fix(orbit): host-side session guards — settings default, maxUsers, push serialisation (#1157)

* fix(orbit): unify session-settings default to the canonical preset

updateOrbitSettings merged patches onto a hand-rolled
{ autoApprove: true, autoShuffle: true } fallback when a legacy session's
blob predated the settings field. That literal disagreed with
ORBIT_DEFAULT_SETTINGS (autoApprove: false), so toggling autoShuffle on
such a session silently flipped autoApprove on, landing guest suggestions
without host approval. Use ORBIT_DEFAULT_SETTINGS as the single source of
truth for the fallback. Add a behavioural test for the settings-less,
preserve-existing, and not-hosting paths.

* fix(orbit): enforce maxUsers host-side in the participant rebuild

The join gate only sees a one-tick-old state blob, so two guests can both
pass the participants.length < maxUsers check and join past the cap at
once; maxUsers was advisory. The host is the single writer of the
canonical state, so enforce the cap there: admit at most maxUsers guests
in the participant rebuild, earliest joiners winning (an established
participant is never displaced by a newcomer) with a deterministic
username tie-break for same-tick joins. Suggestions from an over-cap guest
are ignored, consistent with their absence from the participant list.
Reorders the fold so the admitted set gates the queue-additions loop. Add
tests for the cap, the no-displacement rule, and the ignored over-cap
suggestion.

* fix(orbit): serialise host state pushes to stop overlapping ticks dropping writes

pushState does several awaited round-trips (sweep -> resolve -> write) and
is fired from three sources: the mount, the 2.5 s timer, and a play/pause
subscription. With no in-flight lock these can overlap and race
last-writer-wins: a slow run that already swept and cleared an outbox can
lose its write to a faster run, dropping those suggestions from both the
outbox and the published state. The lastPushedAtRef was only ever written,
never used as a guard.

Add makeCoalescedRunner: a pure helper that serialises an async task and
coalesces any mid-flight request into a single rerun, so no trigger is
lost. The host hook wraps pushState in it; the dead ref is removed. Add a
unit test for the serialise + coalesce semantics.

* docs(changelog): add Orbit host-side session guards (1157)

CHANGELOG.md

@@ -232,6 +232,24 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * A guest who joined while radio was playing could have unrelated radio tracks appended to their queue, drifting them out of sync with the host. Automatic radio top-up is now paused while you are in an Orbit session.
 
+### Orbit — auto-approve could switch on unexpectedly in older sessions
+
+**By [@Psychotoxical](https://github.com/Psychotoxical), PR [#1157](https://github.com/Psychotoxical/psysonic/pull/1157)**
+
+* In a long-running Orbit session created by an older build, toggling auto-shuffle could silently also turn on auto-approve, so guest suggestions started playing without the host approving them. The two settings are now independent again.
+
+### Orbit — sessions could go over their guest limit
+
+**By [@Psychotoxical](https://github.com/Psychotoxical), PR [#1157](https://github.com/Psychotoxical/psysonic/pull/1157)**
+
+* When several people joined at the same moment, a session could end up with more guests than its limit allowed. The host now enforces the limit, keeping the earliest joiners.
+
+### Orbit — an accepted suggestion could occasionally be dropped
+
+**By [@Psychotoxical](https://github.com/Psychotoxical), PR [#1157](https://github.com/Psychotoxical/psysonic/pull/1157)**
+
+* Under load the host's updates could overlap and overwrite each other, occasionally losing a guest suggestion that had just been picked up. Host updates are now serialised so none are lost.
+
 ## [1.48.1] - 2026-06-15
 
 ## Fixed

src/hooks/useOrbitHost.ts

@@ -1,6 +1,6 @@
 import { getSong } from '../api/subsonicLibrary';
 import { songToTrack } from '../utils/playback/songToTrack';
-import { useEffect, useRef } from 'react';
+import { useEffect } from 'react';
 import { useOrbitStore } from '../store/orbitStore';
 import { usePlayerStore } from '../store/playerStore';
 import {
@@ -9,6 +9,7 @@ import {
   applyOutboxSnapshotsToState,
   maybeShuffleQueue,
   effectiveShuffleIntervalMs,
+  makeCoalescedRunner,
   suggestionKey,
 } from '../utils/orbit';
 import {
@@ -49,10 +50,6 @@ export function useOrbitHost(): void {
   const sessionId         = useOrbitStore(s => s.sessionId);
   const hostName          = useOrbitStore(s => s.state?.host);
 
-  // Refs hold the last values we used to build the patch — cheap to
-  // recompute against, no need to subscribe to every playerStore tick.
-  const lastPushedAtRef = useRef(0);
-
   const active = role === 'host' && phase === 'active' && !!sessionPlaylistId;
 
   useEffect(() => {
@@ -138,7 +135,6 @@ export function useOrbitHost(): void {
       useOrbitStore.getState().setState(next);
       try {
         await writeOrbitState(sessionPlaylistId, next);
-        lastPushedAtRef.current = Date.now();
         pushOrbitEvent('host:push', JSON.stringify({
           track: next.currentTrack?.trackId ?? null,
           playing: next.isPlaying,
@@ -221,11 +217,17 @@ export function useOrbitHost(): void {
       }
     };
 
+    // Serialise pushState across its three triggers (mount, timer, play/pause
+    // flip): two bodies must never run concurrently, or a slow run that already
+    // swept+cleared an outbox can lose its write to a faster one. A request
+    // arriving mid-run coalesces into a single rerun so no trigger is lost.
+    const runPush = makeCoalescedRunner(pushState);
+
     // Immediate push on mount so guests see fresh state without waiting
     // a full tick after the host comes online.
-    void pushState();
+    void runPush();
 
-    const id = window.setInterval(() => { void pushState(); }, STATE_TICK_MS);
+    const id = window.setInterval(() => { void runPush(); }, STATE_TICK_MS);
 
     // Event-driven push on play/pause flips. Without this the worst-case
     // delay between "host hits pause" and "guest stops" is two full polling
@@ -239,7 +241,7 @@ export function useOrbitHost(): void {
       if (state.isPlaying === prevIsPlaying) return;
       prevIsPlaying = state.isPlaying;
       pushOrbitEvent('host:event-push', `isPlaying flip → ${state.isPlaying}`);
-      void pushState();
+      void runPush();
     });
 
     return () => {

src/utils/orbit.ts

@@ -27,6 +27,7 @@ export {
 } from './orbit/constants';
 export {
   generateSessionId,
+  makeCoalescedRunner,
   OrbitStateTooLarge,
   serialiseOrbitState,
   suggestionKey,

src/utils/orbit/helpers.test.ts

@@ -7,7 +7,12 @@ import {
   type OrbitQueueItem,
   type OrbitState,
 } from '../../api/orbit';
-import { OrbitStateTooLarge, serialiseOrbitState, serialiseOrbitStateForWire } from './helpers';
+import {
+  makeCoalescedRunner,
+  OrbitStateTooLarge,
+  serialiseOrbitState,
+  serialiseOrbitStateForWire,
+} from './helpers';
 
 function baseState(): OrbitState {
   return makeInitialOrbitState({ sid: 'aaaa1111', host: 'host', name: 'sesh' });
@@ -67,3 +72,34 @@ describe('serialiseOrbitStateForWire', () => {
     expect((parsed!.playQueue ?? []).length).toBeLessThan(400);
   });
 });
+
+describe('makeCoalescedRunner', () => {
+  // Let the microtask queue drain so an awaiting do-while loop can advance.
+  const flush = async () => { await Promise.resolve(); await Promise.resolve(); };
+
+  it('never runs two task bodies concurrently and coalesces mid-flight calls into one rerun', async () => {
+    let starts = 0;
+    const releases: Array<() => void> = [];
+    const task = () => {
+      starts++;
+      return new Promise<void>(res => { releases.push(res); });
+    };
+    const run = makeCoalescedRunner(task);
+
+    void run(); // run #1 starts and blocks
+    void run(); // in-flight → flags a rerun, no second body
+    void run(); // in-flight → still just one pending rerun
+    expect(starts).toBe(1);
+
+    releases[0](); // finish #1 → the coalesced rerun fires exactly one more body
+    await flush();
+    expect(starts).toBe(2);
+
+    releases[1](); // finish #2 → no rerun pending, runner goes idle
+    await flush();
+    expect(starts).toBe(2);
+
+    void run(); // lock released → a fresh call starts a new body
+    expect(starts).toBe(3);
+  });
+});

src/utils/orbit/helpers.ts

@@ -94,6 +94,36 @@ export function serialiseOutboxMeta(meta: OrbitOutboxMeta): string {
 export const suggestionKey = (q: OrbitQueueItem): string =>
   `${q.addedBy}:${q.addedAt}:${q.trackId}`;
 
+/**
+ * Wrap an async task so it never runs concurrently with itself. While a run is
+ * in flight, further calls do NOT start a second run — they flag a rerun, and
+ * exactly one more run fires after the current finishes (any number of
+ * mid-flight requests coalesce into a single catch-up). The host tick uses this
+ * because `pushState` does several awaited round-trips and is fired from a 2.5 s
+ * timer, the mount, and a play/pause subscription that can overlap; without
+ * serialisation a slow run that already swept+cleared an outbox can lose its
+ * write to a faster run (last-writer-wins), dropping those suggestions.
+ */
+export function makeCoalescedRunner(task: () => Promise<void>): () => Promise<void> {
+  let running = false;
+  let rerun = false;
+  return async () => {
+    if (running) {
+      rerun = true;
+      return;
+    }
+    running = true;
+    try {
+      do {
+        rerun = false;
+        await task();
+      } while (rerun);
+    } finally {
+      running = false;
+    }
+  };
+}
+
 /** Extract `<username>` from a filename matching `__psyorbit_<sid>_from_<username>__`. */
 export function parseOutboxPlaylistName(name: string, sid: string): string | null {
   const prefix = `${ORBIT_PLAYLIST_PREFIX}${sid}_from_`;

src/utils/orbit/host.test.ts

@@ -0,0 +1,82 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+import {
+  makeInitialOrbitState,
+  ORBIT_DEFAULT_SETTINGS,
+  type OrbitSettings,
+  type OrbitState,
+} from '../../api/orbit';
+
+const { writeOrbitState } = vi.hoisted(() => ({ writeOrbitState: vi.fn(() => Promise.resolve()) }));
+const { orbitStore } = vi.hoisted(() => ({
+  orbitStore: {
+    role: 'host' as 'host' | 'guest' | null,
+    state: null as OrbitState | null,
+    sessionPlaylistId: 'session-pl' as string | null,
+    setState: vi.fn(),
+  },
+}));
+
+vi.mock('./remote', () => ({
+  writeOrbitState,
+  writeOrbitHeartbeat: vi.fn(() => Promise.resolve()),
+}));
+vi.mock('../../store/orbitStore', () => ({ useOrbitStore: { getState: () => orbitStore } }));
+vi.mock('../../store/authStore', () => ({ useAuthStore: { getState: () => ({}) } }));
+vi.mock('../../store/playerStore', () => ({ usePlayerStore: { getState: () => ({ enqueue: vi.fn() }) } }));
+vi.mock('../../api/subsonicPlaylists', () => ({ createPlaylist: vi.fn(), deletePlaylist: vi.fn() }));
+vi.mock('../../api/subsonicLibrary', () => ({ getSong: vi.fn() }));
+vi.mock('../playback/songToTrack', () => ({ songToTrack: vi.fn() }));
+
+import { updateOrbitSettings } from './host';
+
+function hostStateWith(settings: OrbitSettings | undefined): OrbitState {
+  const base = makeInitialOrbitState({ sid: 'aaaa1111', host: 'host', name: 'sesh' });
+  return { ...base, settings: settings as OrbitSettings };
+}
+
+/** The settings object that updateOrbitSettings persisted on its last call. */
+function writtenSettings(): OrbitSettings {
+  const calls = writeOrbitState.mock.calls as unknown as Array<[string, OrbitState]>;
+  const lastCall = calls[calls.length - 1];
+  return lastCall[1].settings as OrbitSettings;
+}
+
+beforeEach(() => {
+  writeOrbitState.mockClear();
+  orbitStore.setState.mockClear();
+  orbitStore.role = 'host';
+  orbitStore.sessionPlaylistId = 'session-pl';
+});
+
+describe('updateOrbitSettings', () => {
+  it('does not silently flip autoApprove on a legacy settings-less session', async () => {
+    orbitStore.state = hostStateWith(undefined);
+    await updateOrbitSettings({ autoShuffle: false });
+
+    const settings = writtenSettings();
+    // The patch only touched autoShuffle…
+    expect(settings.autoShuffle).toBe(false);
+    // …autoApprove must come from the canonical default (false), not flip true.
+    expect(settings.autoApprove).toBe(ORBIT_DEFAULT_SETTINGS.autoApprove);
+    expect(settings.autoApprove).toBe(false);
+    expect(settings.shuffleIntervalMin).toBe(ORBIT_DEFAULT_SETTINGS.shuffleIntervalMin);
+  });
+
+  it('preserves existing settings when patching one field', async () => {
+    orbitStore.state = hostStateWith({ autoApprove: true, autoShuffle: true, shuffleIntervalMin: 30 });
+    await updateOrbitSettings({ autoShuffle: false });
+
+    const settings = writtenSettings();
+    expect(settings.autoApprove).toBe(true);
+    expect(settings.autoShuffle).toBe(false);
+    expect(settings.shuffleIntervalMin).toBe(30);
+  });
+
+  it('is a no-op when not hosting', async () => {
+    orbitStore.role = 'guest';
+    orbitStore.state = hostStateWith(undefined);
+    await updateOrbitSettings({ autoShuffle: false });
+    expect(writeOrbitState).not.toHaveBeenCalled();
+  });
+});

src/utils/orbit/host.ts

@@ -9,6 +9,7 @@ import {
   orbitOutboxPlaylistName,
   orbitSessionPlaylistName,
   ORBIT_DEFAULT_MAX_USERS,
+  ORBIT_DEFAULT_SETTINGS,
   type OrbitQueueItem,
   type OrbitSettings,
   type OrbitState,
@@ -163,8 +164,13 @@ export async function triggerOrbitShuffleNow(): Promise<void> {
 export async function updateOrbitSettings(patch: Partial<OrbitSettings>): Promise<void> {
   const store = useOrbitStore.getState();
   if (store.role !== 'host' || !store.state || !store.sessionPlaylistId) return;
+  // Fall back to the canonical defaults (not a hand-rolled literal) for
+  // legacy sessions whose blob predates the settings field — otherwise
+  // patching one field on a settings-less session would silently flip
+  // autoApprove on, since the old literal had it true while
+  // ORBIT_DEFAULT_SETTINGS (the popover's source of truth) has it false.
   const mergedSettings: OrbitSettings = {
-    ...(store.state.settings ?? { autoApprove: true, autoShuffle: true }),
+    ...(store.state.settings ?? ORBIT_DEFAULT_SETTINGS),
     ...patch,
   };
   const next: OrbitState = { ...store.state, settings: mergedSettings };

src/utils/orbit/stateMath.test.ts

@@ -57,3 +57,58 @@ describe('applyOutboxSnapshotsToState — queue history cap', () => {
     expect(next.queue.map(q => q.trackId)).toEqual(['t0', 't1']);
   });
 });
+
+describe('applyOutboxSnapshotsToState — maxUsers enforcement', () => {
+  const NOW = 5_000;
+  const fresh = (user: string, trackIds: string[] = []): OutboxSnapshot => ({
+    user,
+    outboxPlaylistId: `ob-${user}`,
+    trackIds,
+    lastHeartbeat: NOW,
+  });
+
+  function hostState(maxUsers: number, participants: OrbitState['participants']): OrbitState {
+    return {
+      ...makeInitialOrbitState({ sid: 'aaaa1111', host: 'host', name: 'sesh', maxUsers }),
+      participants,
+    };
+  }
+
+  it('admits at most maxUsers guests, dropping the surplus', () => {
+    const state = hostState(2, []);
+    const next = applyOutboxSnapshotsToState(
+      state,
+      [fresh('aaa'), fresh('bbb'), fresh('ccc')],
+      NOW,
+    );
+    // Three guests joined on the same tick; the deterministic username
+    // tie-break keeps the first two alphabetically.
+    expect(next.participants.map(p => p.user)).toEqual(['aaa', 'bbb']);
+  });
+
+  it('never displaces an established participant for a newcomer', () => {
+    // `zoe` joined earlier (smaller joinedAt) — she keeps her slot even though
+    // her name sorts last.
+    const state = hostState(2, [{ user: 'zoe', joinedAt: 1_000, lastHeartbeat: 1_000 }]);
+    const next = applyOutboxSnapshotsToState(
+      state,
+      [fresh('zoe'), fresh('aaa'), fresh('bbb')],
+      NOW,
+    );
+    expect(next.participants.map(p => p.user)).toEqual(['zoe', 'aaa']);
+  });
+
+  it('ignores suggestions from an over-cap guest', () => {
+    const state = hostState(2, []);
+    const next = applyOutboxSnapshotsToState(
+      state,
+      [fresh('aaa', ['t-a']), fresh('bbb', ['t-b']), fresh('ccc', ['t-c'])],
+      NOW,
+    );
+    const addedTracks = next.queue.map(q => q.trackId);
+    expect(addedTracks).toContain('t-a');
+    expect(addedTracks).toContain('t-b');
+    // ccc is over the cap → not a participant, suggestion ignored.
+    expect(addedTracks).not.toContain('t-c');
+  });
+});

src/utils/orbit/stateMath.ts

@@ -97,58 +97,70 @@ export function applyOutboxSnapshotsToState(
   snapshots: OutboxSnapshot[],
   nowMs: number = Date.now(),
 ): OrbitState {
-  // ── Queue additions ──
-  // Guest outboxes are append-only from the host's POV — the host reads the
-  // same playlist every sweep, so we must dedupe against anything already in
-  // `state.queue` (or currently playing) by (user, trackId). Without this,
-  // every host tick re-adds every outbox entry and the pending-approval list
-  // balloons indefinitely. A user re-suggesting the same track after it
-  // lands/plays is a rare enough case to live with for now.
-  const existingKeys = new Set<string>(
-    state.queue.map(q => `${q.addedBy} ${q.trackId}`),
-  );
-  if (state.currentTrack) {
-    existingKeys.add(`${state.currentTrack.addedBy} ${state.currentTrack.trackId}`);
-  }
-
-  // Drop any new suggestion from a user the host has muted before the
-  // dedupe scan — they shouldn't count against the queue at all.
-  const blocked = new Set(state.suggestionBlocked ?? []);
-  const newItems: OrbitQueueItem[] = [];
-  for (const snap of snapshots) {
-    if (blocked.has(snap.user)) continue;
-    for (const trackId of snap.trackIds) {
-      const key = `${snap.user} ${trackId}`;
-      if (existingKeys.has(key)) continue;
-      existingKeys.add(key);
-      newItems.push({ trackId, addedBy: snap.user, addedAt: nowMs });
-    }
-  }
-
   // ── Soft-removed list aging ──
   // Drop entries older than the TTL so the list stays bounded and a long-
   // expired marker doesn't kick a freshly-rejoined user back out.
   const removed = (state.removed ?? []).filter(r => nowMs - r.at < ORBIT_REMOVED_TTL_MS);
   const removedUsers = new Set(removed.map(r => r.user));
 
-  // ── Participants rebuild ──
+  // ── Participants rebuild (with maxUsers enforcement) ──
-  // Soft-removed users stay out of `participants` even if their heartbeat is
+  // Anyone with a fresh heartbeat who isn't kicked or soft-removed is
-  // still fresh — gives them up to one read tick (~2.5s) to notice the
+  // *eligible*. Soft-removed users stay out even if their heartbeat is still
-  // `removed`-marker and tear down their guest hooks before the marker ages out.
+  // fresh — gives them up to one read tick (~2.5s) to notice the `removed`
+  // marker and tear down their guest hooks before it ages out.
+  //
+  // The join gate (`joinOrbitSession`) only sees a one-tick-old blob, so two
+  // guests can both pass the `participants.length < maxUsers` check and slip
+  // in past the cap at once. The host is the single writer of the canonical
+  // state, so it's the only place the cap can truly hold — enforce it here by
+  // admitting at most `maxUsers` guests. Earliest joiners win (an established
+  // participant is never displaced by a newcomer; their `joinedAt` is older),
+  // with a deterministic username tie-break for guests that joined on the
+  // same tick. The host runs its own outbox heartbeat but is filtered out by
+  // the sweep, so `participants` is guests only — the cap matches `maxUsers`.
   const prev = new Map(state.participants.map(p => [p.user, p]));
-  const participants: OrbitParticipant[] = [];
+  const eligible: OrbitParticipant[] = [];
   for (const snap of snapshots) {
     if (state.kicked.includes(snap.user)) continue;
     if (removedUsers.has(snap.user)) continue;
     const fresh = snap.lastHeartbeat > 0 && (nowMs - snap.lastHeartbeat) < ORBIT_HEARTBEAT_ALIVE_MS;
     if (!fresh) continue;
     const existing = prev.get(snap.user);
-    participants.push({
+    eligible.push({
       user: snap.user,
       joinedAt: existing?.joinedAt ?? nowMs,
       lastHeartbeat: snap.lastHeartbeat,
     });
   }
+  eligible.sort((a, b) => a.joinedAt - b.joinedAt || a.user.localeCompare(b.user));
+  const participants = eligible.slice(0, Math.max(0, state.maxUsers));
+  const admitted = new Set(participants.map(p => p.user));
+
+  // ── Queue additions ──
+  // Guest outboxes are append-only from the host's POV — the host reads the
+  // same playlist every sweep, so we dedupe against anything already in
+  // `state.queue` (or currently playing) by (user, trackId). Without this,
+  // every host tick re-adds every outbox entry and the pending-approval list
+  // balloons indefinitely. A suggestion only counts from an *admitted* guest
+  // who isn't muted — an over-cap guest's outbox is ignored, consistent with
+  // their absence from `participants`.
+  const existingKeys = new Set<string>(
+    state.queue.map(q => `${q.addedBy} ${q.trackId}`),
+  );
+  if (state.currentTrack) {
+    existingKeys.add(`${state.currentTrack.addedBy} ${state.currentTrack.trackId}`);
+  }
+  const blocked = new Set(state.suggestionBlocked ?? []);
+  const newItems: OrbitQueueItem[] = [];
+  for (const snap of snapshots) {
+    if (blocked.has(snap.user) || !admitted.has(snap.user)) continue;
+    for (const trackId of snap.trackIds) {
+      const key = `${snap.user} ${trackId}`;
+      if (existingKeys.has(key)) continue;
+      existingKeys.add(key);
+      newItems.push({ trackId, addedBy: snap.user, addedAt: nowMs });
+    }
+  }
 
   return {
     ...state,