fix(fleet): serialize restart-lock transitions to close concurrent-breaker race (review #680)

Stale/max-wait takeover was not safe against concurrent breakers: two
breakers could both judge the lock stale and both proceed, re-introducing
the tight-loop. POSIX/Node has no content- or inode-conditional unlink or
rename, so "judge stale, then replace" can never be atomic with pure path
ops.

Serialize ALL lock transitions (acquire, release, takeover) under one
short-lived registry mutex held only across a few fs ops, never across the
restart itself. This makes check-then-mutate atomic, so exactly one breaker
can take over a stale lock while the others wait and re-evaluate.

The mutex itself uses mtime-based staleness (open('wx') creates an empty
inode before the token is written; a content check would reap a lock that is
still being acquired). The mutex populates-or-cleans-up on write failure so a
half-created mutex never leaks.

Regression coverage at two widths: a 2-breaker barrier test (exactly one
takes over, the other waits) and the existing 3-breaker test (maxActive===1,
distinct tokens, final lock released).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/mosaic/src/commands/fleet.spec.ts

@@ -4,6 +4,7 @@ import { dirname, join, resolve } from 'node:path';
 import { Command } from 'commander';
 import { afterEach, describe, expect, it, vi } from 'vitest';
 import {
+  acquireRestartLock,
   addAgentToRoster,
   buildAgentSendCommand,
   buildAgentWatchAttachCommand,
@@ -45,6 +46,8 @@ import {
   removeAgentFromRoster,
   resolveFleetPaths,
   resolvePresetFilename,
+  restartLockPath,
+  RESTART_LOCK_STALE_MS,
   RUNTIME_ACCEPTABLE_COMMANDS,
   serializeRosterToYaml,
   VERIFY_DEFAULT_TIMEOUT_MS,
@@ -678,6 +681,364 @@ describe('fleet command construction', () => {
     }
   });
 
+  it('waits for an in-flight restart to clear before relaunching (re-entry guard)', async () => {
+    const home = await tempDir();
+    const rosterPath = join(home, 'fleet', 'roster.yaml');
+    await mkdir(join(home, 'fleet'), { recursive: true });
+    await writeFile(
+      rosterPath,
+      ['version: 1', 'transport: tmux', 'agents:', '  - name: coder0', '    runtime: codex'].join(
+        '\n',
+      ),
+    );
+
+    // Simulate another `mosaic fleet restart` process mid-teardown: a fresh lock
+    // (recent timestamp, so it is NOT treated as stale) already held.
+    const lockPath = restartLockPath(home);
+    await mkdir(dirname(lockPath), { recursive: true });
+    await writeFile(lockPath, `4242\n${Date.now()}\n`);
+
+    const events: string[] = [];
+    const runner: CommandRunner = async (command, args) => {
+      events.push(`run:${args[args.length - 1]}`);
+      return { stdout: '', stderr: '', exitCode: 0 };
+    };
+    // The injected sleep stands in for time passing while we wait; the in-flight
+    // restart "finishes" (releases its lock) after the first poll.
+    let sleeps = 0;
+    const sleepFn: SleepFn = async () => {
+      sleeps += 1;
+      events.push(`sleep:${sleeps}`);
+      await rm(lockPath, { force: true });
+    };
+
+    const program = new Command();
+    program.exitOverride();
+    registerFleetCommand(program, { runner, sleepFn, mosaicHome: home });
+
+    try {
+      await program.parseAsync(['node', 'mosaic', 'fleet', 'restart']);
+
+      // It must have waited at least once before issuing any systemctl restart.
+      expect(sleeps).toBeGreaterThan(0);
+      const firstSleep = events.findIndex((e) => e.startsWith('sleep:'));
+      const firstRun = events.findIndex((e) => e.startsWith('run:'));
+      expect(firstSleep).toBeGreaterThanOrEqual(0);
+      expect(firstRun).toBeGreaterThan(firstSleep);
+
+      // And it still performs the full restart once the lock clears.
+      expect(events).toContain('run:mosaic-tmux-holder.service');
+      expect(events).toContain('run:mosaic-agent@coder0.service');
+
+      // The lock is released after the restart completes.
+      await expect(readFile(lockPath, 'utf8')).rejects.toMatchObject({ code: 'ENOENT' });
+    } finally {
+      await rm(home, { recursive: true, force: true });
+    }
+  });
+
+  it('breaks a stale restart lock and proceeds without waiting', async () => {
+    const home = await tempDir();
+    const rosterPath = join(home, 'fleet', 'roster.yaml');
+    await mkdir(join(home, 'fleet'), { recursive: true });
+    await writeFile(
+      rosterPath,
+      ['version: 1', 'transport: tmux', 'agents:', '  - name: coder0', '    runtime: codex'].join(
+        '\n',
+      ),
+    );
+
+    // A lock left behind by a crashed owner: timestamp older than the stale window.
+    const lockPath = restartLockPath(home);
+    await mkdir(dirname(lockPath), { recursive: true });
+    await writeFile(lockPath, `4242\n${Date.now() - RESTART_LOCK_STALE_MS - 1_000}\n`);
+
+    const calls: string[][] = [];
+    const runner: CommandRunner = async (command, args) => {
+      calls.push([command, ...args]);
+      return { stdout: '', stderr: '', exitCode: 0 };
+    };
+    const sleepFn = vi.fn<SleepFn>(async () => {});
+
+    const program = new Command();
+    program.exitOverride();
+    registerFleetCommand(program, { runner, sleepFn, mosaicHome: home });
+
+    try {
+      await program.parseAsync(['node', 'mosaic', 'fleet', 'restart']);
+
+      // Stale lock is broken immediately — no waiting.
+      expect(sleepFn).not.toHaveBeenCalled();
+      expect(calls).toEqual([
+        ['systemctl', '--user', 'restart', 'mosaic-tmux-holder.service'],
+        ['systemctl', '--user', 'restart', 'mosaic-agent@coder0.service'],
+      ]);
+      // The stale lock is gone once the restart completes.
+      await expect(readFile(lockPath, 'utf8')).rejects.toMatchObject({ code: 'ENOENT' });
+    } finally {
+      await rm(home, { recursive: true, force: true });
+    }
+  });
+
+  it('releases the restart lock so a subsequent restart is not blocked', async () => {
+    const home = await tempDir();
+    const rosterPath = join(home, 'fleet', 'roster.yaml');
+    await mkdir(join(home, 'fleet'), { recursive: true });
+    await writeFile(
+      rosterPath,
+      ['version: 1', 'transport: tmux', 'agents:', '  - name: coder0', '    runtime: codex'].join(
+        '\n',
+      ),
+    );
+
+    const calls: string[][] = [];
+    const runner: CommandRunner = async (command, args) => {
+      calls.push([command, ...args]);
+      return { stdout: '', stderr: '', exitCode: 0 };
+    };
+    const sleepFn = vi.fn<SleepFn>(async () => {});
+
+    const program = new Command();
+    program.exitOverride();
+    registerFleetCommand(program, { runner, sleepFn, mosaicHome: home });
+
+    try {
+      await program.parseAsync(['node', 'mosaic', 'fleet', 'restart']);
+      await program.parseAsync(['node', 'mosaic', 'fleet', 'restart']);
+
+      // Two sequential restarts both run fully and neither has to wait.
+      expect(sleepFn).not.toHaveBeenCalled();
+      expect(calls).toEqual([
+        ['systemctl', '--user', 'restart', 'mosaic-tmux-holder.service'],
+        ['systemctl', '--user', 'restart', 'mosaic-agent@coder0.service'],
+        ['systemctl', '--user', 'restart', 'mosaic-tmux-holder.service'],
+        ['systemctl', '--user', 'restart', 'mosaic-agent@coder0.service'],
+      ]);
+    } finally {
+      await rm(home, { recursive: true, force: true });
+    }
+  });
+
+  it('guards the single-agent restart path behind the in-flight restart lock', async () => {
+    const home = await tempDir();
+    const rosterPath = join(home, 'fleet', 'roster.yaml');
+    await mkdir(join(home, 'fleet'), { recursive: true });
+    await writeFile(
+      rosterPath,
+      ['version: 1', 'transport: tmux', 'agents:', '  - name: coder0', '    runtime: codex'].join(
+        '\n',
+      ),
+    );
+
+    // A full restart is mid-flight (lock held); a single-agent restart re-enters.
+    const lockPath = restartLockPath(home);
+    await mkdir(dirname(lockPath), { recursive: true });
+    await writeFile(lockPath, `4242\n${Date.now()}\n`);
+
+    const events: string[] = [];
+    const runner: CommandRunner = async (command, args) => {
+      events.push(`run:${args[args.length - 1]}`);
+      return { stdout: '', stderr: '', exitCode: 0 };
+    };
+    let sleeps = 0;
+    const sleepFn: SleepFn = async () => {
+      sleeps += 1;
+      events.push(`sleep:${sleeps}`);
+      await rm(lockPath, { force: true });
+    };
+
+    const program = new Command();
+    program.exitOverride();
+    registerFleetCommand(program, { runner, sleepFn, mosaicHome: home });
+
+    try {
+      await program.parseAsync(['node', 'mosaic', 'fleet', 'restart', 'coder0']);
+
+      // The single-agent restart waits for the in-flight restart before acting.
+      expect(sleeps).toBeGreaterThan(0);
+      const firstSleep = events.findIndex((e) => e.startsWith('sleep:'));
+      const firstRun = events.findIndex((e) => e.startsWith('run:'));
+      expect(firstSleep).toBeGreaterThanOrEqual(0);
+      expect(firstRun).toBeGreaterThan(firstSleep);
+      // Only the named agent is restarted; the holder is untouched.
+      expect(events).toContain('run:mosaic-agent@coder0.service');
+      expect(events).not.toContain('run:mosaic-tmux-holder.service');
+    } finally {
+      await rm(home, { recursive: true, force: true });
+    }
+  });
+
+  it('does not let a timed-out owner drop a lock another restart broke and re-owned', async () => {
+    const home = await tempDir();
+    const runDir = join(home, 'fleet', 'run');
+    await mkdir(runDir, { recursive: true });
+    const lockPath = restartLockPath(home);
+    const tokenOf = async (): Promise<string> => {
+      const raw = await readFile(lockPath, 'utf8');
+      return raw.split('\n')[2]?.trim() ?? '';
+    };
+    const sleepFn = vi.fn<SleepFn>(async () => {});
+
+    // R1 acquires the lock and begins a restart that then hangs.
+    const r1 = await acquireRestartLock(home, sleepFn);
+    const tokenR1 = await tokenOf();
+    expect(tokenR1).not.toBe('');
+
+    // The hung R1 leaves a stale lock: rewrite its timestamp into the past while
+    // preserving R1's token — exactly the on-disk state a stuck owner leaves.
+    await writeFile(lockPath, `4242\n${Date.now() - RESTART_LOCK_STALE_MS - 1_000}\n${tokenR1}\n`);
+
+    // R2 re-enters, sees the stale lock, and atomically takes ownership.
+    const r2 = await acquireRestartLock(home, sleepFn);
+    const tokenR2 = await tokenOf();
+    expect(tokenR2).not.toBe(tokenR1);
+    expect(sleepFn).not.toHaveBeenCalled();
+
+    // R1 finally finishes and releases. It must NOT delete R2's lock — otherwise
+    // a third restart (R3) could acquire and interleave with R2 still running.
+    await r1.release();
+    expect(await tokenOf()).toBe(tokenR2);
+
+    // R2 releases cleanly and the lock is gone.
+    await r2.release();
+    await expect(readFile(lockPath, 'utf8')).rejects.toMatchObject({ code: 'ENOENT' });
+
+    await rm(home, { recursive: true, force: true });
+  });
+
+  it('lets only one of several concurrent breakers proceed past a stale lock', async () => {
+    const home = await tempDir();
+    const lockPath = restartLockPath(home);
+    await mkdir(dirname(lockPath), { recursive: true });
+
+    // A stale lock left by a crashed owner: every concurrent re-entrant restart
+    // will judge it stale and try to break it at the same instant. Breaking must
+    // NOT grant ownership — only the atomic re-create may — so exactly one
+    // contender can ever hold the lock at a time. (The v2 fix wrote our own token
+    // during the break and read it back, so two breakers each saw their own token
+    // and BOTH proceeded; this guards that regression.)
+    await writeFile(
+      lockPath,
+      `4242\n${Date.now() - RESTART_LOCK_STALE_MS - 1_000}\nstale-owner-token\n`,
+    );
+
+    // Yielding sleep so a waiting contender lets the current owner finish and
+    // release before it re-contends, instead of spinning the microtask queue.
+    const sleepFn: SleepFn = async () => {
+      await new Promise((res) => setTimeout(res, 0));
+    };
+
+    let active = 0;
+    let maxActive = 0;
+    const tokens: string[] = [];
+    const tokenOf = async (): Promise<string> => {
+      const raw = await readFile(lockPath, 'utf8');
+      return raw.split('\n')[2]?.trim() ?? '';
+    };
+
+    // One "restart" = acquire the lock, do work in the critical section, release.
+    const restartOnce = async (): Promise<void> => {
+      const guard = await acquireRestartLock(home, sleepFn);
+      active += 1;
+      maxActive = Math.max(maxActive, active);
+      // Record the token we own while we hold it, then yield to interleave with
+      // any other contender that might (wrongly) believe it owns the lock too.
+      tokens.push(await tokenOf());
+      await new Promise((res) => setTimeout(res, 0));
+      active -= 1;
+      await guard.release();
+    };
+
+    try {
+      // Three breakers race the single stale lock simultaneously.
+      await Promise.all([restartOnce(), restartOnce(), restartOnce()]);
+
+      // Mutual exclusion held: never two owners at once despite concurrent breaks.
+      expect(maxActive).toBe(1);
+      // Each acquire owned with its own distinct token — no two ever shared it.
+      expect(new Set(tokens).size).toBe(3);
+      // The lock is fully released at the end.
+      await expect(readFile(lockPath, 'utf8')).rejects.toMatchObject({ code: 'ENOENT' });
+    } finally {
+      await rm(home, { recursive: true, force: true });
+    }
+  });
+
+  it('lets exactly one of two breakers take over a stale lock while the other waits', async () => {
+    const home = await tempDir();
+    const lockPath = restartLockPath(home);
+    await mkdir(dirname(lockPath), { recursive: true });
+
+    // A single stale lock both contenders will judge stale at the same instant.
+    // Every transition runs under the registry mutex, so only one may take the
+    // lock over; the other must observe a now-fresh owner and WAIT/re-evaluate
+    // rather than also taking over. (A content-blind clobber let both believe
+    // they owned it — this asserts the mutex-gated CAS takeover instead.)
+    await writeFile(
+      lockPath,
+      `4242\n${Date.now() - RESTART_LOCK_STALE_MS - 1_000}\nstale-owner-token\n`,
+    );
+
+    // Barrier the winner holds against until the loser has observed the lock
+    // fresh and waited at least once — forcing the exact interleaving where one
+    // proceeds while the other waits, deterministically rather than by timing.
+    let resolveLoserWaited: () => void = () => {};
+    const loserWaited = new Promise<void>((res) => {
+      resolveLoserWaited = res;
+    });
+    let sleeps = 0;
+    const sleepFn: SleepFn = async () => {
+      sleeps += 1;
+      resolveLoserWaited();
+      await new Promise((res) => setTimeout(res, 0));
+    };
+
+    let active = 0;
+    let maxActive = 0;
+    const tokens: string[] = [];
+    const tokenOf = async (): Promise<string> => {
+      const raw = await readFile(lockPath, 'utf8');
+      return raw.split('\n')[2]?.trim() ?? '';
+    };
+
+    let firstOwner = true;
+    const restartOnce = async (): Promise<void> => {
+      const guard = await acquireRestartLock(home, sleepFn);
+      active += 1;
+      maxActive = Math.max(maxActive, active);
+      tokens.push(await tokenOf());
+      if (firstOwner) {
+        // Winner: keep holding the lock until the loser has waited once, so the
+        // loser is guaranteed to see a FRESH owner (not the stale one) and back
+        // off — proving it could not also take over.
+        firstOwner = false;
+        await loserWaited;
+      } else {
+        await new Promise((res) => setTimeout(res, 0));
+      }
+      active -= 1;
+      await guard.release();
+    };
+
+    try {
+      // Exactly two breakers race the single stale lock.
+      await Promise.all([restartOnce(), restartOnce()]);
+
+      // Mutual exclusion: never two owners at once (if both took over the stale
+      // lock, this would be 2).
+      expect(maxActive).toBe(1);
+      // Both eventually owned, each with its own distinct token.
+      expect(new Set(tokens).size).toBe(2);
+      // The loser observed the winner's fresh lock and waited — it did NOT also
+      // take over the stale lock.
+      expect(sleeps).toBeGreaterThanOrEqual(1);
+      // The lock is fully released at the end.
+      await expect(readFile(lockPath, 'utf8')).rejects.toMatchObject({ code: 'ENOENT' });
+    } finally {
+      await rm(home, { recursive: true, force: true });
+    }
+  });
+
   it('attempts every agent and the holder during fleet stop even when an agent stop fails', async () => {
     const home = await tempDir();
     const rosterPath = join(home, 'fleet', 'roster.yaml');

packages/mosaic/src/commands/fleet.ts

@@ -1,5 +1,16 @@
 import { constants } from 'node:fs';
-import { access, chmod, copyFile, mkdir, readFile, unlink, writeFile } from 'node:fs/promises';
+import {
+  access,
+  chmod,
+  copyFile,
+  mkdir,
+  open,
+  readFile,
+  stat,
+  unlink,
+  writeFile,
+} from 'node:fs/promises';
+import { randomUUID } from 'node:crypto';
 import { homedir, hostname, userInfo } from 'node:os';
 import { dirname, join, resolve } from 'node:path';
 import { fileURLToPath } from 'node:url';
@@ -533,6 +544,295 @@ export function buildFleetServiceCommand(action: FleetServiceAction, agentName?:
   return ['systemctl', '--user', action, service];
 }
 
+/** Poll interval (ms) while waiting for an in-flight restart's lock to clear. */
+export const RESTART_LOCK_POLL_INTERVAL_MS = 250;
+/**
+ * Maximum time (ms) a re-entrant restart waits for the in-flight restart to
+ * finish before it breaks the lock and proceeds anyway. A bound is required so
+ * a crashed holder of the lock can never deadlock the fleet permanently.
+ */
+export const RESTART_LOCK_MAX_WAIT_MS = 30_000;
+/**
+ * Age (ms) past which a restart lock is treated as stale (its owner died
+ * without releasing it) and is broken immediately rather than waited on.
+ */
+export const RESTART_LOCK_STALE_MS = 60_000;
+
+/**
+ * Resolves the path of the cross-process restart lock for a given Mosaic home.
+ * Kept strictly under `<mosaicHome>/fleet/run` (not the heartbeat env override)
+ * so the lock is scoped to the same fleet the restart acts on.
+ */
+export function restartLockPath(mosaicHome: string): string {
+  return join(mosaicHome, 'fleet', 'run', 'restart.lock');
+}
+
+/** A held restart lock; `release()` removes the lock file iff we still own it. */
+interface RestartGuard {
+  release(): Promise<void>;
+}
+
+/** Lock-file contents: pid (informational), timestamp, and a unique owner token. */
+function formatRestartLockContent(token: string): string {
+  return `${process.pid}\n${Date.now()}\n${token}\n`;
+}
+
+/**
+ * Reads the owner token (line 3) from a lock file, or null if the file is
+ * missing/unreadable/tokenless. The token is what makes release and break
+ * ownership-safe: a process only ever acts on a lock whose token matches its own.
+ */
+async function readRestartLockToken(lockPath: string): Promise<string | null> {
+  let raw: string;
+  try {
+    raw = await readFile(lockPath, 'utf8');
+  } catch {
+    return null;
+  }
+  const token = raw.split('\n')[2]?.trim();
+  return token ? token : null;
+}
+
+/**
+ * Returns true when a lock's contents are stale: older than RESTART_LOCK_STALE_MS,
+ * or unparseable (a corrupt or partially written lock left by a crashed owner).
+ */
+function isRestartLockContentStale(raw: string, now: number): boolean {
+  const stampLine = raw.split('\n')[1] ?? '';
+  const stamp = Number.parseInt(stampLine.trim(), 10);
+  if (!Number.isFinite(stamp)) {
+    return true;
+  }
+  return now - stamp >= RESTART_LOCK_STALE_MS;
+}
+
+/**
+ * Path of the short-lived registry mutex that guards EVERY transition of the
+ * restart lock (acquire, release, takeover). Held only across a few filesystem
+ * ops — never across the restart itself — so contention clears in microseconds.
+ */
+function restartMutexPath(lockPath: string): string {
+  return `${lockPath}.mutex`;
+}
+
+/** Brief back-off between registry-mutex acquisition attempts (held microseconds). */
+const RESTART_MUTEX_RETRY_MS = 20;
+
+/**
+ * Staleness for the internal mutex / reclaim locks, judged by the file's mtime
+ * rather than its CONTENT. `open(path, 'wx')` creates the inode (with a fresh
+ * mtime) before any token/timestamp is written into it, so a content-based check
+ * would momentarily see that empty file as corrupt-and-stale and could reap a
+ * lock another contender is still acquiring. mtime is set atomically at creation,
+ * so a just-created lock always reads as live; only a lock whose holder died and
+ * stopped touching it ages past the threshold. These locks are never held across
+ * the restart itself (only a couple of filesystem ops), so any mtime this old can
+ * belong only to a dead holder.
+ */
+async function isRestartLockPathStale(path: string, now: number): Promise<boolean> {
+  try {
+    const info = await stat(path);
+    return now - info.mtimeMs >= RESTART_LOCK_STALE_MS;
+  } catch (err) {
+    if ((err as NodeJS.ErrnoException).code === 'ENOENT') {
+      return false; // Gone, not stale — the caller will re-contend.
+    }
+    return false; // Can't stat — treat as live and back off rather than reap.
+  }
+}
+
+/** Path of the reclaim lock that serializes reaping of a crashed-holder mutex. */
+function restartReclaimPath(mutexPath: string): string {
+  return `${mutexPath}.reclaim`;
+}
+
+/**
+ * Reap a registry mutex left behind by a process that CRASHED mid-transition —
+ * one whose file has aged past RESTART_LOCK_STALE_MS. Because the mutex is held
+ * only for a couple of filesystem ops (no sleeps, never across the restart), a
+ * mutex this old can only belong to a dead holder.
+ *
+ * The reap removes the dead mutex but never CREATES/holds it — acquisition stays
+ * the single `open('wx')` create in {@link acquireRestartMutex}, so exactly one
+ * contender wins ownership no matter how the reap and acquires interleave. The
+ * removal is made conditional by a dedicated reclaim lock: while it is held the
+ * dead mutex is stable (its dead holder will never touch it, and no other
+ * reclaimer can race), so re-reading it and removing it only if it is STILL stale
+ * is a true compare — a live holder's fresh mutex is never removed. This closes
+ * the reclaim race a content-blind rename-and-restore left open (a third
+ * contender slipping into the gap while a fresh mutex was moved aside).
+ */
+async function reclaimStaleRestartMutex(mutexPath: string): Promise<void> {
+  const reclaimPath = restartReclaimPath(mutexPath);
+  let handle: Awaited<ReturnType<typeof open>>;
+  try {
+    handle = await open(reclaimPath, 'wx');
+  } catch (err) {
+    if ((err as NodeJS.ErrnoException).code !== 'EEXIST') {
+      throw err;
+    }
+    // Someone is already reclaiming. If their reclaim lock is itself stale by
+    // mtime, its holder crashed mid-reap (the lock spans only a stat + unlink,
+    // microseconds) — clear it so a later pass can retry. Otherwise a live
+    // reclaimer has it; back off. Either way we do not reap the mutex this pass.
+    if (await isRestartLockPathStale(reclaimPath, Date.now())) {
+      await unlink(reclaimPath).catch(() => {});
+    }
+    return;
+  }
+  try {
+    // Re-check the mutex UNDER the reclaim lock and remove it only if it is STILL
+    // stale by mtime. A live holder's mutex is fresh and is left untouched; a dead
+    // holder's mutex is stable here (its holder is gone and no other reclaimer can
+    // race us), so this re-check is authoritative.
+    if (await isRestartLockPathStale(mutexPath, Date.now())) {
+      await unlink(mutexPath).catch(() => {});
+    }
+  } finally {
+    await handle.close();
+    await unlink(reclaimPath).catch(() => {});
+  }
+}
+
+/**
+ * Acquire the registry mutex, BLOCKING (with brief back-offs) until held, and
+ * return a token-gated release. This is the single point of mutual exclusion for
+ * the restart lock: acquire, release, and stale/timeout takeover all run under it,
+ * so "read the lock, then mutate it" is atomic — no acquirer, releaser, or breaker
+ * can ever interleave with another. A mutex left by a crashed holder is reclaimed
+ * once it ages past the stale threshold.
+ */
+async function acquireRestartMutex(
+  mutexPath: string,
+  token: string,
+): Promise<RestartGuard['release']> {
+  for (;;) {
+    let handle: Awaited<ReturnType<typeof open>>;
+    try {
+      handle = await open(mutexPath, 'wx');
+    } catch (err) {
+      if ((err as NodeJS.ErrnoException).code !== 'EEXIST') {
+        throw err;
+      }
+      // Staleness is judged by mtime, not content, so a mutex that exists but has
+      // not yet had its token written (the open-before-write window) reads as live
+      // and is never wrongly reaped.
+      if (!(await isRestartLockPathStale(mutexPath, Date.now()))) {
+        // A live holder has it — it will be gone in microseconds. Back off briefly.
+        await new Promise((resolve) => setTimeout(resolve, RESTART_MUTEX_RETRY_MS));
+        continue;
+      }
+      await reclaimStaleRestartMutex(mutexPath);
+      continue;
+    }
+    // We created the mutex. Populate it with our token; if writing fails, clean up
+    // our own file so we never leak an empty mutex that a peer would have to reap.
+    try {
+      await handle.writeFile(formatRestartLockContent(token));
+      await handle.close();
+    } catch (err) {
+      await handle.close().catch(() => {});
+      await unlink(mutexPath).catch(() => {});
+      throw err;
+    }
+    return async (): Promise<void> => {
+      if ((await readRestartLockToken(mutexPath)) !== token) return;
+      await unlink(mutexPath).catch(() => {});
+    };
+  }
+}
+
+/**
+ * Acquire the fleet restart lock, serializing concurrent `mosaic fleet restart`
+ * invocations across processes. Each restart tears the tmux holder (and the
+ * agent sessions inside it) down and back up; without this guard a re-entrant
+ * restart relaunches agents against a half-torn-down holder, which fails and
+ * tight-loops. A re-entrant caller waits for the in-flight restart to release
+ * the lock (clean shutdown settled) before proceeding, breaks a stale lock left
+ * by a crashed owner, and after RESTART_LOCK_MAX_WAIT_MS breaks the lock to
+ * avoid a permanent deadlock.
+ *
+ * Correctness rests on a single invariant: EVERY transition of the lock — taking
+ * a free lock, taking over a stale/timed-out one, and releasing — happens under
+ * the registry mutex. Because the check ("is the lock free / stale / fresh?") and
+ * the mutation that follows it both run while the mutex is held, they are atomic:
+ * no other acquirer, releaser, or breaker can slip in between. That is what makes
+ * takeover a true compare-and-swap rather than a content-blind clobber — a normal
+ * `open('wx')` acquirer cannot create a fresh lock in a gap, and the original
+ * owner's `release()` (also mutex-gated and token-checked) cannot drop a lock a
+ * breaker already took over. So no interleaving lets two restarts both own the
+ * lock and run concurrently.
+ */
+export async function acquireRestartLock(
+  mosaicHome: string,
+  sleepFn: SleepFn,
+): Promise<RestartGuard> {
+  const token = randomUUID();
+  const lockPath = restartLockPath(mosaicHome);
+  const mutexPath = restartMutexPath(lockPath);
+  await mkdir(dirname(lockPath), { recursive: true });
+  const release = async (): Promise<void> => {
+    // Mutex-gated and token-gated: only remove the lock if it is still ours. If
+    // another caller took it over (after a stale/timeout break) the token no
+    // longer matches and we leave their lock intact.
+    const releaseMutex = await acquireRestartMutex(mutexPath, token);
+    try {
+      if ((await readRestartLockToken(lockPath)) === token) {
+        await unlink(lockPath).catch(() => {});
+      }
+    } finally {
+      await releaseMutex();
+    }
+  };
+  const deadline = Date.now() + RESTART_LOCK_MAX_WAIT_MS;
+  for (;;) {
+    let owned = false;
+    const releaseMutex = await acquireRestartMutex(mutexPath, token);
+    try {
+      // Read and (if appropriate) mutate the lock atomically under the mutex.
+      let current: string | null = null;
+      let absent = false;
+      try {
+        current = await readFile(lockPath, 'utf8');
+      } catch (readErr) {
+        if ((readErr as NodeJS.ErrnoException).code === 'ENOENT') {
+          absent = true;
+        } else {
+          current = null; // Unreadable/corrupt: treat as stale.
+        }
+      }
+      const now = Date.now();
+      if (absent) {
+        // Lock is free — take it.
+        await writeFile(lockPath, formatRestartLockContent(token));
+        owned = true;
+      } else {
+        const stale = current === null || isRestartLockContentStale(current, now);
+        const timedOut = now >= deadline;
+        if (stale || timedOut) {
+          process.stderr.write(
+            stale
+              ? 'Breaking stale fleet restart lock.\n'
+              : `Timed out after ${RESTART_LOCK_MAX_WAIT_MS}ms waiting for the in-flight fleet ` +
+                  'restart; breaking the lock.\n',
+          );
+          // Takeover is just an overwrite — safe because we hold the mutex, so no
+          // acquirer or releaser can touch the lock between our read and this write.
+          await writeFile(lockPath, formatRestartLockContent(token));
+          owned = true;
+        }
+        // else: a fresh restart owns it — wait below and re-evaluate.
+      }
+    } finally {
+      await releaseMutex();
+    }
+    if (owned) {
+      return { release };
+    }
+    await sleepFn(RESTART_LOCK_POLL_INTERVAL_MS);
+  }
+}
+
 /**
  * Returns the systemctl --user enable command for a given unit.
  * Used by the install auto-enable step to persist units across reboots.
@@ -1172,6 +1472,7 @@ export function isSendAccepted(capturedOutput: string): SendVerifyResult {
 
 export function registerFleetCommand(program: Command, deps: FleetCommandDeps = {}): Command {
   const runner = deps.runner ?? runCommand;
+  const sleepFn = deps.sleepFn ?? defaultSleep;
   const paths = resolveFleetPaths(deps.mosaicHome);
   const frameworkRoot = deps.frameworkRoot ?? resolveFrameworkRoot();
 
@@ -1285,9 +1586,22 @@ export function registerFleetCommand(program: Command, deps: FleetCommandDeps =
       .command(`${action} [agent]`)
       .description(`${action} the fleet holder or one agent`)
       .action(async (agent?: string) => {
+        const commandOpts = cmd.opts<{ mosaicHome: string; roster?: string }>();
+        const activePaths = resolveFleetPaths(commandOpts.mosaicHome);
         const roster = await loadRosterForCommand(cmd);
         if (agent) {
           getRosterAgent(roster, agent);
+          // Single-agent restart is guarded too: it can race a full restart that
+          // is tearing the shared holder down.
+          if (action === 'restart') {
+            const guard = await acquireRestartLock(activePaths.mosaicHome, sleepFn);
+            try {
+              await runChecked(runner, buildFleetServiceCommand(action, agent));
+            } finally {
+              await guard.release();
+            }
+            return;
+          }
           await runChecked(runner, buildFleetServiceCommand(action, agent));
           return;
         }
@@ -1298,6 +1612,21 @@ export function registerFleetCommand(program: Command, deps: FleetCommandDeps =
           );
           return;
         }
+        if (action === 'restart') {
+          // Serialize the holder+agents teardown/relaunch behind the restart lock
+          // so a re-entrant restart waits for clean shutdown before relaunching,
+          // instead of racing a half-torn-down holder into a tight loop.
+          const guard = await acquireRestartLock(activePaths.mosaicHome, sleepFn);
+          try {
+            await runChecked(runner, buildFleetServiceCommand(action));
+            for (const rosterAgent of roster.agents) {
+              await runChecked(runner, buildFleetServiceCommand(action, rosterAgent.name));
+            }
+          } finally {
+            await guard.release();
+          }
+          return;
+        }
         await runChecked(runner, buildFleetServiceCommand(action));
         for (const rosterAgent of roster.agents) {
           await runChecked(runner, buildFleetServiceCommand(action, rosterAgent.name));