Go concurrency

Go Concurrency

Goroutines are cheap, but every one you spawn is a resource you must own. The goal is structured concurrency: each goroutine has a clear owner, a predictable exit, and a way for the caller to wait and collect errors.

Core Rules

1. Never start a goroutine without knowing how it will stop. A blocked goroutine is not garbage-collected — it leaks.
2. The caller must be able to wait. Use sync.WaitGroup, errgroup.Group, or an explicit done channel.
3. No goroutines in init(). Expose Start/Stop/Shutdown so callers control the lifecycle.
4. Share by communicating. Default to channels; reach for sync.Mutex only when the problem is genuinely "protect a shared field".
5. Only the sender closes a channel. Closing from the receiver side panics on the next send.
6. Specify channel direction (chan<-, <-chan) at function boundaries — the compiler catches misuse.
7. Always include ctx.Done() in select. Without it, the goroutine cannot be cancelled.
8. Test for leaks with go.uber.org/goleak.

Primitive Decision

Need	Use	Why
Pass a value from producer to consumer	Channel	Transfers ownership explicitly
Wait for N fire-and-forget goroutines	sync.WaitGroup (Go 1.25: wg.Go)	No error needed
Wait + collect first error + cancel siblings	errgroup.WithContext	Structured failure
Bound concurrency (worker pool)	errgroup.SetLimit(n)	Replaces hand-rolled pools
Protect a shared field	sync.Mutex / sync.RWMutex	Short critical section
Counter / flag	typed sync/atomic (atomic.Int64, atomic.Bool)	Lock-free, type-safe
Read-heavy concurrent map	sync.Map	Concurrent map read/write otherwise crashes
One-shot init	sync.Once (or OnceFunc/OnceValue in 1.21+)	Idempotent setup
Deduplicate concurrent calls	x/sync/singleflight	Cache stampede prevention

Read references/sync-primitives.md when picking between mutex, atomic, sync.Map, sync.Pool, or singleflight, or when designing the field layout of a struct that protects shared state.

Goroutine Lifetimes

// Good: bounded WaitGroup, deterministic exit
var wg sync.WaitGroup
for item := range queue {
    wg.Add(1)
    go func(it Item) { defer wg.Done(); process(ctx, it) }(item)
}
wg.Wait()

// Bad: no stop signal, no wait — classic leak
go func() { for { flush(); time.Sleep(delay) } }()

Go 1.25+ exposes wg.Go(fn) which folds Add/Done into one call. Always call wg.Add before go — otherwise wg.Wait may return before the goroutine even starts.

errgroup: Errors and Cancellation

errgroup.WithContext is the right default when sibling goroutines should cancel each other on the first failure:

g, ctx := errgroup.WithContext(ctx)
g.SetLimit(8)
for _, url := range urls {
    g.Go(func() error { return fetch(ctx, url) })
}
if err := g.Wait(); err != nil {
    return fmt.Errorf("fetching urls: %w", err)
}

g.Wait returns the first non-nil error; ctx is cancelled as soon as any worker fails. See references/errgroup-and-pools.md.

Channels

func produce(out chan<- int)                  { /* send-only */ }
func consume(in <-chan int)                   { /* receive-only */ }
func transform(in <-chan int, out chan<- int) { /* ownership crosses */ }

Buffer size is 0 or 1. Anything larger must be justified (what bounds it under load, what happens when writers block).

In every long-running select, include <-ctx.Done(). Avoid time.After in hot loops — it allocates a timer per iteration; hoist a time.NewTimer and Reset it instead.

Read references/channels-and-select.md when implementing pipelines, fan-in/fan-out, broadcast via close, or non-blocking sends with default.

Mutexes and Atomics

The zero value of sync.Mutex/RWMutex is valid — almost never use a pointer. Do not embed mutexes; keep them as an unexported mu field so Lock/Unlock aren't public API. Keep critical sections short; never hold a lock across I/O. Prefer typed atomics (atomic.Bool, atomic.Int64) over raw sync/atomic on int32/int64 fields.

Testing: goleak and synctest

Wire go.uber.org/goleak into every package that spawns goroutines (goleak.VerifyTestMain(m) or defer goleak.VerifyNone(t)). For timer-dependent tests, use testing/synctest (Go 1.25+) so synthetic time advances deterministically. Go 1.26 adds an experimental goroutineleak pprof profile for production diagnosis — it is not a substitute for goleak in tests. See references/leaks-and-synctest.md.

Anti-Patterns

Anti-pattern	Why it hurts	Do this instead
Fire-and-forget go func() with no signal	Leaks on shutdown; can outlive its inputs	Pass ctx, use errgroup, or own a done channel
Closing a channel from the receiver	Panics on the next send	Only the sender closes
time.After in a hot loop	Allocates a timer per iteration	time.NewTimer + Reset
select without ctx.Done()	Cannot be cancelled	Always include the cancel case
wg.Add(1) inside the goroutine	Wait may return before Add runs	Add before go, or use wg.Go (Go 1.25+)
Buffered channel sized "to be safe"	Hides backpressure, masks bugs	Size 0 or 1; justify anything larger
Concurrent read+write on map	Hard runtime crash, not a race warning	sync.Map or sync.RWMutex + map
Mutex held across I/O / RPC	Serializes the whole service	Copy what you need under the lock; release before the call
Sending a pointer through a channel	Re-introduces shared memory	Send a copy or an immutable value
Forgetting -race in CI	Races ship to prod	go test -race ./... always

Verification Checklist

Before finishing a concurrency change:

• Every go has a documented exit (ctx, done channel, or bounded loop)
• Every long-running select has a <-ctx.Done() case
• wg.Add is called before go, or wg.Go is used (Go 1.25+)
• Channels are sized 0 or 1, or the size has a comment justifying it
• Only the sender closes channels; receivers use for v := range ch or v, ok := <-ch
• No mutex is held across network/disk I/O
• go test -race ./... is clean
• Packages that spawn goroutines wire goleak.VerifyTestMain or per-test VerifyNone

References

• references/sync-primitives.md — mutex vs atomic vs sync.Map/Pool/Once/singleflight
• references/channels-and-select.md — channel ownership, direction, pipelines, non-blocking sends
• references/errgroup-and-pools.md — errgroup, SetLimit, worker pools, fan-out/fan-in
• references/leaks-and-synctest.md — goleak, testing/synctest, Go 1.26 experimental leak profile