and the whole log vocabulary", () => {
  let a: Instance;
  let b: Instance;
  const sockets: WebSocket[] = [];
 
  const mint = async (user: string) => {
    const res = await fetch(`${api.url}/auth/dev-token`, {
      method: "POST",
      headers: {
        "content-type": "application/json",
        authorization: `Bearer ${api.credential}`,
      },
      body: JSON.stringify({ user, ttl_seconds: 3600 }),
    });
    if (!res.ok) throw new Error(`dev-token: ${res.status}`);
    return ((await res.json()) as { token: string }).token;
  };
  const open = async (user: string, instance: Instance = a) => {
    const socket = new WebSocket(`${instance.url}/v1/ws?token=${await mint(user)}`);
    sockets.push(socket);
    await waitForFrame(socket, "connection.ack");
    return socket;
  };
 
  beforeAll(async () => {
    a = await startInstance(api.url);
    b = await startInstance(api.url);
  }, 60_000);
 
  afterAll(async () => {
    for (const socket of sockets) socket.close();
    await a?.close();
    await b?.close();
  });
 
  // T072, FR-026. ADR-10: the correct amount of durability for a green circle is
  // none. The lane runs with `RELAY_OUTBOX_RELAY=off`, so rows accumulate rather
  // than drain — if a transition wrote one it would still be there to count.
  it("writes no outbox row for a transition", async () => {
    const before = await api.outboxCount();
    const who = takeSubject();
    await open(who, b);
    await quiet(450);
    expect(await api.outboxCount()).toBe(before);
  }, 30_000);
 
  // T073, FR-025 and constitution VI. `channels` is a COUNT, not a list: the number
  // is useful in an incident and the list is a membership graph in a log file.
  it("logs presence.published with a channel count and no content", async () => {
    const who = takeSubject();
    await open(who, b);
    await quiet(450);
 
    const published = b.logs.filter(
      (l) => l.msg === "presence.published" && l.fields.user === who,
    );
    expect(published).toHaveLength(1);
    const line = published[0] as LogLine;
    expect(line.fields.state).toBe("online");
    expect(line.fields.channels).toBe(3);
    // No text, no token, no channel list — the fields are exactly these.
    expect(Object.keys(line.fields).sort()).toEqual([
      "channels",
      "level",
      "msg",
      "service",
      "state",
      "time",
      "user",
    ]);
  }, 30_000);
 
  // T074, FR-030. The two events FR-024 and FR-025 do not cover. They were specified
  // in the contract, implemented, and asserted nowhere until this test.
  it("logs presence.suppressed when a second connection changes nothing", async () => {
    const who = takeSubject();
    await open(who, a);
    await quiet(400);
    await open(who, b);
    await quiet(400);
 
    const suppressed = b.logs.filter(
      (l) => l.msg === "presence.suppressed" && l.fields.user === who,
    );
    expect(suppressed).toHaveLength(1);
    expect(suppressed[0]?.fields.reason).toBe("already online");
  }, 30_000);
 
  // RENAMED IN PHASE 9, because the title said the opposite of the assertion. This
  // test publishes a MESSAGE on a MESSAGE subject and asserts presence never sees
  // it — FR-029 from the other side — and `toEqual([])` is right for that. It was
  // titled "logs presence.invalid_payload", which is a claim nothing here checks:
  // the coverage run showed both rejection arms at zero while this test was green.
  // The real ones are in the last describe.
  it("does not reach the presence parser with a message payload", async () => {
    const who = takeSubject();
    await open("linh", a);
    await quiet(300);
    // Published straight onto a presence subject with neither module's code — the
    // only way to put a malformed payload on the fabric. The subject is derived the
    // same way the module derives it.
    const raw = createFanout({ logger: silent });
    await raw.publish({
      id: randomUUID(),
      channel: api.channelIds[0] as string,
      seq: 1,
      user: who,
      text: "not a presence payload",
      created_at: new Date().toISOString(),
    });
    await quiet(500);
    await raw.close();
 
    // The message subject is not the presence subject, so this proves the reverse of
    // FR-029 too: a message payload does not reach the presence parser at all.
    expect(a.logs.filter((l) => l.msg === "presence.invalid_payload")).toEqual([]);
  }, 30_000);
});
 
// THE BRANCHES A PASSING SUITE DOES NOT REACH — PHASE 9, FR-032.
//
// NFR-MNT-02 asks 100% branch coverage of tenant-isolation code and `presence.ts`
// measured 81.81 with every test above green. That is the whole argument for a
// ratchet: six arms of this module had never executed, and one of them — the
// malformed-payload rejection — had a test whose TITLE claimed it.
//
// Each test here names the arm it exists for. None of them is a scenario a user
// performs; they are the paths a failing store, a lost key or a caller using the
// module's own interface can produce, and the module documents all of them.
describe("presence: the arms a green suite left alone (FR-032)", () => {
  let a: Instance;
  const sockets: WebSocket[] = [];
 
  const mint = async (user: string) => {
    const res = await fetch(`${api.url}/auth/dev-token`, {
      method: "POST",
      headers: {
        "content-type": "application/json",
        authorization: `Bearer ${api.credential}`,
      },
      body: JSON.stringify({ user, ttl_seconds: 3600 }),
    });
    if (!res.ok) throw new Error(`dev-token: ${res.status}`);
    return ((await res.json()) as { token: string }).token;
  };
  const open = async (user: string, instance: Instance = a) => {
    const socket = new WebSocket(`${instance.url}/v1/ws?token=${await mint(user)}`);
    sockets.push(socket);
    await waitForFrame(socket, "connection.ack");
    return socket;
  };
 
  beforeAll(async () => {
    a = await startInstance(api.url);
    // `linh` is the fixture's watcher and a member of all three channels, so this
    // one connection is what makes the instance a subscriber on `presence:{c}`.
    // The first draft opened a user named "watcher" — the dev-token endpoint mints
    // a token for any id, so the socket opened, the user was a member of nothing,
    // the instance subscribed to nothing, and both publishes below reached nobody.
    // The tests failed for a reason that had nothing to do with what they check.
    await open("linh");
    await quiet(300);
  }, 60_000);
 
  afterAll(async () => {
    for (const socket of sockets) socket.close();
    await a?.close();
  });
 
  // The `catch` around `JSON.parse`. A subscriber receives bytes, not objects, and
  // "not JSON at all" is a different failure from "JSON of the wrong shape" —
  // `fanout.ts` separates them the same way and its own arm is uncovered too.
  it("logs presence.invalid_payload for a body that is not JSON", async () => {
    const raw = new Redis(process.env.RELAY_REDIS_URL ?? "redis://localhost:6379");
    await raw.publish(subjectForPresence(api.channelIds[0] as string), "{not json");
    await quiet(400);
    await raw.quit();
 
    const rejected = a.logs.filter((l) => l.msg === "presence.invalid_payload");
    expect(rejected).toHaveLength(1);
    expect(rejected[0]?.fields.subject).toBe(
      subjectForPresence(api.channelIds[0] as string),
    );
  }, 30_000);
 
  // The `safeParse` arm. `presenceFabricSchema` is a `strictObject`, so a field this
  // module has never published is a rejection rather than a silent ignore — which is
  // what makes adding a field to the fabric a decision on both sides of a deploy.
  it("logs presence.invalid_payload for JSON that is not a transition", async () => {
    const before = a.logs.filter((l) => l.msg === "presence.invalid_payload").length;
    const raw = new Redis(process.env.RELAY_REDIS_URL ?? "redis://localhost:6379");
    await raw.publish(
      subjectForPresence(api.channelIds[0] as string),
      JSON.stringify({ user: "someone", state: "online" }),
    );
    await quiet(400);
    await raw.quit();
 
    // No `transition`, so a receiver could not dedup it — and `strictObject` refuses
    // it before that becomes anybody's problem.
    expect(
      a.logs.filter((l) => l.msg === "presence.invalid_payload").length - before,
    ).toBe(1);
  }, 30_000);
 
  // FR-031's self-healing duplicate, and the only arm here that a REAL incident
  // produces: a Redis restart or an eviction takes the key out from under a live
  // connection. Provoked without touching the store — a TTL shorter than the refresh
  // interval reaches the same state, because `XX` answers null for a key that is gone.
  it("re-elects and logs when the key vanishes under a live connection", async () => {
    const instance = await startInstance(api.url, {
      ttlMs: 200,
      refreshMs: 400,
      graceMs: 200,
      marginMs: 50,
    });
    try {
      const who = takeSubject();
      const socket = new WebSocket(
        `${instance.url}/v1/ws?token=${await mint(who)}`,
      );
      await waitForFrame(socket, "connection.ack");
      // Two refresh intervals: the key expires at 200 ms, the refresh at 400 ms finds
      // it gone, and the re-election runs.
      await quiet(1_000);
      socket.close();
 
      const reelected = instance.logs.filter(
        (l) =>
          l.msg === "presence.suppressed" &&
          l.fields.reason === "key vanished under a live connection; re-electing",
      );
      expect(reelected.length).toBeGreaterThanOrEqual(1);
      expect(reelected[0]?.fields.user).toBe(who);
    } finally {
      await instance.close();
    }
  }, 30_000);
 
  // `unsubscribe` for a channel this instance never subscribed to. Not reachable
  // through `session.ts`, which unsubscribes exactly the set it subscribed — but it
  // is on the `Presence` interface, so a caller can do it, and the reference count
  // must not go negative or throw.
  it("tolerates an unsubscribe for a channel never subscribed", async () => {
    const presence = createPresence({ logger: silent });
    try {
      await expect(presence.unsubscribe(randomUUID())).resolves.toBeUndefined();
    } finally {
      await presence.close();
    }
  }, 30_000);
 
  // The no-op `deliver` the module starts with. `onTransition` is called by
  // `attachSessions` at wiring time, and a `Presence` built without sessions still
  // receives on its subscriptions — it must drop them rather than throw inside an
  // ioredis event handler, where a throw is an unhandled rejection.
  it("drops a transition when no handler has been registered", async () => {
    const listener = createPresence({ logger: silent });
    const publisher = createPresence({ logger: silent });
    try {
      const channel = api.channelIds[1] as string;
      await listener.subscribe(channel);
      await quiet(200);
      await publisher.connected("env-x", takeSubject(), [channel]);
      await quiet(400);
      // Nothing to assert but the absence of a crash: an unhandled rejection here
      // fails the file, which is the assertion.
      expect(true).toBe(true);
    } finally {
      await listener.close();
      await publisher.close();
    }
  }, 30_000);
 
  // `close()` while a grace check is pending. A draining instance abandons its
  // pending offlines — stated in the chapter rather than discovered — and the timer
  // must be cleared, or a suite standing up two instances leaks one into the next
  // file.
  it("clears a pending grace check when the instance closes", async () => {
    const instance = await startInstance(api.url, {
      graceMs: 10_000,
      marginMs: 1_000,
    });
    const who = takeSubject();
    const socket = new WebSocket(`${instance.url}/v1/ws?token=${await mint(who)}`);
    await waitForFrame(socket, "connection.ack");
    await quiet(300);
    // Closing the socket arms the check ten seconds out; closing the instance while
    // it is pending is the path.
    socket.close();
    await quiet(400);
    await instance.close();
 
    expect(
      instance.logs.filter(
        (l) => l.msg === "presence.published" && l.fields.state === "offline",
      ),
    ).toEqual([]);
  }, 30_000);
 
  // `DEFAULT_REDIS_URL`. `main.ts` builds the module with no `url`, so the default
  // is what a real gateway uses and no test had ever taken it — every instance here
  // passes one, and `RELAY_REDIS_URL` is set in the lane.
  it("falls back to the default url when neither is supplied", async () => {
    const saved = process.env.RELAY_REDIS_URL;
    delete process.env.RELAY_REDIS_URL;
    try {
      const presence = createPresence({ logger: silent });
      await presence.subscribe(api.channelIds[2] as string);
      await presence.close();
    } finally {
      if (saved !== undefined) process.env.RELAY_REDIS_URL = saved;
    }
    expect(DEFAULT_REDIS_URL).toBe("redis://localhost:6379");
  }, 30_000);
});