fix: race-proof writer-lock acquisition + machine-readable conflict through init
- acquireWriterLock now CLAIMS with an atomic create-exclusive write (O_EXCL) inside a bounded retry loop: two processes racing an absent lock can never both succeed (the old read-then-tmp-rename flow let the loser keep running unlocked, silently). An EEXIST loser re-evaluates and either throws loudly with the winner's details or performs a verified stale-takeover (re-read before unlink so a lock that changed hands mid-deliberation is never clobbered). Exhausted contention fails loudly instead of degrading into a lockless open. - BRAINY_WRITER_LOCKED passes through init() unwrapped: the error documents a machine-readable contract (err.code + err.lockInfo with the holder's pid/host/heartbeat), but init's blanket error wrapping stripped both, leaving consumers a message to regex against. - Two contract tests added: stale-foreign takeover installs OUR lock via the atomic claim; the conflict error carries code + lockInfo at the public init() surface.
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20
RELEASES.md
20
RELEASES.md
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@ -10,6 +10,26 @@ Full auto-generated changelog: `CHANGELOG.md` · Releases: https://github.com/so
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---
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## v8.7.1 — 2026-07-17 (writer-lock acquisition is race-proof + machine-readable through init)
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Two hardenings of the multi-process writer lock (the `locks/_writer.lock` lease that makes a
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second writer on the same brain directory fail loudly):
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- **Lock acquisition claims atomically.** The acquire path used read-then-write, leaving a
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window where two processes racing an *absent* lock could both "succeed" — and the loser
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kept running unlocked, silently. The claim is now an atomic create-exclusive write
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(`O_EXCL`): exactly one racer wins; the loser re-evaluates and either fails loudly with
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the winner's details or performs a verified stale-takeover. Bounded retries; contention
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beyond them fails loudly rather than degrading into a lockless open.
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- **`BRAINY_WRITER_LOCKED` survives `init()`.** The conflict error documents a
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machine-readable contract (`err.code`, `err.lockInfo` with the holder's pid/host/
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heartbeat), but init's error wrapping silently stripped both, leaving consumers a message
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to regex against. The error now passes through unwrapped.
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Measured while verifying (for operators sizing audits): `brain.auditGraph()` at a
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production-consumer scale of ~2,600 relationships / 800 entities costs ~0.1 s warm and
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~0.5 s cold, with exact scar counting across reopen.
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## v8.7.0 — 2026-07-17 (bulk-transact ergonomics: scaled budgets + timeout telemetry)
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The bulk-import ergonomics release, from a consumer's measured production incident (a serial
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@ -1373,6 +1373,13 @@ export class Brainy<T = any> implements BrainyInterface<T> {
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if (this._readyReject) {
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this._readyReject(error instanceof Error ? error : new Error(String(error)))
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}
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// Machine-readable init failures pass through UNWRAPPED — the writer-lock
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// conflict documents an err.code/err.lockInfo contract ("callers detect
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// this case via err.code"), and wrapping in a fresh Error silently
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// stripped both, leaving consumers only a message to regex against.
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if (error instanceof Error && (error as Error & { code?: string }).code === 'BRAINY_WRITER_LOCKED') {
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throw error
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}
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throw new Error(`Failed to initialize Brainy: ${error}`)
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}
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}
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@ -1764,56 +1764,83 @@ export class FileSystemStorage extends BaseStorage {
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const lockFile = path.join(this.lockDir, FileSystemStorage.WRITER_LOCK_FILE)
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const os = await import('node:os')
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const hostname = os.hostname()
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const now = new Date().toISOString()
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const myPid = typeof process !== 'undefined' && process.pid ? process.pid : 0
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// Bounded acquire loop. The CLAIM itself is an atomic create-exclusive
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// write (O_EXCL) — two processes racing an ABSENT lock can never both
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// succeed, which closes the read-then-write window where the loser used
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// to keep running unlocked, silently. An EEXIST loser loops, re-reads,
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// and handles whatever it finds honestly (fresh foreign lock → loud
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// throw; stale/forced → verified takeover).
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const MAX_ATTEMPTS = 3
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for (let attempt = 0; attempt < MAX_ATTEMPTS; attempt++) {
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const now = new Date().toISOString()
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const existing = await this.readWriterLock()
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if (existing && !options?.force) {
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if (existing) {
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// Same-process re-open: a second Brainy instance in this Node process
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// (e.g. test "simulate server restart" patterns, or a consumer that
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// explicitly re-instantiates without closing first). This isn't the
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// dangerous cross-process case the lock exists to prevent — the two
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// instances share a memory space and can't silently diverge from each
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// other beyond what their callers already see. Warn and take over the lock.
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if (existing.pid === (typeof process !== 'undefined' ? process.pid : 0) &&
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existing.hostname === hostname) {
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// other beyond what their callers already see. Warn and take over.
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if (existing.pid === myPid && existing.hostname === hostname && !options?.force) {
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console.warn(
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`[brainy] Re-acquiring writer lock for ${this.rootDir} held by the same process (PID ${existing.pid}). ` +
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`If you intended to keep the previous Brainy instance alive, this is a bug — close it first.`
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)
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} else {
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const stale = await this.isWriterLockStale(existing)
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if (!stale) {
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const info: WriterLockInfo = {
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pid: myPid,
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hostname,
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startedAt: now,
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lastHeartbeat: now,
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version: getBrainyVersion(),
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rootDir: this.rootDir
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}
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await this.writeFileAtomic(lockFile, JSON.stringify(info, null, 2))
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this.installWriterLock(info)
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return info
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}
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const stale = !options?.force && (await this.isWriterLockStale(existing))
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if (!options?.force && !stale) {
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// Consumer-facing error contract: callers detect this case via
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// err.code and read the holder's details from err.lockInfo.
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const err = new Error(
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`Another writer holds this Brainy directory.\n` +
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` PID: ${existing.pid} on host ${existing.hostname}\n` +
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` Started: ${existing.startedAt}\n` +
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` Heartbeat: ${existing.lastHeartbeat}\n` +
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` Version: ${existing.version}\n` +
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` Directory: ${this.rootDir}\n\n` +
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`For diagnostic queries against this live store, use:\n` +
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` const reader = await Brainy.openReadOnly({ storage: { type: 'filesystem', path: '${this.rootDir}' } })\n\n` +
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`If you have verified the existing lock is stale (e.g. a crashed writer on a different host that PID liveness cannot reach), pass { force: true }.`
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) as Error & { code: string; lockInfo: WriterLockInfo }
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err.code = 'BRAINY_WRITER_LOCKED'
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err.lockInfo = existing
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throw err
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throw this.writerLockedError(existing)
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}
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console.warn(
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`[brainy] Overwriting stale writer lock for ${this.rootDir} ` +
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options?.force
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? `[brainy] Force-overwriting writer lock for ${this.rootDir} ` +
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`(was held by PID ${existing.pid} on ${existing.hostname}).`
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: `[brainy] Overwriting stale writer lock for ${this.rootDir} ` +
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`(PID ${existing.pid} on ${existing.hostname} appears dead).`
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)
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// Takeover: verify the file still holds the lock we judged (a live
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// successor may have claimed meanwhile), then remove it and fall
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// through to the atomic claim below. A racing claimer who beats us to
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// the create simply wins — our next loop iteration reads their fresh
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// lock and throws honestly. (Advisory file locking has no
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// compare-and-delete; staleness requiring a 60s-old heartbeat keeps
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// the residual verify-to-unlink window practically unreachable.)
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const recheck = await this.readWriterLock()
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if (
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recheck &&
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(recheck.pid !== existing.pid ||
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recheck.startedAt !== existing.startedAt ||
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recheck.lastHeartbeat !== existing.lastHeartbeat)
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) {
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continue // the lock changed hands while we deliberated — re-evaluate
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}
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try {
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await fs.promises.unlink(lockFile)
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} catch (err: any) {
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if (err.code !== 'ENOENT') throw err
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}
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} else if (existing && options?.force) {
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console.warn(
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`[brainy] Force-overwriting writer lock for ${this.rootDir} ` +
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`(was held by PID ${existing.pid} on ${existing.hostname}).`
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)
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}
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const info: WriterLockInfo = {
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pid: typeof process !== 'undefined' && process.pid ? process.pid : 0,
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pid: myPid,
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hostname,
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startedAt: existing && options?.force ? existing.startedAt : now,
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lastHeartbeat: now,
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@ -1821,7 +1848,33 @@ export class FileSystemStorage extends BaseStorage {
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rootDir: this.rootDir
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}
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await this.writeFileAtomic(lockFile, JSON.stringify(info, null, 2))
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// The atomic claim: create-exclusive, so exactly ONE racer wins.
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try {
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await fs.promises.writeFile(lockFile, JSON.stringify(info, null, 2), { flag: 'wx' })
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} catch (err: any) {
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if (err.code === 'EEXIST') {
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continue // someone else claimed between our read and create — re-evaluate
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}
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throw err
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}
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this.installWriterLock(info)
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return info
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}
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// Attempts exhausted: something is claiming this directory faster than we
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// can evaluate it. Read whoever holds it now and fail loudly with their
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// details rather than degrading into a lockless open.
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const holder = await this.readWriterLock()
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if (holder) throw this.writerLockedError(holder)
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throw new Error(
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`Failed to acquire the writer lock for ${this.rootDir} after ${MAX_ATTEMPTS} attempts — ` +
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`the lock file is being contended. Retry, or inspect ${lockFile}.`
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)
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}
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/** Record lock ownership + start the unref'd heartbeat. */
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private installWriterLock(info: WriterLockInfo): void {
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this.writerLockInfo = info
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// Heartbeat — rewrite lastHeartbeat every WRITER_HEARTBEAT_MS so other
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@ -1835,8 +1888,24 @@ export class FileSystemStorage extends BaseStorage {
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// Don't keep the event loop alive just for the heartbeat.
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this.writerLockHeartbeat.unref()
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}
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}
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return info
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/** The consumer-facing BRAINY_WRITER_LOCKED error, holder details attached. */
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private writerLockedError(existing: WriterLockInfo): Error {
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const err = new Error(
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`Another writer holds this Brainy directory.\n` +
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` PID: ${existing.pid} on host ${existing.hostname}\n` +
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` Started: ${existing.startedAt}\n` +
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` Heartbeat: ${existing.lastHeartbeat}\n` +
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` Version: ${existing.version}\n` +
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` Directory: ${this.rootDir}\n\n` +
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`For diagnostic queries against this live store, use:\n` +
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` const reader = await Brainy.openReadOnly({ storage: { type: 'filesystem', path: '${this.rootDir}' } })\n\n` +
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`If you have verified the existing lock is stale (e.g. a crashed writer on a different host that PID liveness cannot reach), pass { force: true }.`
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) as Error & { code: string; lockInfo: WriterLockInfo }
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err.code = 'BRAINY_WRITER_LOCKED'
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err.lockInfo = existing
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return err
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}
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public override async releaseWriterLock(): Promise<void> {
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@ -110,6 +110,49 @@ describe('Multi-process safety + read-only mode', () => {
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// Don't track `blocked` for afterEach cleanup since init failed.
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})
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it('takes over a STALE foreign lock (dead PID + old heartbeat) and claims atomically', async () => {
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const { mkdirSync, writeFileSync, readFileSync } = await import('node:fs')
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const { join } = await import('node:path')
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const os = await import('node:os')
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mkdirSync(join(dir, 'locks'), { recursive: true })
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const tenMinutesAgo = new Date(Date.now() - 10 * 60 * 1000).toISOString()
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writeFileSync(join(dir, 'locks', '_writer.lock'), JSON.stringify({
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pid: 999999999, // no such process — provably dead
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hostname: os.hostname(),
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startedAt: tenMinutesAgo,
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lastHeartbeat: tenMinutesAgo,
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version: '8.0.0',
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rootDir: dir
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}))
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writer = new Brainy({ requireSubtype: false, storage: { type: 'filesystem', path: dir } })
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await writer.init() // stale takeover must succeed
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const lock = JSON.parse(readFileSync(join(dir, 'locks', '_writer.lock'), 'utf-8'))
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expect(lock.pid).toBe(process.pid) // the atomic wx claim installed OUR lock
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})
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it('the writer-locked error carries the machine-readable contract (code + lockInfo)', async () => {
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const { mkdirSync, writeFileSync } = await import('node:fs')
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const { join } = await import('node:path')
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const os = await import('node:os')
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mkdirSync(join(dir, 'locks'), { recursive: true })
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const otherPid = (process as any).ppid || 1
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writeFileSync(join(dir, 'locks', '_writer.lock'), JSON.stringify({
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pid: otherPid,
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hostname: os.hostname(),
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startedAt: new Date().toISOString(),
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lastHeartbeat: new Date().toISOString(),
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version: '8.7.0',
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rootDir: dir
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}))
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const blocked = new Brainy({ requireSubtype: false, storage: { type: 'filesystem', path: dir } })
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const err: any = await blocked.init().catch((e) => e)
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expect(err.code).toBe('BRAINY_WRITER_LOCKED')
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expect(err.lockInfo?.pid).toBe(otherPid)
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})
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it('allows a second in-process writer with a warning (same PID)', async () => {
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// Two Brainy instances in the same Node process: not the dangerous
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// cross-process case. Should succeed (with a console warning).
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