// Bad: Sending on closed channel causes panic
ch := make(chan int)
ch <- 42
close(ch)
ch <- 13 // panic

// Good: Goroutine lifetimes are clear
func (w *Worker) Run(ctx context.Context) error {
    var wg sync.WaitGroup
    for item := range w.q {
        wg.Add(1)
        go func() {
            defer wg.Done()
            process(ctx, item) // Returns when context is cancelled
        }()
    }
    wg.Wait() // Prevent spawned goroutines from outliving this function
}

// Bad: Careless about when goroutines finish
func (w *Worker) Run() {
    for item := range w.q {
        go process(item) // When does this finish? What if it never does?
    }
}

// Bad: No way to stop or wait for this goroutine
go func() {
    for {
        flush()
        time.Sleep(delay)
    }
}()

// Good: Stop/done channel pattern for controlled shutdown
var (
    stop = make(chan struct{}) // tells the goroutine to stop
    done = make(chan struct{}) // tells us that the goroutine exited
)
go func() {
    defer close(done)
    ticker := time.NewTicker(delay)
    defer ticker.Stop()
    for {
        select {
        case <-ticker.C:
            flush()
        case <-stop:
            return
        }
    }
}()

// To shut down:
close(stop)  // signal the goroutine to stop
<-done       // and wait for it to exit

var wg sync.WaitGroup
for i := 0; i < N; i++ {
    wg.Add(1)
    go func() {
        defer wg.Done()
        // work...
    }()
}
wg.Wait()

done := make(chan struct{})
go func() {
    defer close(done)
    // work...
}()
<-done // wait for goroutine to finish

// Bad: Spawns uncontrollable background goroutine
func init() {
    go doWork()
}

// Good: Explicit lifecycle management
type Worker struct {
    stop chan struct{}
    done chan struct{}
}

func NewWorker() *Worker {
    w := &Worker{
        stop: make(chan struct{}),
        done: make(chan struct{}),
    }
    go w.doWork()
    return w
}

func (w *Worker) Shutdown() {
    close(w.stop)
    <-w.done
}

// Bad: Unnecessary pointer
mu := new(sync.Mutex)
mu.Lock()

// Good: Zero-value is valid
var mu sync.Mutex
mu.Lock()

// Bad: Embedded mutex exposes Lock/Unlock as part of API
type SMap struct {
    sync.Mutex // Lock() and Unlock() become methods of SMap
    data map[string]string
}

func (m *SMap) Get(k string) string {
    m.Lock()
    defer m.Unlock()
    return m.data[k]
}

// Good: Named field keeps mutex as implementation detail
type SMap struct {
    mu   sync.Mutex
    data map[string]string
}

func (m *SMap) Get(k string) string {
    m.mu.Lock()
    defer m.mu.Unlock()
    return m.data[k]
}

// Good: Synchronous function - caller controls concurrency
func ProcessItems(items []Item) ([]Result, error) {
    var results []Result
    for _, item := range items {
        result, err := processItem(item)
        if err != nil {
            return nil, err
        }
        results = append(results, result)
    }
    return results, nil
}

// Caller can add concurrency if needed:
go func() {
    results, err := ProcessItems(items)
    // handle results
}()

// Good: Direction specified - clear ownership
func sum(values <-chan int) int {
    total := 0
    for v := range values {
        total += v
    }
    return total
}

// Bad: No direction - allows accidental misuse
func sum(values chan int) (out int) {
    for v := range values {
        out += v
    }
    close(values) // Bug! This compiles but shouldn't happen.
}

// Bad: Arbitrary buffer size
c := make(chan int, 64) // "Ought to be enough for anybody!"

// Good: Deliberate sizing
c := make(chan int, 1) // Size of one
c := make(chan int)    // Unbuffered, size of zero

func produce(out chan<- int) { /* send-only */ }
func consume(in <-chan int)  { /* receive-only */ }
func transform(in <-chan int, out chan<- int) { /* both directions */ }

// Bad: Easy to forget atomic operation
type foo struct {
    running int32 // atomic
}

func (f *foo) start() {
    if atomic.SwapInt32(&f.running, 1) == 1 {
        return // already running
    }
    // start the Foo
}

func (f *foo) isRunning() bool {
    return f.running == 1 // race! forgot atomic.LoadInt32
}

// Good: Type-safe atomic operations
type foo struct {
    running atomic.Bool
}

func (f *foo) start() {
    if f.running.Swap(true) {
        return // already running
    }
    // start the Foo
}

func (f *foo) isRunning() bool {
    return f.running.Load() // can't accidentally read non-atomically
}