var todo … var background … var Canceled … var DeadlineExceeded … // WithCancel returns a copy of parent with a new Done channel. The returned // context's Done channel is closed when the returned cancel function is called // or when the parent context's Done channel is closed, whichever happens first. // // Canceling this context releases resources associated with it, so code should // call cancel as soon as the operations running in this Context complete. func WithCancel(parent Context) (ctx Context, cancel CancelFunc) { … } // WithDeadline returns a copy of the parent context with the deadline adjusted // to be no later than d. If the parent's deadline is already earlier than d, // WithDeadline(parent, d) is semantically equivalent to parent. The returned // context's Done channel is closed when the deadline expires, when the returned // cancel function is called, or when the parent context's Done channel is // closed, whichever happens first. // // Canceling this context releases resources associated with it, so code should // call cancel as soon as the operations running in this Context complete. func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) { … } // WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)). // // Canceling this context releases resources associated with it, so code should // call cancel as soon as the operations running in this Context complete: // // func slowOperationWithTimeout(ctx context.Context) (Result, error) { // ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond) // defer cancel() // releases resources if slowOperation completes before timeout elapses // return slowOperation(ctx) // } func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) { … } // WithValue returns a copy of parent in which the value associated with key is // val. // // Use context Values only for request-scoped data that transits processes and // APIs, not for passing optional parameters to functions. func WithValue(parent Context, key interface{ … }