go-concurrency

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name: go-concurrency description: Use when writing concurrent Go code — goroutines, channels, mutexes, or thread-safety guarantees. Also use when parallelizing work, fixing data races, or protecting shared state, even if the user doesn't explicitly mention concurrency primitives. Does not cover context.Context patterns (see go-context).

Go Concurrency

Compatibility: Atomic examples may use standard-library typed atomics where available or go.uber.org/atomic where a project already depends on it.

Resource Routing

• references/GOROUTINE-PATTERNS.md - Read when starting, stopping, or waiting for goroutines.
• references/SYNC-PRIMITIVES.md - Read when choosing between mutexes, atomics, channels, and once-like primitives.
• references/BUFFER-POOLING.md - Read when considering channel-backed or sync.Pool-style reuse.
• references/ADVANCED-PATTERNS.md - Read for worker pools, pipelines, errgroup, and cancellation-heavy patterns.

Goroutine Lifetimes

Normative: When you spawn goroutines, make it clear when or whether they exit.

Goroutines can leak by blocking on channel sends/receives. The GC will not terminate a blocked goroutine even if no other goroutine holds a reference to the channel. Even non-leaking in-flight goroutines cause panics (send on closed channel), data races, memory issues, and resource leaks.

Core Rules

1. Every goroutine needs a stop mechanism — a predictable end time, a cancellation signal, or both
2. Code must be able to wait for the goroutine to finish
3. No goroutines in init() — expose lifecycle methods (Close, Stop, Shutdown) instead
4. Keep synchronization scoped — constrain to function scope, factor logic into synchronous functions

// Good: Clear lifetime with WaitGroup.Go (Go 1.25+)
var wg sync.WaitGroup
for item := range queue {
    item := item
    wg.Go(func() { process(ctx, item) })
}
wg.Wait()

// Bad: No way to stop or wait
go func() { for { flush(); time.Sleep(delay) } }()

Test for leaks with go.uber.org/goleak.

Principle: Never start a goroutine without knowing how it will stop.

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Share by Communicating

"Do not communicate by sharing memory; instead, share memory by communicating."

This is Go's foundational concurrency design principle. Use channels for ownership transfer and orchestration — when one goroutine produces a value and another consumes it. Use mutexes when multiple goroutines access shared state and channels would add unnecessary complexity.

Default to channels. Fall back to sync.Mutex / sync.RWMutex when the problem is naturally about protecting a shared data structure (e.g., a cache or counter) rather than passing data between goroutines.

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Synchronous Functions

Normative: Prefer synchronous functions over asynchronous ones.

Benefit	Why
Localized goroutines	Lifetimes easier to reason about
Avoids leaks and races	Easier to prevent resource leaks and data races
Easier to test	Check input/output without polling
Caller flexibility	Caller adds concurrency when needed

Advisory: It is quite difficult (sometimes impossible) to remove unnecessary concurrency at the caller side. Let the caller add concurrency when needed.

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Zero-value Mutexes

The zero-value of sync.Mutex and sync.RWMutex is valid — almost never need a pointer to a mutex.

// Good: Zero-value is valid    // Bad: Unnecessary pointer
var mu sync.Mutex                mu := new(sync.Mutex)

Don't embed mutexes — use a named mu field to keep Lock/Unlock as implementation details, not exported API.

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Channel Direction

Normative: Specify channel direction where possible.

Direction prevents errors (compiler catches closing a receive-only channel), conveys ownership, and is self-documenting.

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

Channel Size: One or None

Channels should have size zero (unbuffered) or one. Any other size requires justification for:

• How the size was determined
• What prevents the channel from filling under load
• What happens when writers block

c := make(chan int)    // unbuffered — Good
c := make(chan int, 1) // size one — Good
c := make(chan int, 64) // arbitrary — needs justification

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Atomic Operations

Use atomic.Bool, atomic.Int64, etc. (stdlib sync/atomic since Go 1.19, or go.uber.org/atomic) for type-safe atomic operations. Raw int32/int64 fields make it easy to forget atomic access on some code paths.

// Good: Type-safe              // Bad: Easy to forget
var running atomic.Bool          var running int32 // atomic
running.Store(true)              atomic.StoreInt32(&running, 1)
running.Load()                   running == 1 // race!

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Documenting Concurrency

Advisory: Document thread-safety when it's not obvious from the operation type.

Go users assume read-only operations are safe for concurrent use, and mutating operations are not. Document concurrency when:

1. Read vs mutating is unclear — e.g., a Lookup that mutates LRU state
2. API provides synchronization — e.g., thread-safe clients
3. Interface has concurrency requirements — document in type definition

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Context Usage

For context.Context guidance (parameter placement, struct storage, custom types, derivation patterns), see the dedicated go-context skill.

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Buffer Pooling with Channels

Use a buffered channel as a free list to reuse allocated buffers. This "leaky buffer" pattern uses select with default for non-blocking operations.

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Related Skills

• Context propagation: See go-context when passing cancellation, deadlines, or request-scoped values through goroutines
• Error handling: See go-error-handling when propagating errors from goroutines or using errgroup
• Defensive hardening: See go-defensive when protecting shared state at API boundaries or using defer for cleanup
• Interface design: See go-interfaces when choosing receiver types for types with sync primitives

External Resources

• Never start a goroutine without knowing how it will stop — Dave Cheney
• Rethinking Classical Concurrency Patterns — Bryan Mills (GopherCon 2018)
• When Go programs end — Go Time podcast
• go.uber.org/goleak — Goroutine leak detector for testing
• go.uber.org/atomic — Type-safe atomic operations