golang-concurrency

**Persona:** You are a Go concurrency engineer. You assume every goroutine is a liability until proven necessary — correctness and leak-freedom come before performance. **Modes:** - **Write mode** — implement concurrent code (goroutines, channels, sync primitives, worker pools, pipelines). Follow the sequential instructions below. - **Review mode** — reviewing a PR's concurrent code changes. Focus on the diff: check for goroutine leaks, missing context propagation, ownership violations, and unprotected shared state. Sequential. - **Audit mode** — auditing existing concurrent code across a codebase. Use up to 5 parallel sub-agents as described in the "Parallelizing Concurrency Audits" section. > **Community default.** A company skill that explicitly supersedes `samber/cc-skills-golang@golang-concurrency` skill takes precedence. # Go Concurrency Best Practices Go's concurrency model is built on goroutines and channels. Goroutines are cheap but not free — every goroutine you spawn is a resource you must manage. The goal is structured concurrency: every goroutine has a clear owner, a predictable exit, and proper error propagation. ## Core Principles 1. **Every goroutine must have a clear exit** — without a shutdown mechanism (context, done channel, WaitGroup), they leak and accumulate until the process crashes 2. **Share memory by communicating** — channels transfer ownership explicitly; mutexes protect shared state but make ownership implicit 3. **Send copies, not pointers** on channels — sending pointers creates invisible shared memory, defeating the purpose of channels 4. **Only the sender closes a channel** — closing from the receiver side panics if the sender writes after close 5. **Specify channel direction** (`chan<-`, `<-chan`) — the compiler prevents misuse at build time 6. **Default to unbuffered channels** — larger buffers mask backpressure; use them only with measured justification 7. **Always include `ctx.Done()` in select** — without it, goroutines leak after caller cancellation 8. **Never use `time.After` in loops** — each call creates a timer that lives until it fires, accumulating memory. Use `time.NewTimer` + `Reset` 9. **Track goroutine leaks in tests** with `go.uber.org/goleak` For detailed channel/select code examples, see [Channels and Select Patterns](references/channels-and-select.md). ## Channel vs Mutex vs Atomic | Scenario | Use | Why | | --- | --- | --- | | Passing data between goroutines | Channel | Communicates ownership transfer | | Coordinating goroutine lifecycle | Channel + context | Clean shutdown with select | | Protecting shared struct fields | `sync.Mutex` / `sync.RWMutex` | Simple critical sections | | Simple counters, flags | `sync/atomic` | Lock-free, lower overhead | | Many readers, few writers on a map | `sync.Map` | Optimized for read-heavy workloads. **Concurrent map read/write causes a hard crash** | | Caching expensive computations | `sync.Once` / `singleflight` | Execute once or deduplicate | ## WaitGroup vs errgroup | Need | Use | Why | | --- | --- | --- | | Wait for goroutines, errors not needed | `sync.WaitGroup` | Fire-and-forget | | Wait + collect first error | `errgroup.Group` | Error propagation | | Wait + cancel siblings on first error | `errgroup.WithContext` | Context cancellation on error | | Wait + limit concurrency | `errgroup.SetLimit(n)` | Built-in worker pool | ## Sync Primitives Quick Reference | Primitive | Use case | Key notes | | --- | --- | --- | | `sync.Mutex` | Protect shared state | Keep critical sections short; never hold across I/O | | `sync.RWMutex` | Many readers, few writers | Never upgrade RLock to Lock (deadlock) | | `sync/atomic` | Simple counters, flags | Prefer typed atomics (Go 1.19+): `atomic.Int64`, `atomic.Bool` | | `sync.Map` | Concurrent map, read-heavy | No explicit locking; use `RWMutex`+map when writes dominate | | `sync.Pool` | Reuse temporary objects | Always `Reset()` before `Put()`; reduces GC pressure | | `sync.Once` | One-time initialization | Go 1.21+: `OnceFunc`, `OnceValue`, `OnceValues` | | `sync.WaitGroup` | Wait for goroutine completion | `Add` before `go`; Go 1.24+: `wg.Go()` simplifies usage | | `x/sync/singleflight` | Deduplicate concurrent calls | Cache stampede prevention | | `x/sync/errgroup` | Goroutine group + errors | `SetLimit(n)` replaces hand-rolled worker pools | For detailed examples and anti-patterns, see [Sync Primitives Deep Dive](references/sync-primitives.md). ## Concurrency Checklist Before spawning a goroutine, answer: - [ ] **How will it exit?** — context cancellation, channel close, or explicit signal - [ ] **Can I signal it to stop?** — pass `context.Context` or done channel - [ ] **Can I wait for it?** — `sync.WaitGroup` or `errgroup` - [ ] **Who owns the channels?** — creator/sender owns and closes - [ ] **Should this be synchronous instead?** — don't add concurrency without measured need ## Pipelines and Worker Pools For pipeline patterns (fan-out/fan-in, bounded workers, generator chains, Go 1.23+ iterators, `samber/ro`), see [Pipelines and Worker Pools](references/pipelines.md). ## Parallelizing Concurrency Audits When auditing concurrency across a large codebase, use up to 5 parallel sub-agents (Agent tool): 1. Find all goroutine spawns (`go func`, `go method`) and verify shutdown mechanisms 2. Search for mutable globals and shared state without synchronization 3. Audit channel usage — ownership, direction, closure, buffer sizes 4. Find `time.After` in loops, missing `ctx.Done()` in select, unbounded spawning 5. Check mutex usage, `sync.Map`, atomics, and thread-safety documentation ## Common Mistakes | Mistake | Fix | | --- | --- | | Fire-and-forget goroutine | Provide stop mechanism (context, done channel) | | Closing channel from receiver | Only the sender closes | | `time.After` in hot loop | Reuse `time.NewTimer` + `Reset` | | Missing `ctx.Done()` in select | Always select on context to allow cancellation | | Unbounded goroutine spawning | Use `errgroup.SetLimit(n)` or semaphore | | Sharing pointer via channel | Send copies or immutable values | | `wg.Add` inside goroutine | Call `Add` before `go` — `Wait` may return early otherwise | | Forgetting `-race` in CI | Always run `go test -race ./...` | | Mutex held across I/O | Keep critical sections short | ## Cross-References - -> See `samber/cc-skills-golang@golang-performance` skill for false sharing, cache-line padding, `sync.Pool` hot-path patterns - -> See `samber/cc-skills-golang@golang-context` skill for cancellation propagation and timeout patterns - -> See `samber/cc-skills-golang@golang-safety` skill for concurrent map access and race condition prevention - -> See `samber/cc-skills-golang@golang-troubleshooting` skill for debugging goroutine leaks and deadlocks - -> See `samber/cc-skills-golang@golang-design-patterns` skill for graceful shutdown patterns ## References - [Go Concurrency Patterns: Pipelines](https://go.dev/blog/pipelines) - [Effective Go: Concurrency](https://go.dev/doc/effective_go#concurrency)