refactor(auth): improve authentication flow for oauth 2.0 with local webserver

cmd/gateclient/client.go

@@ -388,10 +388,14 @@ func authenticateOAuth2(output func(string), httpClient *http.Client, endpoint s
 			return false, errors.New("incorrect OAuth2 auth configuration")
 		}
 
+		redirectURL := url.URL{
+			Scheme: "http",
+			Host:   localWebServer,
+		}
 		config := &oauth2.Config{
 			ClientID:     OAuth2.ClientId,
 			ClientSecret: OAuth2.ClientSecret,
-			RedirectURL:  "http://localhost:8085",
+			RedirectURL:  redirectURL.String(),
 			Scopes:       OAuth2.Scopes,
 			Endpoint: oauth2.Endpoint{
 				AuthURL:  OAuth2.AuthUrl,
@@ -409,14 +413,6 @@ func authenticateOAuth2(output func(string), httpClient *http.Client, endpoint s
 				return false, errors.Wrapf(err, "Could not refresh token from source: %v", tokenSource)
 			}
 		} else {
-			// Do roundtrip.
-			http.Handle("/", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-				code := r.FormValue("code")
-				fmt.Fprintln(w, code)
-			}))
-			go http.ListenAndServe(":8085", nil)
-			// Note: leaving server connection open for scope of request, will be reaped on exit.
-
 			verifier, verifierCode, err := generateCodeVerifier()
 			if err != nil {
 				return false, err
@@ -427,10 +423,8 @@ func authenticateOAuth2(output func(string), httpClient *http.Client, endpoint s
 			challengeMethod := oauth2.SetAuthURLParam("code_challenge_method", "S256")
 
 			authURL := config.AuthCodeURL("state-token", oauth2.AccessTypeOffline, oauth2.ApprovalForce, challengeMethod, codeChallenge)
-			output(fmt.Sprintf("Navigate to %s and authenticate", authURL))
-			code := prompt(output, "Paste authorization code:")
-
-			newToken, err = config.Exchange(context.Background(), code, codeVerifier)
+			output("Trying to get token from web")
+			newToken, err = getTokenFromWeb(output, config, authURL, codeVerifier)
 			if err != nil {
 				return false, err
 			}

cmd/gateclient/oauth.go

@@ -0,0 +1,85 @@
+package gateclient
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"net"
+	"net/http"
+	"os/exec"
+	"runtime"
+
+	"golang.org/x/oauth2"
+)
+
+const localWebServer = "localhost:8085"
+
+func startWebServer() (codeCh chan string, err error) {
+	listener, err := net.Listen("tcp", localWebServer)
+	if err != nil {
+		return nil, err
+	}
+	codeCh = make(chan string)
+
+	go http.Serve(listener, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// TODO: add handle of `error`
+		//error := r.FormValue("error")
+		code := r.FormValue("code")
+		codeCh <- code // send code to OAuth flow
+		listener.Close()
+		w.Header().Set("Content-Type", "text/plain")
+		fmt.Fprintf(w, "Received code: %v\r\nYou can now safely close this browser window.", code)
+	}))
+
+	return codeCh, nil
+}
+
+// getTokenFromWeb uses Config to request a Token.
+// It returns the retrieved Token.
+func getTokenFromWeb(output func(string), config *oauth2.Config, authURL string, opts ...oauth2.AuthCodeOption) (*oauth2.Token, error) {
+	codeCh, err := startWebServer()
+	if err != nil {
+		output("Unable to start a web server.")
+		return nil, err
+	}
+
+	err = openURL(authURL)
+	if err != nil {
+		log.Fatalf("Unable to open authorization URL in web server: %v", err)
+	} else {
+		output("Your browser has been opened to an authorization URL.\n" +
+			" This program will resume once authorization has been provided.\n")
+		output(authURL)
+	}
+
+	// Wait for the web server to get the code.
+	code := <-codeCh
+	return exchangeToken(config, code, opts...)
+}
+
+// Exchange the authorization code for an access token
+func exchangeToken(config *oauth2.Config, code string, opts ...oauth2.AuthCodeOption) (*oauth2.Token, error) {
+	tok, err := config.Exchange(context.Background(), code, opts...)
+	if err != nil {
+		return nil, fmt.Errorf("Unable to retrieve token %v", err)
+	}
+	return tok, nil
+}
+
+// openURL opens a browser window to the specified location.
+// This code originally appeared at:
+//   http://stackoverflow.com/questions/10377243/how-can-i-launch-a-process-that-is-not-a-file-in-go
+func openURL(url string) error {
+	var err error
+	switch runtime.GOOS {
+	case "linux":
+		err = exec.Command("xdg-open", url).Start()
+	case "windows":
+		err = exec.Command("rundll32", "url.dll,FileProtocolHandler", url).Start()
+	case "darwin":
+		err = exec.Command("open", url).Start()
+	default:
+		err = fmt.Errorf("Cannot open URL %s on this platform", url)
+	}
+	return err
+}