projax/web/auth.go

package web

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"net/url"
	"strings"
	"time"
)

// AuthConfig backs projax's own /login. Same Supabase backend the other m/*
// tools use, but cookies are per-host (no Domain attribute) so projax does not
// share its session with any other subdomain. Matches the mBrian / flexsiebels
// pattern.
type AuthConfig struct {
	SupabaseURL string // e.g. https://supa.flexsiebels.de
	AnonKey     string
	HTTPClient  *http.Client
}

// client returns a usable *http.Client even when HTTPClient is unset, so
// helpers can be called directly (e.g. from /login GET) without explicit init.
func (cfg AuthConfig) client() *http.Client {
	if cfg.HTTPClient != nil {
		return cfg.HTTPClient
	}
	return &http.Client{Timeout: 5 * time.Second}
}

const (
	accessTokenCookie  = "access_token"
	refreshTokenCookie = "refresh_token"
	cookieMaxAge       = 365 * 24 * 60 * 60
	loginPath          = "/login"
	logoutPath         = "/logout"
)

// supabaseUser is the minimum slice of GET /auth/v1/user we read.
type supabaseUser struct {
	ID    string `json:"id"`
	Email string `json:"email"`
}

// supabaseSession is what /auth/v1/token returns for either grant type
// (password or refresh_token).
type supabaseSession struct {
	AccessToken  string `json:"access_token"`
	RefreshToken string `json:"refresh_token"`
	User         struct {
		ID string `json:"id"`
	} `json:"user"`
}

// supabaseAuthError is the {error, error_description, msg} shape Supabase
// returns on a 4xx from /auth/v1/*.
type supabaseAuthError struct {
	Error            string `json:"error"`
	ErrorDescription string `json:"error_description"`
	Msg              string `json:"msg"`
}

func (e supabaseAuthError) Message() string {
	switch {
	case e.ErrorDescription != "":
		return e.ErrorDescription
	case e.Msg != "":
		return e.Msg
	case e.Error != "":
		return e.Error
	default:
		return "Login failed"
	}
}

// authMiddleware gates every request except /healthz, /login and /logout.
// On invalid session it 302s to /login?redirectTo=<safe-path>.
func authMiddleware(cfg AuthConfig, logger *slog.Logger, next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Always-open routes: probe and auth endpoints themselves.
		switch r.URL.Path {
		case "/healthz", loginPath, logoutPath:
			next.ServeHTTP(w, r)
			return
		}

		access := tokenFromBearer(r)
		if access == "" {
			if c, err := r.Cookie(accessTokenCookie); err == nil {
				access = c.Value
			}
		}

		ctx := r.Context()
		if access != "" {
			if _, err := cfg.validateAccessToken(ctx, access); err == nil {
				next.ServeHTTP(w, r)
				return
			}
		}

		if c, err := r.Cookie(refreshTokenCookie); err == nil && c.Value != "" {
			sess, err := cfg.refreshSession(ctx, c.Value)
			if err == nil {
				cfg.setSessionCookies(w, sess)
				next.ServeHTTP(w, r)
				return
			}
			logger.Debug("auth: refresh failed", "err", err)
		}

		http.Redirect(w, r, loginRedirectURL(r), http.StatusFound)
	})
}

// tokenFromBearer extracts a Bearer token from the Authorization header.
func tokenFromBearer(r *http.Request) string {
	h := r.Header.Get("Authorization")
	const prefix = "Bearer "
	if !strings.HasPrefix(h, prefix) {
		return ""
	}
	return strings.TrimSpace(h[len(prefix):])
}

// validateAccessToken calls GET <SUPABASE_URL>/auth/v1/user with the bearer.
func (cfg AuthConfig) validateAccessToken(ctx context.Context, token string) (*supabaseUser, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, cfg.SupabaseURL+"/auth/v1/user", nil)
	if err != nil {
		return nil, err
	}
	req.Header.Set("Authorization", "Bearer "+token)
	req.Header.Set("apikey", cfg.AnonKey)
	resp, err := cfg.client().Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		body, _ := io.ReadAll(resp.Body)
		return nil, fmt.Errorf("supabase /auth/v1/user: %d %s", resp.StatusCode, strings.TrimSpace(string(body)))
	}
	var u supabaseUser
	if err := json.NewDecoder(resp.Body).Decode(&u); err != nil {
		return nil, err
	}
	if u.ID == "" {
		return nil, errors.New("supabase /auth/v1/user: empty user id")
	}
	return &u, nil
}

// refreshSession swaps a refresh token for a fresh access/refresh pair.
func (cfg AuthConfig) refreshSession(ctx context.Context, refresh string) (*supabaseSession, error) {
	body, _ := json.Marshal(map[string]string{"refresh_token": refresh})
	return cfg.tokenRequest(ctx, "refresh_token", body)
}

// passwordSignIn exchanges email+password for a session via /auth/v1/token.
func (cfg AuthConfig) passwordSignIn(ctx context.Context, email, password string) (*supabaseSession, error) {
	body, _ := json.Marshal(map[string]string{"email": email, "password": password})
	return cfg.tokenRequest(ctx, "password", body)
}

func (cfg AuthConfig) tokenRequest(ctx context.Context, grant string, body []byte) (*supabaseSession, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodPost,
		cfg.SupabaseURL+"/auth/v1/token?grant_type="+grant,
		bytes.NewReader(body))
	if err != nil {
		return nil, err
	}
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("apikey", cfg.AnonKey)
	resp, err := cfg.client().Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		raw, _ := io.ReadAll(resp.Body)
		var ae supabaseAuthError
		_ = json.Unmarshal(raw, &ae)
		return nil, fmt.Errorf("supabase /auth/v1/token (%s): %s", grant, ae.Message())
	}
	var s supabaseSession
	if err := json.NewDecoder(resp.Body).Decode(&s); err != nil {
		return nil, err
	}
	if s.AccessToken == "" || s.RefreshToken == "" {
		return nil, errors.New("supabase /auth/v1/token: empty token in response")
	}
	return &s, nil
}

// setSessionCookies writes per-host access/refresh cookies. No Domain attribute
// — scope is projax.msbls.de only, matching mbrian + flexsiebels.
func (cfg AuthConfig) setSessionCookies(w http.ResponseWriter, s *supabaseSession) {
	for _, c := range []*http.Cookie{
		sessionCookie(accessTokenCookie, s.AccessToken),
		sessionCookie(refreshTokenCookie, s.RefreshToken),
	} {
		http.SetCookie(w, c)
	}
}

func sessionCookie(name, value string) *http.Cookie {
	return &http.Cookie{
		Name:     name,
		Value:    value,
		Path:     "/",
		MaxAge:   cookieMaxAge,
		HttpOnly: true,
		Secure:   true,
		SameSite: http.SameSiteLaxMode,
	}
}

// clearCookie returns a Set-Cookie that erases `name` on the browser (Max-Age=0).
func clearCookie(name string) *http.Cookie {
	return &http.Cookie{
		Name:     name,
		Value:    "",
		Path:     "/",
		MaxAge:   -1,
		HttpOnly: true,
		Secure:   true,
		SameSite: http.SameSiteLaxMode,
	}
}

// loginRedirectURL builds /login?redirectTo=<safe-path>.
func loginRedirectURL(r *http.Request) string {
	target := safeRedirect(r.URL.RequestURI())
	if target == "" || target == loginPath {
		return loginPath
	}
	q := url.Values{}
	q.Set("redirectTo", target)
	return loginPath + "?" + q.Encode()
}

// safeRedirect rejects anything that is not a same-origin path. Mirrors the
// mgmt safeRedirect: must start with "/", must not start with "//", must not
// contain "\" (Windows-style URL trick).
func safeRedirect(value string) string {
	v := strings.TrimSpace(value)
	if v == "" {
		return ""
	}
	if !strings.HasPrefix(v, "/") {
		return ""
	}
	if strings.HasPrefix(v, "//") {
		return ""
	}
	if strings.ContainsAny(v, "\\\r\n\t") {
		return ""
	}
	return v
}