go-concurrency

SKILL.md

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

Every goroutine needs a stop mechanism — a predictable end time, a
cancellation signal, or both

Code must be able to wait for the goroutine to finish

No goroutines in init() — expose lifecycle methods (Close, Stop,
Shutdown) instead

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.

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.

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.

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.

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.

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.

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:

Read vs mutating is unclear — e.g., a Lookup that mutates LRU state

API provides synchronization — e.g., thread-safe clients

Interface has concurrency requirements — document in type definition

Context Usage

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

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.

Advanced Patterns

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

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