Bun 1.2 Worker Thread SharedArrayBuffer Atomics Mutex Timeout Guardrail

--- description: Bun 1.2 Worker Thread SharedArrayBuffer Atomics Mutex Timeout Guardrail - Enforces deterministic bounded timeout wrappers around all Atomics.wait calls within multi-threaded Bun 1.2 worker pools. Unbounded Atomics.wait invocations in worker threads cause unrecoverable thread deadlocks, resource exhaustion, and persistent event loop starvation when locking worker threads fail to release shared memory locks. globs: src/**/*.{ts,js,mjs}, workers/**/*.{ts,js,mjs}, lib/**/*.{ts,js,mjs} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/bun-worker-atomics-mutex-timeout-guardrail" --- # Bun 1.2 Worker Thread SharedArrayBuffer Atomics Mutex Timeout Guardrail > **Severity**: CRITICAL | **Category**: backend > **Anti-Pattern Warning**: Unbounded Atomics.wait Invocations Causing Permanent Worker Thread Deadlocks and Zombie Pool Exhaustion ## System Constraints You are a Principal Backend Systems Architect enforcing strict concurrency invariants in Bun 1.2 worker pools. Mandate bounded wait timeouts, error-state unwinding, and poison-pill detection on all SharedArrayBuffer Atomics operations to prevent worker thread deadlocks and pool starvation. ## Strict Guidelines - Engineers MUST ALWAYS pass an explicit, bounded non-negative timeout parameter (in milliseconds) to Atomics.wait when synchronizing on SharedArrayBuffer instances. - Engineers MUST NOT invoke Atomics.wait without a timeout argument or pass Infinity as the wait duration. - Workers MUST handle the 'timed-out' return value from Atomics.wait by unwinding thread state, releasing acquired partial locks, and throwing or handling a typed LockTimeoutError. - Workers MUST wrap critical sections between mutex acquisition and release in try...finally blocks to guarantee Atomics.store and Atomics.notify execution even during unhandled exceptions. - Engineers MUST verify that Atomics.wait is only executed on dedicated worker threads (Bun Worker or node:worker_threads), as invoking Atomics.wait on the main thread throws a TypeError in conformant JavaScript runtimes. ## Bad vs Good Implementations ### Unbounded Mutex Acquisition vs. Bounded Reentrant Mutex with Timeout Fallback #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Atomics.wait(this.state, 0, 1) omits the timeout parameter. If the thread holding the lock encounters an unhandled exception or termination, waiting threads deadlock forever, causing silent zombie worker threads and pool starvation in Bun. ```typescript export class UnsafeWorkerMutex { private state: Int32Array; constructor(sharedBuffer: SharedArrayBuffer, offset: number = 0) { this.state = new Int32Array(sharedBuffer, offset, 1); } lock(): void { // ANTI-PATTERN: Indefinite wait with no timeout parameter. // If another worker thread crashes or exits unexpectedly while holding the lock, // this thread blocks permanently, starving the Bun worker pool. while (Atomics.compareExchange(this.state, 0, 0, 1) !== 0) { Atomics.wait(this.state, 0, 1); } } unlock(): void { Atomics.store(this.state, 0, 0); Atomics.notify(this.state, 0, 1); } } ``` #### Production Standard (Correct) > **Standard**: The bounded mutex recalculates remaining time against an absolute deadline and passes it to Atomics.wait. A 'timed-out' return raises an explicit LockTimeoutError, preventing worker deadlocks. The runLocked helper ensures deterministic lock release via try...finally. ```typescript export class LockTimeoutError extends Error { constructor(timeoutMs: number) { super(`Failed to acquire lock within ${timeoutMs}ms: potential worker deadlock.`); this.name = 'LockTimeoutError'; } } export class BoundedWorkerMutex { private static readonly UNLOCKED = 0; private static readonly LOCKED = 1; private readonly state: Int32Array; constructor(sharedBuffer: SharedArrayBuffer, byteOffset: number = 0) { this.state = new Int32Array(sharedBuffer, byteOffset, 1); } acquire(timeoutMs: number = 5000): void { const deadline = performance.now() + timeoutMs; while (Atomics.compareExchange(this.state, 0, BoundedWorkerMutex.UNLOCKED, BoundedWorkerMutex.LOCKED) !== BoundedWorkerMutex.UNLOCKED) { const remainingMs = deadline - performance.now(); if (remainingMs <= 0) { throw new LockTimeoutError(timeoutMs); } const result = Atomics.wait(this.state, 0, BoundedWorkerMutex.LOCKED, remainingMs); if (result === 'timed-out') { throw new LockTimeoutError(timeoutMs); } } } release(): void { const previous = Atomics.swap(this.state, 0, BoundedWorkerMutex.UNLOCKED); if (previous === BoundedWorkerMutex.LOCKED) { Atomics.notify(this.state, 0, 1); } } runLocked<T>(fn: () => T, timeoutMs: number = 5000): T { this.acquire(timeoutMs); try { return fn(); } finally { this.release(); } } } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Bun 1.2 Worker Thread SharedArrayBuffer Atomics Mutex Timeout Guardrail

Bun 1.2 Worker Thread SharedArrayBuffer Atomics Mutex Timeout Guardrail defines an industrial-grade behavioral standard for backend engineering. Enforces deterministic bounded timeout wrappers around all Atomics.wait calls within multi-threaded Bun 1.2 worker pools. Unbounded Atomics.wait invocations in worker threads cause unrecoverable thread deadlocks, resource exhaustion, and persistent event loop starvation when locking worker threads fail to release shared memory locks. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Unbounded Atomics.wait Invocations Causing Permanent Worker Thread Deadlocks and Zombie Pool Exhaustion Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Unbounded Atomics.wait Invocations Causing Permanent Worker Thread Deadlocks and Zombie Pool Exhaustion	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

Engineers MUST ALWAYS pass an explicit, bounded non-negative timeout parameter (in milliseconds) to Atomics.wait when synchronizing on SharedArrayBuffer instances.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.