Detach a Context分离一个 Context

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// Detach returns a context that keeps all the values of its parent context
// but detaches from the cancellation and error handling.
func Detach(ctx context.Context) context.Context { return detachedContext{ctx} }

type detachedContext struct{ parent context.Context }

func (v detachedContext) Deadline() (time.Time, bool)       { return time.Time{}, false }
func (v detachedContext) Done() <-chan struct{}             { return nil }
func (v detachedContext) Err() error                        { return nil }
func (v detachedContext) Value(key interface{}) interface{} { return v.parent.Value(key) }

func TestDetach(t *testing.T) {
	type ctxKey string
	var key = ctxKey("key")

	ctx, cancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
	defer cancel()
	ctx = context.WithValue(ctx, key, "value")
	dctx := Detach(ctx)
	// Detached context has the same values.
	got, ok := dctx.Value(key).(string)
	if !ok || got != "value" {
		t.Errorf("Value: got (%v, %t), want 'value', true", got, ok)
	}
	// Detached context doesn't time out.
	time.Sleep(500 * time.Millisecond)
	if err := ctx.Err(); err != context.DeadlineExceeded {
		t.Fatalf("original context Err: got %v, want DeadlineExceeded", err)
	}
	if err := dctx.Err(); err != nil {
		t.Errorf("detached context Err: got %v, want nil", err)
	}
}