REC-57: open browser automatically

When calling `engflow_auth login $cluster`, the browser will now automatically open to the correct url with the necessary query params appended.

README.md

@@ -30,7 +30,7 @@ This repository provides `engflow_auth`, a [Bazel credential helper](https://blo
 
 ## Use
 
-1. Run `engflow_auth login [CLUSTER URL]` to obtain a credential. This prints a URL to visit in your browser.
+1. Run `engflow_auth login [CLUSTER URL]` to obtain a credential. This opens a URL in your browser.
 1. Visit the URL to complete the process, logging in if necessary. `engflow_auth` will download and store a credential in on your system's encrypted keyring.
 
 This process needs to be repeated after the credential expires, typically every 90 days.

cmd/engflow_auth/main.go

@@ -47,7 +47,7 @@ const (
 )
 
 type appState struct {
-	// These vars are initialized by `build()` iff they are not pre-populated;
+	// These vars are initialized by `build()` if they are not pre-populated;
 	// they should be pre-populated in tests and left nil otherwise.
 	userConfigDir string
 	browserOpener browser.Opener

internal/browser/browser.go

@@ -18,6 +18,8 @@ import (
 	"fmt"
 	"net/url"
 	"os"
+	"os/exec"
+	"runtime"
 )
 
 type Opener interface {
@@ -26,11 +28,36 @@ type Opener interface {
 
 type StderrPrint struct{}
 
+func openURL(url string) error {
+	var err error
+	switch runtime.GOOS {
+	case "darwin": // macOS
+		err = exec.Command("open", url).Start()
+	case "linux":
+		providers := []string{"xdg-open", "x-www-browser", "www-browser"}
+
+		// There are multiple possible providers to open a browser on linux
+		// One of them is xdg-open, another is x-www-browser, then there's www-browser, etc.
+		// Look for one that exists and run it
+		for _, provider := range providers {
+			if _, err := exec.LookPath(provider); err == nil {
+				err = exec.Command(provider, url).Start()
+			}
+		}
+	case "windows":
+		err = exec.Command("rundll32", "url.dll,FileProtocolHandler", url).Start()
+	default:
+		err = fmt.Errorf("unsupported platform")
+	}
+	return err
+}
+
 func (p *StderrPrint) Open(u *url.URL) error {
 	fmt.Fprintf(
-		os.Stderr,
-		"Please open the following URL in your web browser to authenticate:\n\n\t%s\n\n",
+		os.Stdout,
+		"Opening the following URL in your web browser to authenticate:\n\n\t%s\n\n",
 		u,
 	)
+	openURL(u.String())
 	return nil
 }