go-nil-interface-service-wiring

Answer

Two independent fixes: (1) wire the real MFAService in main.go (repo + service + config all existed — only the wiring was missing), and (2) make LoginService fail loudly instead of dereferencing a nil interface.

The bug (before):

// main.go — buggy wiring
func main() {
    cfg := config.Load()
    db := sqlite.Open(cfg.DSN)
    if err := migrate.Up(db); err != nil { log.Fatal(err) }

    userRepo := user.NewRepo(db)
    // mfaRepo, mfa.NewService, cfg.MFA all existed — but were never wired:
    loginSvc := auth.NewLoginService(userRepo, nil) // ← nil mfaService

    http.HandleFunc("/login", loginSvc.HandleLogin)
    ...
}

// auth/login.go — unconditional call
func (s *LoginService) Login(ctx context.Context, in LoginInput) (LoginResult, error) {
    u, err := s.users.FindByEmail(ctx, in.Email)
    if err != nil { return LoginResult{}, ErrInvalidCredentials }

    if s.mfaService.IsEnabled(ctx, u.ID) { // ← SIGSEGV when mfaService is nil
        ...
    }
}

The fix (after):

// main.go — fixed wiring: repo + service + config all already existed
func main() {
    cfg := config.Load()
    db := sqlite.Open(cfg.DSN)
    if err := migrate.Up(db); err != nil { log.Fatal(err) }

    userRepo := user.NewRepo(db)
    mfaRepo := mfa.NewRepo(db)
    mfaSvc := mfa.NewService(mfaRepo, cfg.MFA) // ← real service, fully constructed

    loginSvc, err := auth.NewLoginService(userRepo, mfaSvc)
    if err != nil { log.Fatalf("wire login: %v", err) } // fail fast at boot, not at 2am

    http.HandleFunc("/register", registerSvc.HandleRegister)
    http.HandleFunc("/login", loginSvc.HandleLogin)
    ...
}

// auth/login.go — constructor guarantees a callable service; Login nil-guards anyway
var ErrMFAUnwired = errors.New("mfa service not wired")

// isNilInterface catches BOTH the nil interface and the classic Go gotcha:
// a typed nil pointer stored in an interface is != nil but still panics on call.
func isNilInterface(v any) bool {
    if v == nil { return true }
    rv := reflect.ValueOf(v)
    switch rv.Kind() {
    case reflect.Ptr, reflect.Map, reflect.Slice, reflect.Chan, reflect.Func, reflect.Interface:
        return rv.IsNil()
    }
    return false
}

func NewLoginService(users userStore, mfa MFAVerifier) (*LoginService, error) {
    if isNilInterface(mfa) {
        return nil, fmt.Errorf("%w: main.go must wire mfa.NewService(mfaRepo, cfg.MFA)", ErrMFAUnwired)
    }
    return &LoginService{users: users, mfaService: mfa}, nil
}

func (s *LoginService) Login(ctx context.Context, in LoginInput) (LoginResult, error) {
    u, err := s.users.FindByEmail(ctx, in.Email)
    if err != nil { return LoginResult{}, ErrInvalidCredentials }

    // Defensive nil-guard: a mis-wired service returns a 500 with a reason,
    // never SIGSEGV. (Fail closed: if MFA can't be evaluated, don't skip it.)
    if isNilInterface(s.mfaService) {
        return LoginResult{}, fmt.Errorf("login: %w", ErrMFAUnwired)
    }

    if s.mfaService.IsEnabled(ctx, u.ID) {
        ok, err := s.mfaService.Verify(ctx, u.ID, in.MFACode)
        if err != nil || !ok { return LoginResult{}, ErrMFARequired }
    }
    return issueSession(u)
}

Why the bug survived: the register endpoint 500'd first on schema drift (a missing column the migration never added), so login was unreachable live — 100% of traffic died at /register. Fix the masking layer too: align migrations/*.sql with the schema user.NewRepo expects, so the battery reaches login at all.

The E2E-001 battery fix (the actual lesson): the battery stopped at register 201 / duplicate 409. Extend it to the full auth path:

// e2e_test.go — full-auth-path battery, not a register-only battery
func TestE2E_001_FullAuthBattery(t *testing.T) {
    db := sqlite.Open(filepath.Join(t.TempDir(), "app.db"))
    require.NoError(t, migrate.Up(db))          // fresh sqlite migrate

    email := "e2e-" + randID() + "@example.com"

    // 1. register 201
    resp := mustPost(t, "/register", map[string]string{"email": email, "password": "hunter2!"})
    require.Equal(t, http.StatusCreated, resp.StatusCode)

    // 2. duplicate register 409
    resp = mustPost(t, "/register", map[string]string{"email": email, "password": "hunter2!"})
    require.Equal(t, http.StatusConflict, resp.StatusCode)

    // 3. LOGIN — the step that was missing; this is where the SIGSEGV fired
    resp = mustPost(t, "/login", map[string]string{"email": email, "password": "hunter2!"})
    require.Equal(t, http.StatusOK, resp.StatusCode)   // ← panicked: 500/SIGSEGV before the fix
    require.NotEmpty(t, resp.Header.Get("Set-Cookie"))

    // 4. (optional) authenticated /me with the session cookie
    resp = mustGet(t, "/me", resp.Cookies())
    require.Equal(t, http.StatusOK, resp.StatusCode)
}