Code Review: remove RoundTripper

We do not want to make the interface only satisfy the RoundTripper interface
because that alone cannot make Websockets work. We need to expose an
interface that ensures currently Client requirements are met.

client/client.go

@@ -52,7 +52,7 @@ type Requester interface {
 
 	// Transport provides direct access to the transport instance for
 	// specialist operations.
-	Transport() http.RoundTripper
+	Transport() *http.Transport
 }
 
 // RequestOptions allows setting up a specific request.
@@ -198,16 +198,11 @@ func (client *Client) getTaskWebsocket(taskID, websocketID string) (clientWebsoc
 	return client.getWebsocket(url)
 }
 
-func getWebsocket(transport http.RoundTripper, url string) (clientWebsocket, error) {
-	httpTransport, ok := transport.(*http.Transport)
-	if !ok {
-		return nil, fmt.Errorf("cannot create websocket: transport not compatible")
-	}
-
+func getWebsocket(transport *http.Transport, url string) (clientWebsocket, error) {
 	dialer := websocket.Dialer{
-		NetDial:          httpTransport.Dial,
-		Proxy:            httpTransport.Proxy,
-		TLSClientConfig:  httpTransport.TLSClientConfig,
+		NetDial:          transport.Dial,
+		Proxy:            transport.Proxy,
+		TLSClientConfig:  transport.TLSClientConfig,
 		HandshakeTimeout: 5 * time.Second,
 	}
 	conn, _, err := dialer.Dial(url, nil)
@@ -216,16 +211,7 @@ func getWebsocket(transport http.RoundTripper, url string) (clientWebsocket, err
 
 // CloseIdleConnections closes any API connections that are currently unused.
 func (client *Client) CloseIdleConnections() {
-	transport := client.Requester().Transport()
-	// The following is taken from net/http/client.go because
-	// we are directly going to try and close idle connections and
-	// we must make sure the transport supports this.
-	type closeIdler interface {
-		CloseIdleConnections()
-	}
-	if tr, ok := transport.(closeIdler); ok {
-		tr.CloseIdleConnections()
-	}
+	client.Requester().Transport().CloseIdleConnections()
 }
 
 // Maintenance returns an error reflecting the daemon maintenance status or nil.
@@ -589,7 +575,7 @@ type DefaultRequester struct {
 	baseURL   url.URL
 	doer      doer
 	userAgent string
-	transport http.RoundTripper
+	transport *http.Transport
 	decoder   DecoderFunc
 }
 
@@ -625,6 +611,6 @@ func (br *DefaultRequester) SetDecoder(decoder DecoderFunc) {
 	br.decoder = decoder
 }
 
-func (br *DefaultRequester) Transport() http.RoundTripper {
+func (br *DefaultRequester) Transport() *http.Transport {
 	return br.transport
 }