func httpDo(ctx context.Context, req *http.Request, tlsConfig *tls.Config, f func(*http.Response, error) error) error {
	tr := &http.Transport{DisableKeepAlives: true, TLSClientConfig: tlsConfig}
	client := &http.Client{Transport: tr}
	c := make(chan error, 1)
	go func() { c <- f(client.Do(req)) }()
	select {
	case <-ctx.Done():
		tr.CancelRequest(req)
		<-c
		return ctx.Err()
	case err := <-c:
		return err
	}
}

func (c *httpClusterClient) AutoSync(ctx context.Context, interval time.Duration) error {
	ticker := time.NewTicker(interval)
	defer ticker.Stop()
	for {
		err := c.Sync(ctx)
		if err != nil {
			return err
		}
		select {
		case <-ctx.Done():
			return ctx.Err()
		case <-ticker.C:
		}
	}
}

func httpDo(ctx context.Context, req *http.Request, tlsConfig *tls.Config, f func(*http.Response, error) error) error {
	// Run the HTTP request in a goroutine and pass the response to f.
	tr := &http.Transport{DisableKeepAlives: true, TLSClientConfig: tlsConfig}
	client := &http.Client{Transport: tr}
	c := make(chan error, 1)
	go func() { c <- f(client.Do(req)) }()
	select {
	case <-ctx.Done():
		tr.CancelRequest(req)
		<-c // Wait for f to return.
		return ctx.Err()
	case err := <-c:
		return err
	}
}

func Query(ctx context.Context, db *sql.DB, query string, args ...interface{}) (*sql.Rows, error) {
	ch := make(chan struct {
		rows *sql.Rows
		err  error
	})
	go func() {
		rows, err := db.Query(query, args...)
		ch <- struct {
			rows *sql.Rows
			err  error
		}{rows, err}
	}()

	select {
	case <-ctx.Done():
		return nil, ctx.Err()
	case out := <-ch:
		return out.rows, out.err
	}
}

func Exec(ctx context.Context, db *sql.DB, query string, args ...interface{}) (sql.Result, error) {
	ch := make(chan struct {
		res sql.Result
		err error
	})
	go func() {
		res, err := db.Exec(query, args...)
		ch <- struct {
			res sql.Result
			err error
		}{res, err}
	}()

	select {
	case <-ctx.Done():
		return nil, ctx.Err()
	case out := <-ch:
		return out.res, out.err
	}
}

// Do sends an HTTP request with the provided http.Client and returns an HTTP response.
// If the client is nil, http.DefaultClient is used.
// If the context is canceled or times out, ctx.Err() will be returned.
func Do(ctx context.Context, client *http.Client, req *http.Request) (*http.Response, error) {
	if client == nil {
		client = http.DefaultClient
	}

	// Request cancelation changed in Go 1.5, see cancelreq.go and cancelreq_go14.go.
	cancel := canceler(client, req)

	type responseAndError struct {
		resp *http.Response
		err  error
	}
	result := make(chan responseAndError, 1)

	go func() {
		resp, err := client.Do(req)
		result <- responseAndError{resp, err}
	}()

	var resp *http.Response

	select {
	case <-ctx.Done():
		cancel()
		return nil, ctx.Err()
	case r := <-result:
		var err error
		resp, err = r.resp, r.err
		if err != nil {
			return resp, err
		}
	}

	c := make(chan struct{})
	go func() {
		select {
		case <-ctx.Done():
			cancel()
		case <-c:
			// The response's Body is closed.
		}
	}()
	resp.Body = &notifyingReader{resp.Body, c}

	return resp, nil
}

func (c *simpleHTTPClient) Do(ctx context.Context, act httpAction) (*http.Response, []byte, error) {
	req := act.HTTPRequest(c.endpoint)

	if err := printcURL(req); err != nil {
		return nil, nil, err
	}

	var hctx context.Context
	var hcancel context.CancelFunc
	if c.headerTimeout > 0 {
		hctx, hcancel = context.WithTimeout(ctx, c.headerTimeout)
	} else {
		hctx, hcancel = context.WithCancel(ctx)
	}
	defer hcancel()

	reqcancel := requestCanceler(c.transport, req)

	rtchan := make(chan roundTripResponse, 1)
	go func() {
		resp, err := c.transport.RoundTrip(req)
		rtchan <- roundTripResponse{resp: resp, err: err}
		close(rtchan)
	}()

	var resp *http.Response
	var err error

	select {
	case rtresp := <-rtchan:
		resp, err = rtresp.resp, rtresp.err
	case <-hctx.Done():
		// cancel and wait for request to actually exit before continuing
		reqcancel()
		rtresp := <-rtchan
		resp = rtresp.resp
		switch {
		case ctx.Err() != nil:
			err = ctx.Err()
		case hctx.Err() != nil:
			err = fmt.Errorf("client: endpoint %s exceeded header timeout", c.endpoint.String())
		default:
			panic("failed to get error from context")
		}
	}

	// always check for resp nil-ness to deal with possible
	// race conditions between channels above
	defer func() {
		if resp != nil {
			resp.Body.Close()
		}
	}()

	if err != nil {
		return nil, nil, err
	}

	var body []byte
	done := make(chan struct{})
	go func() {
		body, err = ioutil.ReadAll(resp.Body)
		done <- struct{}{}
	}()

	select {
	case <-ctx.Done():
		resp.Body.Close()
		<-done
		return nil, nil, ctx.Err()
	case <-done:
	}

	return resp, body, err
}