/golang-concurrency

name: golang-concurrency
description: Write safe concurrent Go code with goroutines, channels, and context. Use when implementing concurrency with goroutines, channels, or context in Go.
metadata:
  triggers:
    keywords:
    - goroutine
    - go keyword
    - channel
    - mutex
    - waitgroup
    - context
    - errgroup
    - race condition

Golang Concurrency

**Priority: P0 (CRITICAL)**

Principles

• **Share Memory by Communicating**: Use channels or synchronization primitives instead of unprotected shared memory.
• **Context Propagation**: Always pass `ctx` to manage cancellation, deadlines, and graceful termination.
• **Throttle Database Load**: Reserve goroutines for independent external API calls or CPU work. **Avoid unconstrained goroutines hitting databases** to prevent connection pool exhaustion.
• **Prevent Leaks**: Never start a goroutine without a deterministic shutdown mechanism.
• **Race Detection**: Always verify concurrent code with `go test -race`.

Implementation Workflow

1. **Choose primitive** — `errgroup.Group` for parallel tasks with error propagation, channels for pipelines, `sync.Mutex` for simple shared state. 2. **Bound Concurrency with Semaphore** — Cap maximum active goroutines using a buffered channel semaphore:

   sem := make(chan struct{}, maxWorkers)
   for _, task := range tasks {
       task := task // prevent loop pointer capture
       sem <- struct{}{}
       g.Go(func() error {
           defer func() { <-sem }()
           return task.Execute(ctx)
       })
   }

3. **Respect Context Cancellation** — Goroutines performing I/O must honor `ctx.Done()`. 4. **Avoid Loop Pointer Capture** — Capture iteration variables explicitly before launching goroutines. 5. **Test with race detector** — Run `go test -race` in local dev and CI.

See [ErrGroup and concurrency patterns](references/concurrency-patterns.md) and [context timeout examples](references/context-usage.md)

Anti-Patterns

• **No DB connection spam**: avoid spawning unbounded goroutines to query the database in parallel.
• **No goroutine leaks**: ensure every goroutine has an exit path via `ctx.Done()` or channel closure.
• **No loop variable pointer trap**: never pass the address of a loop variable (`&item`) into a concurrent closure.
• **No unbuffered goroutine spawn storms**: always cap concurrency with worker pools or semaphores.
• **No bare goroutines**: use `errgroup` or `sync.WaitGroup` for lifecycle tracking.

References

• [Concurrency Patterns](references/concurrency-patterns.md)
• [Context Usage](references/context-usage.md)