g, ctx := errgroup.WithContext(ctx)

g.Go(func() error {
    return fetchUsers(ctx)
})

g.Go(func() error {
    return fetchOrders(ctx)
})

if err := g.Wait(); err != nil {
    return fmt.Errorf("fetch data: %w", err)
}

func (w *Worker) Run(ctx context.Context) error {
    for {
        select {
        case <-ctx.Done():
            return ctx.Err()
        case job := <-w.jobs:
            if err := w.process(job); err != nil {
                w.logger.Error("process job", zap.Error(err))
            }
        }
    }
}

// ✅ Good — caller controls lifecycle
go worker.Run(ctx)

// ❌ Bad — function secretly starts goroutine
func NewWorker() *Worker {
    w := &Worker{}
    go w.run() // hidden goroutine — caller has no control
    return w
}

// Unbuffered — synchronization point
ch := make(chan Result)

// Buffered with size 1 — single-item handoff
ch := make(chan Result, 1)

// Larger buffers need explicit justification with documented reasoning
ch := make(chan Result, 100) // requires comment explaining why

done := make(chan struct{})
close(done) // broadcast signal to all receivers

func produce(ctx context.Context) <-chan Item {
    ch := make(chan Item)
    go func() {
        defer close(ch)
        for {
            item, err := fetchNext(ctx)
            if err != nil {
                return
            }
            select {
            case ch <- item:
            case <-ctx.Done():
                return
            }
        }
    }()
    return ch
}

// ✅ Good — zero value works
type Cache struct {
    mu    sync.RWMutex
    items map[string]Item
}

// ❌ Bad — unnecessary pointer
type Cache struct {
    mu    *sync.RWMutex // never do this
}

type SafeMap struct {
    mu sync.RWMutex // mutex guards the fields below
    items map[string]string
    count int
}

// ✅ Good — minimal lock scope
func (c *Cache) Get(key string) (Item, bool) {
    c.mu.RLock()
    item, ok := c.items[key]
    c.mu.RUnlock()
    return item, ok
}

// ✅ Also good — defer for methods that return early
func (c *Cache) GetOrCreate(key string) Item {
    c.mu.Lock()
    defer c.mu.Unlock()

    if item, ok := c.items[key]; ok {
        return item
    }
    item := newItem(key)
    c.items[key] = item
    return item
}

// ❌ BLOCKER — copying a mutex copies its lock state
cache2 := *cache1 // this copies the mutex!

// ✅ Good — type-safe atomics
import "go.uber.org/atomic"

type Server struct {
    running atomic.Bool
    reqCount atomic.Int64
}

func (s *Server) HandleRequest() {
    s.reqCount.Inc()
    // ...
}

func (s *Service) Process(ctx context.Context, req Request) error {
    // Derive context with timeout for external call
    fetchCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
    defer cancel() // ALWAYS defer cancel

    data, err := s.client.Fetch(fetchCtx, req.ID)
    if err != nil {
        return fmt.Errorf("fetch %s: %w", req.ID, err)
    }
    // ...
}

// ✅ Good
select {
case result := <-ch:
    return result, nil
case <-ctx.Done():
    return nil, ctx.Err()
}

// ❌ Bad — blocks forever if context cancelled
result := <-ch

// ❌ Bad — mutable global, not safe for concurrent access
var db *sql.DB

// ✅ Good — pass as dependency
type Server struct {
    db *sql.DB
}

type Client struct {
    initOnce sync.Once
    conn     *grpc.ClientConn
}

func (c *Client) getConn() *grpc.ClientConn {
    c.initOnce.Do(func() {
        c.conn = dial()
    })
    return c.conn
}

go test -race ./...