go-concurrency
Use when writing, reviewing, or debugging concurrent Go code. Covers goroutine lifecycle management, channels, errgroup, mutexes, atomics, sync.Map, and synchronous-first design. Based on Google and Uber style guides.
What this skill does
# 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.
```go
// 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.
```go
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.
```go
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.
```go
// 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.
```go
// 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.
```go
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.
```go
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`.
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