`context.WithCancel(parent)` creates a child context that can be canceled manually by calling the returned `cancel()` function.

Signature: `ctx, cancel := context.WithCancel(parent)`

• Stop background goroutines when you’re done.
• Cancel a group of tasks if one fails.
• Implement graceful shutdown logic.

ctx, cancel := context.WithCancel(context.Background())
go func() {
    <-ctx.Done()
    // cleanup or stop work
}()
cancel()

• Always call `cancel()` to release resources.
• Cancellation propagates: canceling parent cancels children.
• Use `select { case ←ctx.Done(): … }` inside goroutines.

• context.Context
• WithTimeout
• WithDeadline

• manual /ˈmænjuəl/: thủ công
• cleanup /ˈkliːnʌp/: dọn dẹp tài nguyên
• propagate /ˈprɑːpəɡeɪt/: truyền tiếp
• graceful shutdown /ˈɡreɪsfəl ˈʃʌtdaʊn/: tắt êm