powerlock

This repository contains alternative lock implementations to help with debugging and monitoring Go applications.

• CancelRWMutex - Cancellable locks that can reject all current and future waiters
• MaxRWMutex - Locks that limit the number of waiting goroutines
• ContextRWMutex - Locks that return when a context is cancelled or times out
• ObservedRWMutex - Structured acquisition, release, contention, and rejection events
• WatchdogRWMutex - Reports slow lock acquisitions and long-held locks, including where they originated
• KeyedMutex - Independent exclusion by comparable key with bounded key counts

Overview

A Go library providing enhanced read-write mutex implementations with advanced features for monitoring, debugging, and resource management.

See Choosing a lock for a side-by-side comparison of failure behavior, observation, and zero-value support.

Features

This library offers specialized read-write and exclusive mutex types for different concurrent programming challenges:

CancelRWMutex - Cancellable Locking

A read-write mutex that can be cancelled to reject all current and future lock attempts.

Key capabilities:

• Cancellation support - Cancel() method rejects all waiters and future attempts
• Lock() and RLock() panic when cancelled; try-methods return false
• Non-blocking try-lock operations for both read and write locks
• Compatible with standard sync.RWMutex interface

Usage:

import "github.com/donomii/powerlock"

mutex := powerlock.NewCancelRWMutex("database-access")

// Normal locking operations
mutex.Lock()
defer mutex.Unlock()
// Critical section

// Non-blocking operations
if mutex.TryLock() {
    defer mutex.Unlock()
    // Critical section
}

// Cancel the mutex to reject all current and future waiters
mutex.Cancel()
// After cancellation:
// - Lock() and RLock() will panic
// - TryLock() and TryRLock() will return false

MaxRWMutex - Limited Waiter Queue

A read-write mutex that limits the number of goroutines that can wait for the lock. Excess goroutines will be rejected immediately.

Key capabilities:

• Configurable maximum number of waiting goroutines
• Fails fast when wait limit is exceeded
• Prevents system overload from unbounded lock queues

Usage:

mutex := powerlock.NewMaxRWMutexWithLimit("api-handler", 64)

// Will fail immediately if too many goroutines are already waiting
if mutex.TryLock() {
    defer mutex.Unlock()
    // Critical section
} else {
    // Handle the lock being unavailable without joining the queue.
}

NewMaxRWMutex(location) uses a default limit of 1.

ObservedRWMutex and WatchdogRWMutex - Diagnostics and Prometheus

Observed locks emit structured state transitions. Watchdog locks add configurable slow-wait and long-hold reports with acquisition stacks. The optional powerlock/prometheus adapter converts those events into bounded metrics without adding Prometheus imports to the core package.

Key capabilities:

• Read/write waiter and holder gauges
• Acquisition result and contention counters
• Wait and exact-hold duration histograms
• Wait and hold threshold counters
• Cached metric handles for stable lock names
• Runtime trace annotations with an optional application-owned flight-recorder callback

Usage:

import (
    powerlockprometheus "github.com/donomii/powerlock/prometheus"
    "github.com/prometheus/client_golang/prometheus"
)

reg := prometheus.NewRegistry()
metrics := powerlockprometheus.MustNew(reg)
mutex := metrics.NewWatchdogRWMutex("cache-update")

mutex.Lock()
defer mutex.Unlock()
// Structured events and Prometheus metrics are updated automatically.

ContextRWMutex - Context-Aware Locking

A read-write mutex whose blocking lock operations return if the supplied context is cancelled or reaches its deadline.

Key capabilities:

• LockContext(ctx) and RLockContext(ctx) return typed acquisition errors that preserve ctx.Err() through errors.Is
• Lock() and RLock() remain available for standard blocking use
• Non-blocking try-lock operations for both read and write locks

Usage:

mutex := powerlock.NewContextRWMutex("request-state")

ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
defer cancel()

if err := mutex.LockContext(ctx); err != nil {
    // Handle timeout or cancellation
    return
}
defer mutex.Unlock()

Installation

go get github.com/donomii/powerlock@latest

Requirements

• Go 1.23 or later
• Prometheus client library only when importing the optional powerlock/prometheus adapter

Use Cases

• CancelRWMutex: Graceful shutdown scenarios, resource cleanup, or any situation where you need to cancel pending operations
• MaxRWMutex: High-throughput services where you want to prevent lock queue buildup
• ContextRWMutex: Request-scoped work where waiting for a lock must respect cancellation and deadlines
• ObservedRWMutex: Structured diagnostics, metrics adapters, and exact acquisition guards
• WatchdogRWMutex: Slow-wait and long-hold incidents with acquisition callers
• KeyedMutex: Per-customer, per-resource, or per-job exclusion without a permanent lock map

Common Interface

All read-write mutex types provide these methods:

• Lock() / Unlock() - Exclusive write access
• RLock() / RUnlock() - Shared read access
• TryLock() / TryRLock() - Non-blocking lock attempts
• SetLocation(string) - Set the stable diagnostic name before first use

Each type extends this base interface with its specialized capabilities:

• CancelRWMutex adds: Cancel() method for rejecting waiters
• MaxRWMutex adds: waiter limiting behavior
• ContextRWMutex adds: LockContext(context.Context) and RLockContext(context.Context)
• ObservedRWMutex adds: structured events and exact acquisition guards
• WatchdogRWMutex adds: wait and exact-hold threshold reports

Exclusive-only forms omit the read methods. KeyedMutex accepts a key on acquisition and release.

Examples

go run ./examples/cancel
go run ./examples/max
go run ./examples/metrics
go run ./examples/context
go run ./examples/keyed
go run ./examples/watchdog

Benchmarks

go test -bench=.

License and notices

See LICENSE for Powerlock's license and THIRD_PARTY_NOTICES.md for retained upstream notices.