[auth] save Github username to local file (#429)

## Summary

During the `devbox cloud shell` command execution, we now save the
user's
Github username in `~/.config/devbox/ssh/github_username`. This should
improve the UX since they won't need to keep re-entering this
information.

We wait for the `gateway auth` to successfully return a VM before saving
the info
in the `github_username` file. Because the gateway verifies that the
user's github username
is valid.

In case the user wishes to override the cached github username, they can
use the new `--username` or `-u` flag in `devbox cloud shell`.

## How was it tested?


```
> devbox cloud shell
....

> cat ~/.config/devbox/ssh/github_username
savil
```

I then edited `github_username` to have an incorrect value like
`savil2`.
And cleared the socker in `~/.config/devbox/ssh/sockets` to ensure the
CLI client calls the gateway again.
This time `devbox cloud shell` gave an error about using a bad username
`savil2`.

I then did: `devbox cloud shell -u savil` to fix the situation.

Author: savil

internal/boxcli/cloud.go

@@ -15,6 +15,8 @@ import (
 
 type cloudShellCmdFlags struct {
 	config configFlags
+
+	githubUsername string
 }
 
 func CloudCmd() *cobra.Command {
@@ -43,6 +45,9 @@ func cloudShellCmd() *cobra.Command {
 	}
 
 	flags.config.register(command)
+	command.Flags().StringVarP(
+		&flags.githubUsername, "username", "u", "", "Github username to use for ssh",
+	)
 	return command
 }
 
@@ -74,5 +79,5 @@ func runCloudShellCmd(cmd *cobra.Command, flags *cloudShellCmdFlags) error {
 	if err != nil {
 		return errors.WithStack(err)
 	}
-	return cloud.Shell(box.ProjectDir(), cmd.ErrOrStderr())
+	return cloud.Shell(cmd.ErrOrStderr(), box.ProjectDir(), flags.githubUsername)
 }

internal/cloud/cloud.go

@@ -27,25 +27,20 @@ import (
 	"go.jetpack.io/devbox/internal/debug"
 )
 
-func Shell(projectDir string, w io.Writer) error {
+func Shell(w io.Writer, projectDir string, githubUsername string) error {
 	c := color.New(color.FgMagenta).Add(color.Bold)
 	c.Fprintln(w, "Devbox Cloud")
 	fmt.Fprintln(w, "Remote development environments powered by Nix")
 	fmt.Fprint(w, "\n")
 
 	username, vmHostname := parseVMEnvVar()
+	// The flag for githubUsername overrides any env-var, since flags are a more
+	// explicit action compared to an env-var which could be latently present.
+	if githubUsername != "" {
+		username = githubUsername
+	}
 	if username == "" {
-		stepGithubUsername := stepper.Start("Detecting your Github username...")
-		var err error
-		username, err = queryGithubUsername()
-		if err == nil && username != "" {
-			stepGithubUsername.Success("Username: %s", username)
-		} else {
-			stepGithubUsername.Fail("Unable to resolve username")
-			// The query for Github username is best effort, and if it fails to resolve
-			// we fallback to prompting the user, and suggesting the local computer username.
-			username = promptUsername()
-		}
+		username = getGithubUsername()
 	}
 	debug.Log("username: %s", username)
 
@@ -80,6 +75,14 @@ func Shell(projectDir string, w io.Writer) error {
 			var region string
 			vmHostname, region = getVirtualMachine(sshClient)
 			stepVM.Success("Created a virtual machine in %s", fly.RegionName(region))
+
+			// We save the username to local file only after we get a successful response
+			// from the gateway, because the gateway will verify that the user's SSH keys
+			// match their claimed username from github.
+			err = openssh.SaveGithubUsernameToLocalFile(username)
+			if err != nil {
+				debug.Log("Failed to save username: %v", err)
+			}
 		}
 	}
 	debug.Log("vm_hostname: %s", vmHostname)
@@ -109,17 +112,39 @@ func PortForward(local, remote string) error {
 	return exec.Command("ssh", "-N", vmHostname, "-L", portMap).Run()
 }
 
+func getGithubUsername() string {
+
+	username, err := openssh.GithubUsernameFromLocalFile()
+	if err != nil || username == "" {
+		if err != nil {
+			debug.Log("failed to get auth.Username. Error: %v", err)
+		}
+
+		username, err = queryGithubUsername()
+		if err == nil && username != "" {
+			debug.Log("Username from ssh -T [email protected]: %s", username)
+		} else {
+			// The query for Github username is best effort, and if it fails to resolve
+			// we fallback to prompting the user, and suggesting the local computer username.
+			username = promptUsername()
+		}
+	} else {
+		debug.Log("Username from locally-cached file: %s", username)
+	}
+	return username
+}
+
 func promptUsername() string {
 	username := ""
 	prompt := &survey.Input{
 		Message: "What is your github username?",
 		Default: os.Getenv("USER"),
 	}
-	debug.Log("Failed to get username from Github. Falling back to suggesting $USER: %s", prompt.Default)
 	err := survey.AskOne(prompt, &username, survey.WithValidator(survey.Required))
 	if err != nil {
 		log.Fatal(err)
 	}
+	debug.Log("Username from prompting user: %s", username)
 	return username
 }
 

internal/cloud/openssh/config.go

@@ -224,6 +224,7 @@ func EnsureDirExists(path string, perm fs.FileMode, chmod bool) error {
 	return nil
 }
 
+// returns path to ~/.config/devbox/ssh
 func devboxSSHDir() (string, error) {
 	home, err := os.UserHomeDir()
 	if err != nil {

internal/cloud/openssh/username.go

@@ -0,0 +1,45 @@
+package openssh
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/pkg/errors"
+	"go.jetpack.io/devbox/internal/fileutil"
+)
+
+func GithubUsernameFromLocalFile() (string, error) {
+	filePath, err := usernameFilePath()
+	if err != nil {
+		return "", err
+	}
+	if !fileutil.Exists(filePath) {
+		return "", nil
+	}
+
+	username, err := os.ReadFile(filePath)
+	if err != nil {
+		return "", errors.WithStack(err)
+	}
+	return string(username), nil
+}
+
+func SaveGithubUsernameToLocalFile(username string) error {
+
+	filePath, err := usernameFilePath()
+	if err != nil {
+		return errors.WithStack(err)
+	}
+
+	err = os.WriteFile(filePath, []byte(username), 0600)
+	return errors.WithStack(err)
+}
+
+func usernameFilePath() (string, error) {
+	sshDir, err := devboxSSHDir()
+	if err != nil {
+		return "", errors.WithStack(err)
+	}
+
+	return filepath.Join(sshDir, "github_username"), nil
+}