tracker: add -changepw to change a user passphrase

Mirrors -adduser: prompts for a new passphrase (min 8 chars) or reads
TRACKER_PASSWORD, hashes with bcrypt, and updates the stored hash.
Returns an error if the user does not exist.

README.md

@@ -23,6 +23,8 @@ go build -o tracker .
 ```
 
 Non-interactive variant: `TRACKER_PASSWORD=... ./tracker -adduser kyle`.
+Change an existing user's passphrase the same way:
+`./tracker -changepw kyle` (or `TRACKER_PASSWORD=... ./tracker -changepw kyle`).
 
 Defaults: listens on `:8420` (all interfaces, so it's reachable over Tailscale),
 database at `~/.local/share/tracker/tracker.db`.

internal/store/auth.go

@@ -46,6 +46,14 @@ func (s *Store) UserByName(username string) (User, error) {
 	return u, err
 }
 
+func (s *Store) SetPassword(username, passwordHash string) error {
+	res, err := s.db.Exec(`UPDATE users SET password_hash = ? WHERE username = ?`, passwordHash, username)
+	if err != nil {
+		return err
+	}
+	return checkAffected(res)
+}
+
 // --- api keys ---
 
 func (s *Store) CreateAPIKey(name, keyHash string) (int64, error) {

main.go

@@ -22,6 +22,7 @@ func main() {
 	dbPath := flag.String("db", defaultDB, "sqlite database path")
 	addr := flag.String("addr", ":8420", "listen address")
 	addUser := flag.String("adduser", "", "create a login user, then exit")
+	changepw := flag.String("changepw", "", "change a login user's passphrase, then exit")
 	flag.Parse()
 
 	if err := os.MkdirAll(filepath.Dir(*dbPath), 0o755); err != nil {
@@ -35,6 +36,13 @@ func main() {
 		return
 	}
 
+	if *changepw != "" {
+		if err := runChangePassword(*dbPath, *changepw); err != nil {
+			log.Fatal(err)
+		}
+		return
+	}
+
 	st, err := store.Open(*dbPath)
 	if err != nil {
 		log.Fatal(err)
@@ -106,3 +114,61 @@ func runAddUser(dbPath, username string) error {
 	fmt.Printf("user %q created. log in at /login\n", username)
 	return nil
 }
+
+func runChangePassword(dbPath, username string) error {
+	username = strings.TrimSpace(username)
+	if username == "" {
+		return errors.New("username cannot be empty")
+	}
+	if strings.ContainsAny(username, " \t") {
+		return errors.New("username cannot contain whitespace")
+	}
+
+	st, err := store.Open(dbPath)
+	if err != nil {
+		return err
+	}
+	defer st.Close()
+
+	if _, err := st.UserByName(username); err != nil {
+		if errors.Is(err, store.ErrNotFound) {
+			return fmt.Errorf("user %q not found", username)
+		}
+		return err
+	}
+
+	password := os.Getenv("TRACKER_PASSWORD")
+	if password == "" {
+		fmt.Print("new passphrase: ")
+		pw, err := term.ReadPassword(int(os.Stdin.Fd()))
+		fmt.Println()
+		if err != nil {
+			return err
+		}
+		fmt.Print("confirm: ")
+		confirmPw, err := term.ReadPassword(int(os.Stdin.Fd()))
+		fmt.Println()
+		if err != nil {
+			return err
+		}
+		password = string(pw)
+		if string(confirmPw) != password {
+			return errors.New("passphrases do not match")
+		}
+	}
+
+	if len(password) < 8 {
+		return errors.New("passphrase must be at least 8 characters")
+	}
+
+	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		return err
+	}
+
+	if err := st.SetPassword(username, string(hash)); err != nil {
+		return err
+	}
+	fmt.Printf("passphrase for %q updated\n", username)
+	return nil
+}