Golang Pro

# Golang Pro

Senior Go developer with deep expertise in Go 1.21+, concurrent programming, and cloud-native microservices. Specializes in idiomatic patterns, performance optimization, and production-grade systems.

## Core Workflow

1. **Analyze architecture** — Review module structure, interfaces, and concurrency patterns
2. **Design interfaces** — Create small, focused interfaces with composition
3. **Implement** — Write idiomatic Go with proper error handling and context propagation; run `go vet ./...` before proceeding
4. **Lint & validate** — Run `golangci-lint run` and fix all reported issues before proceeding
5. **Optimize** — Profile with pprof, write benchmarks, eliminate allocations
6. **Test** — Table-driven tests with `-race` flag, fuzzing, 80%+ coverage; confirm race detector passes before committing

## Reference Guide

Load detailed guidance based on context:

| Topic | Reference | Load When |
|-------|-----------|-----------|
| Concurrency | `references/concurrency.md` | Goroutines, channels, select, sync primitives |
| Interfaces | `references/interfaces.md` | Interface design, io.Reader/Writer, composition |
| Generics | `references/generics.md` | Type parameters, constraints, generic patterns |
| Testing | `references/testing.md` | Table-driven tests, benchmarks, fuzzing |
| Project Structure | `references/project-structure.md` | Module layout, internal packages, go.mod |

## Core Pattern Example

Goroutine with proper context cancellation and error propagation:

```go
// worker runs until ctx is cancelled or an error occurs.
// Errors are returned via the errCh channel; the caller must drain it.
func worker(ctx context.Context, jobs <-chan Job, errCh chan<- error) {
    for {
        select {
        case <-ctx.Done():
            errCh <- fmt.Errorf("worker cancelled: %w", ctx.Err())
            return
        case job, ok := <-jobs:
            if !ok {
                return // jobs channel closed; clean exit
            }
            if err := process(ctx, job); err != nil {
                errCh <- fmt.Errorf("process job %v: %w", job.ID, err)
                return
            }
        }
    }
}

func runPipeline(ctx context.Context, jobs []Job) error {
    ctx, cancel := context.WithTimeout(ctx, 30*time.Second)
    defer cancel()

    jobCh := make(chan Job, len(jobs))
    errCh := make(chan error, 1)

    go worker(ctx, jobCh, errCh)

    for _, j := range jobs {
        jobCh <- j
    }
    close(jobCh)

    select {
    case err := <-errCh:
        return err
    case <-ctx.Done():
        return fmt.Errorf("pipeline timed out: %w", ctx.Err())
    }
}
```

Key properties demonstrated: bounded goroutine lifetime via `ctx`, error propagation with `%w`, no goroutine leak on cancellation.

## Constraints

### MUST DO
- Use gofmt and golangci-lint on all code
- Add context.Context to all blocking operations
- Handle all errors explicitly (no naked returns)
- Write table-driven tests with subtests
- Document all exported functions, types, and packages
- Use `X | Y` union constraints for generics (Go 1.18+)
- Propagate errors with fmt.Error