Imported from saisudhir14/claude-skills (skills/go-concurrency/SKILL.md). Install upstream with npx skills add saisudhir14/claude-skills --skill go-concurrency. Copyright stays with the author (MIT).

Go Concurrency

Patterns for safe, efficient concurrent Go code.

Channel Size

Channels should have size zero (unbuffered) or one. Any other size requires justification about what prevents filling under load.

// Wrong: arbitrary buffer
c := make(chan int, 64)

// Correct
c := make(chan int)    // unbuffered: synchronous handoff
c := make(chan int, 1) // buffered: allows one pending send

Goroutine Lifetimes

Document when and how goroutines exit. Goroutines blocked on channels will not be garbage collected even if the channel is unreachable.

func (w *Worker) Run(ctx context.Context) error {
    for {
        select {
        case <-ctx.Done():
            return ctx.Err()
        case job := <-w.jobs:
            w.process(job)
        }
    }
}

Use errgroup for Concurrent Operations

Prefer errgroup.Group over manual sync.WaitGroup for error-returning goroutines.

import "golang.org/x/sync/errgroup"

func processItems(ctx context.Context, items []Item) error {
    g, ctx := errgroup.WithContext(ctx)

    for _, item := range items {
        g.Go(func() error {
            return process(ctx, item)
        })
    }

    return g.Wait() // returns first error, cancels others via ctx
}

// With concurrency limit
func processItemsLimited(ctx context.Context, items []Item) error {
    g, ctx := errgroup.WithContext(ctx)
    g.SetLimit(10) // max 10 concurrent goroutines

    for _, item := range items {
        g.Go(func() error {
            return process(ctx, item)
        })
    }

    return g.Wait()
}

Prefer Synchronous Functions

Synchronous functions are easier to reason about and test. Let callers add concurrency when needed.

// Wrong: forces concurrency on caller
func Fetch(url string) <-chan Result

// Correct: caller can wrap in goroutine if needed
func Fetch(url string) (Result, error)

Zero Value Mutexes

The zero value of sync.Mutex is valid. Do not use pointers to mutexes or embed them in exported structs.

// Wrong
mu := new(sync.Mutex)

// Wrong: exposes Lock/Unlock in API
type SMap struct {
    sync.Mutex
    data map[string]string
}

// Correct
type SMap struct {
    mu   sync.Mutex
    data map[string]string
}

Atomic Operations (Go 1.19+)

Use the standard library's typed atomics.

import "sync/atomic"

type Counter struct {
    value atomic.Int64
}

func (c *Counter) Inc() { c.value.Add(1) }
func (c *Counter) Value() int64 { return c.value.Load() }

// Also available: atomic.Bool, atomic.Pointer[T], atomic.Uint32, etc.

sync.Map Performance (Go 1.24+)

The sync.Map implementation was significantly improved in Go 1.24. Modifications of disjoint sets of keys are much less likely to contend on larger maps, and there is no longer any ramp-up time required to achieve low-contention loads.

Context Cancellation

Always select on ctx.Done() in long-running goroutines to allow clean cancellation.

func longOperation(ctx context.Context) error {
    resultCh := make(chan result, 1)

    go func() {
        // Note: if ctx is canceled, this goroutine still runs to completion
        // and sends to the buffered channel (then gets GC'd).
        // Pass ctx into the work function if it supports cancellation.
        resultCh <- expensiveWork()
    }()

    select {
    case <-ctx.Done():
        return ctx.Err()
    case r := <-resultCh:
        return r.err
    }
}

For graceful server shutdown patterns, see references/patterns.md.