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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). license: Apache-2.0 compatibility: Requires go.uber.org/atomic for atomic operation wrappers metadata: sources: "Effective Go, Google Style Guide, Uber Style Guide"

Go Concurrency

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
var wg sync.WaitGroup
for item := range queue {
    wg.Add(1)
    go func() { defer wg.Done(); 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.

Read references/GOROUTINE-PATTERNS.md when implementing stop/done channel patterns, goroutine waiting strategies, or lifecycle-managed workers.

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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.

Read references/GOROUTINE-PATTERNS.md when writing synchronous-first APIs that callers may wrap in goroutines.

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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.

Read references/SYNC-PRIMITIVES.md when implementing mutex-protected structs or deciding how to structure mutex fields.

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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

Read references/SYNC-PRIMITIVES.md when reviewing detailed channel direction examples with error-prone patterns.

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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!

Read references/SYNC-PRIMITIVES.md when choosing between sync/atomic and go.uber.org/atomic, or implementing atomic state flags in structs.

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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.

Read references/BUFFER-POOLING.md when implementing a worker pool with reusable buffers or choosing between channel-based pools and sync.Pool.

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Advanced Patterns

Read references/ADVANCED-PATTERNS.md when implementing request-response multiplexing with channels of channels, or CPU-bound parallel computation across cores.

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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