Google OAuth connect and disconnect (Feature 16) (#125)

Co-authored-by: orchestrator-agent <20+orchestrator-agent@code.cubecraftcreations.com>

internal/gcal/connect.go

@@ -0,0 +1,401 @@
+// Package gcal connects the household's Google account for Push to Calendar.
+// The service is a confidential OAuth web client: the authorization code,
+// tokens and client secret stay on the server. The refresh token is stored
+// sealed by secretbox in integrations_google and is never logged or
+// returned.
+package gcal
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/subtle"
+	"database/sql"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"slices"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/endpoints"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/auth"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/db"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/secretbox"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/store"
+)
+
+// CallbackPath is where Google returns the browser after consent. It is
+// served under the signed-in /api/v1 router, so the redirect URI registered
+// with Google is MEALQUEST_PUBLIC_URL's origin followed by this path.
+const CallbackPath = "/api/v1/integrations/google/callback"
+
+// Scopes requested at connect: create and change the events MealQuest pushes,
+// and list the calendars the household can push to.
+var Scopes = []string{
+	"https://www.googleapis.com/auth/calendar.events",
+	"https://www.googleapis.com/auth/calendar.calendarlist.readonly",
+}
+
+// GoogleRevokeURL is Google's OAuth token revocation endpoint.
+const GoogleRevokeURL = "https://oauth2.googleapis.com/revoke"
+
+// GoogleEndpoint is Google's OAuth authorization and token endpoint.
+var GoogleEndpoint = oauth2.Endpoint{
+	AuthURL:   endpoints.Google.AuthURL,
+	TokenURL:  endpoints.Google.TokenURL,
+	AuthStyle: oauth2.AuthStyleInParams,
+}
+
+const (
+	connectCookieName = "__Host-mealquest_google"
+	connectCookieTTL  = 10 * time.Minute
+	// connectCookieKeyLabel separates the connect cookie's signing key from
+	// the login cookie's and every other key derived from the server secret.
+	connectCookieKeyLabel = "mealquest google connect cookie v1"
+	// googleTimeout bounds every request to Google.
+	googleTimeout = 10 * time.Second
+	// maxRevokeBody caps how much of a revoke response is read.
+	maxRevokeBody = 4 << 10
+)
+
+var (
+	// ErrBadState means the callback's state is missing, does not match the
+	// connect cookie, or the cookie is missing, expired or belongs to another
+	// session.
+	ErrBadState = errors.New("gcal: connect state is missing, expired or does not match this session")
+	// ErrDenied means Google returned an authorization error, usually because
+	// the household declined consent.
+	ErrDenied = errors.New("gcal: Google did not grant access")
+	// ErrNoCode means the callback carries no authorization code.
+	ErrNoCode = errors.New("gcal: callback has no authorization code")
+	// ErrNoRefreshToken means Google granted access without a refresh token,
+	// so MealQuest could not push later; the household must connect again.
+	ErrNoRefreshToken = errors.New("gcal: Google returned no refresh token")
+	// ErrScopes means the household did not grant every scope in Scopes.
+	ErrScopes = errors.New("gcal: Google did not grant every Calendar permission")
+	// ErrNotConnected means no Google account is connected.
+	ErrNotConnected = errors.New("gcal: Google Calendar is not connected")
+)
+
+// Config is what NewService needs to talk to Google.
+type Config struct {
+	ClientID     string
+	ClientSecret string
+	// PublicURL is the service's public URL; its origin prefixes CallbackPath
+	// to form the redirect URI.
+	PublicURL *url.URL
+	// SecretKey is the server key the connect cookie's signing key derives
+	// from.
+	SecretKey []byte
+	// Endpoint is Google's authorization and token endpoint; production uses
+	// GoogleEndpoint and tests a fake.
+	Endpoint oauth2.Endpoint
+	// RevokeURL is Google's revocation endpoint; production uses
+	// GoogleRevokeURL and tests a fake.
+	RevokeURL string
+}
+
+// Service connects and disconnects the household's Google account.
+// MealQuest runs as one process, so the service's in-memory lock and pending
+// states see every request.
+type Service struct {
+	st        *store.Store
+	box       *secretbox.Box
+	oauth     oauth2.Config
+	revokeURL string
+	client    *http.Client
+	cookie    auth.FlowCookie
+	// now is the clock; tests replace it.
+	now func() time.Time
+
+	// account serializes every change to the connected account — a
+	// connect's exchange and store, a disconnect's read, revoke and delete —
+	// across the request to Google and the database write. Google revokes the
+	// whole grant, so a revoke overlapping a connect would also kill the
+	// token the connect stores.
+	account sync.Mutex
+
+	// pending holds the state of every connect started and not yet
+	// completed, with when it expires. Complete consumes a state once, so a
+	// replayed cookie finds it gone.
+	pendingMu sync.Mutex
+	pending   map[string]time.Time
+}
+
+// NewService returns the service over st, sealing refresh tokens with box.
+func NewService(st *store.Store, box *secretbox.Box, cfg Config) (*Service, error) {
+	switch {
+	case st == nil:
+		return nil, errors.New("gcal: nil store")
+	case box == nil:
+		return nil, errors.New("gcal: nil secret box")
+	case cfg.ClientID == "", cfg.ClientSecret == "":
+		return nil, errors.New("gcal: client ID and client secret are required")
+	case cfg.PublicURL == nil:
+		return nil, errors.New("gcal: public URL is required")
+	case len(cfg.SecretKey) == 0:
+		return nil, errors.New("gcal: secret key is required")
+	case cfg.Endpoint.AuthURL == "", cfg.Endpoint.TokenURL == "", cfg.RevokeURL == "":
+		return nil, errors.New("gcal: authorization, token and revoke endpoints are required")
+	}
+	return &Service{
+		st:  st,
+		box: box,
+		oauth: oauth2.Config{
+			ClientID:     cfg.ClientID,
+			ClientSecret: cfg.ClientSecret,
+			Endpoint:     cfg.Endpoint,
+			RedirectURL:  cfg.PublicURL.Scheme + "://" + cfg.PublicURL.Host + CallbackPath,
+			Scopes:       slices.Clone(Scopes),
+		},
+		revokeURL: cfg.RevokeURL,
+		client:    &http.Client{Timeout: googleTimeout, CheckRedirect: auth.CheckRedirect},
+		cookie:    auth.NewFlowCookie(connectCookieName, connectCookieKeyLabel, connectCookieTTL, cfg.SecretKey),
+		now:       time.Now,
+		pending:   map[string]time.Time{},
+	}, nil
+}
+
+// connectState travels from Connect to Complete in the signed connect
+// cookie.
+type connectState struct {
+	State    string `json:"s"`
+	Verifier string `json:"v"`
+	// Session binds the state to the session that started the connect: the
+	// SHA-256 of its CSRF token, which is unique to the session and never
+	// changes during it.
+	Session string `json:"b"`
+	auth.FlowExpiry
+}
+
+// sessionBinding is the value connectState.Session must hold for s.
+func sessionBinding(s auth.Session) string {
+	sum := sha256.Sum256([]byte(s.CSRFToken))
+	return hex.EncodeToString(sum[:])
+}
+
+// Connect sets the connect cookie on w, bound to session, and returns
+// Google's consent URL: offline access, consent always prompted so Google
+// always issues a refresh token, and PKCE (S256).
+func (s *Service) Connect(w http.ResponseWriter, session auth.Session) (string, error) {
+	if session.CSRFToken == "" {
+		return "", errors.New("gcal: connect needs a session")
+	}
+	state := connectState{
+		State:    rand.Text(),
+		Verifier: oauth2.GenerateVerifier(),
+		Session:  sessionBinding(session),
+	}
+	now := s.now()
+	if err := s.cookie.Set(w, &state, now); err != nil {
+		return "", fmt.Errorf("gcal: set connect cookie: %w", err)
+	}
+	s.addPending(state.State, now)
+	return s.oauth.AuthCodeURL(state.State,
+		oauth2.AccessTypeOffline,
+		oauth2.ApprovalForce,
+		oauth2.S256ChallengeOption(state.Verifier),
+	), nil
+}
+
+// Complete finishes the connect Google redirected r back with. It clears the
+// connect cookie, checks the state against it and session, consumes the
+// state, exchanges the code and stores the refresh token, sealed, with
+// connected_at now. A reconnect replaces the stored token and keeps the
+// chosen calendar.
+func (s *Service) Complete(w http.ResponseWriter, r *http.Request, session auth.Session) error {
+	ctx := r.Context()
+	s.cookie.Clear(w)
+
+	var state connectState
+	if err := s.cookie.Read(r, &state, s.now()); err != nil {
+		return fmt.Errorf("%w: %v", ErrBadState, err)
+	}
+	q := r.URL.Query()
+	if subtle.ConstantTimeCompare([]byte(q.Get("state")), []byte(state.State)) != 1 {
+		return fmt.Errorf("%w: state mismatch", ErrBadState)
+	}
+	if subtle.ConstantTimeCompare([]byte(sessionBinding(session)), []byte(state.Session)) != 1 {
+		return fmt.Errorf("%w: started by another session", ErrBadState)
+	}
+	// A copy of the cookie verifies until it expires, so the state is
+	// single-use only because it is consumed here.
+	if !s.consumePending(state.State) {
+		return fmt.Errorf("%w: already used", ErrBadState)
+	}
+	if q.Get("error") != "" {
+		// error and error_description are Google-supplied text; neither is
+		// kept.
+		return ErrDenied
+	}
+	code := q.Get("code")
+	if code == "" {
+		return ErrNoCode
+	}
+
+	s.account.Lock()
+	defer s.account.Unlock()
+	token, err := s.oauth.Exchange(context.WithValue(ctx, oauth2.HTTPClient, s.client), code,
+		oauth2.VerifierOption(state.Verifier))
+	if err != nil {
+		return fmt.Errorf("gcal: %w", auth.ExchangeError(err))
+	}
+	if token.RefreshToken == "" {
+		return ErrNoRefreshToken
+	}
+	if err := checkScopes(token); err != nil {
+		return err
+	}
+	sealed, err := s.box.Encrypt([]byte(token.RefreshToken))
+	if err != nil {
+		return fmt.Errorf("gcal: seal refresh token: %w", err)
+	}
+	return s.store(ctx, sealed)
+}
+
+// addPending records state, started at now, as pending until the connect
+// cookie expires, and forgets expired ones.
+func (s *Service) addPending(state string, now time.Time) {
+	s.pendingMu.Lock()
+	defer s.pendingMu.Unlock()
+	for k, expires := range s.pending {
+		if !now.Before(expires) {
+			delete(s.pending, k)
+		}
+	}
+	s.pending[state] = now.Add(connectCookieTTL)
+}
+
+// consumePending removes state from the pending states, reporting whether
+// it was there.
+func (s *Service) consumePending(state string) bool {
+	s.pendingMu.Lock()
+	defer s.pendingMu.Unlock()
+	_, ok := s.pending[state]
+	delete(s.pending, state)
+	return ok
+}
+
+// checkScopes fails with ErrScopes unless token's granted scopes include
+// every scope in Scopes. Google lets the household untick a permission on
+// the consent screen and reports what was granted in "scope".
+func checkScopes(token *oauth2.Token) error {
+	granted, _ := token.Extra("scope").(string)
+	have := strings.Fields(granted)
+	for _, want := range Scopes {
+		if !slices.Contains(have, want) {
+			return ErrScopes
+		}
+	}
+	return nil
+}
+
+// store saves sealed as the refresh token with connected_at now, clearing
+// any earlier sync error.
+func (s *Service) store(ctx context.Context, sealed []byte) error {
+	return s.st.WithTx(ctx, func(tx *sql.Tx) error {
+		q := db.New(tx)
+		row, err := q.GetIntegrationsGoogle(ctx)
+		exists := err == nil
+		if err != nil && !errors.Is(err, sql.ErrNoRows) {
+			return fmt.Errorf("gcal: read Google integration: %w", err)
+		}
+		now := store.Timestamp{Time: s.now().UTC().Truncate(time.Millisecond)}.String()
+		created := now
+		if exists {
+			created = row.CreatedAt
+		}
+		if _, err := q.UpsertIntegrationsGoogle(ctx, db.UpsertIntegrationsGoogleParams{
+			RefreshTokenCt: sealed,
+			CalendarID:     row.CalendarID,
+			CalendarName:   row.CalendarName,
+			ConnectedAt:    now,
+			CreatedAt:      created,
+			UpdatedAt:      now,
+		}); err != nil {
+			return fmt.Errorf("gcal: save Google integration: %w", err)
+		}
+		return nil
+	})
+}
+
+// Disconnect revokes the stored refresh token at Google, then deletes the
+// connection. A token Google no longer knows (the household already removed
+// MealQuest's access in their Google account) counts as revoked. Any other
+// revoke failure keeps the connection so the household can retry.
+func (s *Service) Disconnect(ctx context.Context) error {
+	s.account.Lock()
+	defer s.account.Unlock()
+	row, err := db.New(s.st.Reader).GetIntegrationsGoogle(ctx)
+	if errors.Is(err, sql.ErrNoRows) {
+		return ErrNotConnected
+	}
+	if err != nil {
+		return fmt.Errorf("gcal: read Google integration: %w", err)
+	}
+	token, err := s.box.Decrypt(row.RefreshTokenCt)
+	if err != nil {
+		return fmt.Errorf("gcal: open refresh token: %w", err)
+	}
+	if err := s.revoke(ctx, string(token)); err != nil {
+		return err
+	}
+	if err := db.New(s.st.Writer).DeleteIntegrationsGoogle(ctx); err != nil {
+		return fmt.Errorf("gcal: delete Google integration: %w", err)
+	}
+	return nil
+}
+
+// RevokeError is a revoke Google refused or that failed in transit. Its text
+// never holds the token or Google's response body.
+type RevokeError struct {
+	msg string
+}
+
+func (e *RevokeError) Error() string { return e.msg }
+
+// revoke revokes token at Google. HTTP 200 and HTTP 400 invalid_token (the
+// token is already revoked or expired) both leave it unusable.
+func (s *Service) revoke(ctx context.Context, token string) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, s.revokeURL,
+		strings.NewReader(url.Values{"token": {token}}.Encode()))
+	if err != nil {
+		return fmt.Errorf("gcal: build revoke request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	resp, err := s.client.Do(req)
+	if err != nil {
+		var transport *url.Error
+		if errors.As(err, &transport) {
+			// The URL is dropped: after a refused redirect it is the
+			// Google-supplied Location.
+			return &RevokeError{msg: fmt.Sprintf("gcal: revoke request failed: %s: %v", transport.Op, transport.Err)}
+		}
+		return &RevokeError{msg: "gcal: revoke request failed"}
+	}
+	defer resp.Body.Close()
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxRevokeBody))
+	if err != nil {
+		return &RevokeError{msg: "gcal: read revoke response failed"}
+	}
+	if resp.StatusCode == http.StatusOK {
+		return nil
+	}
+	if resp.StatusCode == http.StatusBadRequest {
+		var reply struct {
+			Error string `json:"error"`
+		}
+		if json.Unmarshal(body, &reply) == nil && reply.Error == "invalid_token" {
+			return nil
+		}
+	}
+	return &RevokeError{msg: fmt.Sprintf("gcal: Google refused the revoke (HTTP %d)", resp.StatusCode)}
+}

internal/gcal/connect_test.go

@@ -0,0 +1,586 @@
+package gcal_test
+
+import (
+	"bytes"
+	"context"
+	"crypto/rand"
+	"database/sql"
+	"encoding/base64"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"slices"
+	"strings"
+	"testing"
+	"time"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/auth"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/db"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/gcal"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/gcal/gcaltest"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/secretbox"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/store"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/migrations"
+)
+
+const publicURL = "https://meals.example.com/app"
+
+var (
+	redirectURL = "https://meals.example.com" + gcal.CallbackPath
+	session     = auth.Session{Subject: "user-1", CSRFToken: "csrf-session-one"}
+	other       = auth.Session{Subject: "user-1", CSRFToken: "csrf-session-two"}
+)
+
+type harness struct {
+	svc    *gcal.Service
+	google *gcaltest.Google
+	st     *store.Store
+	box    *secretbox.Box
+	clock  time.Time
+}
+
+func newHarness(t *testing.T) *harness {
+	t.Helper()
+	ctx := context.Background()
+	st, err := store.Open(ctx, t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if err := st.Close(); err != nil {
+			t.Error(err)
+		}
+	})
+	if err := store.Migrate(ctx, st.Writer, migrations.FS); err != nil {
+		t.Fatal(err)
+	}
+	key := make([]byte, secretbox.KeySize)
+	rand.Read(key)
+	box, err := secretbox.New(base64.StdEncoding.EncodeToString(key))
+	if err != nil {
+		t.Fatal(err)
+	}
+	pub, err := url.Parse(publicURL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	google := gcaltest.New(t, redirectURL)
+	svc, err := gcal.NewService(st, box, gcal.Config{
+		ClientID:     gcaltest.ClientID,
+		ClientSecret: gcaltest.ClientSecret,
+		PublicURL:    pub,
+		SecretKey:    []byte("server-secret-key"),
+		Endpoint:     google.Endpoint(),
+		RevokeURL:    google.RevokeURL(),
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	h := &harness{svc: svc, google: google, st: st, box: box, clock: time.Date(2026, 10, 7, 18, 30, 0, 0, time.UTC)}
+	gcal.SetClock(svc, func() time.Time { return h.clock })
+	return h
+}
+
+// connect is a started connect: Google's consent URL and the connect
+// cookie.
+type connect struct {
+	consent *url.URL
+	cookie  *http.Cookie
+}
+
+func (h *harness) connect(t *testing.T, s auth.Session) connect {
+	t.Helper()
+	rec := httptest.NewRecorder()
+	raw, err := h.svc.Connect(rec, s)
+	if err != nil {
+		t.Fatal(err)
+	}
+	consent, err := url.Parse(raw)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cookies := rec.Result().Cookies()
+	if len(cookies) != 1 {
+		t.Fatalf("Connect set %d cookies, want 1", len(cookies))
+	}
+	return connect{consent: consent, cookie: cookies[0]}
+}
+
+// callback completes c as session s with query, returning the error and the
+// recorder Complete wrote to.
+func (h *harness) callback(c connect, s auth.Session, query url.Values) (*httptest.ResponseRecorder, error) {
+	req := httptest.NewRequest(http.MethodGet, gcal.CallbackPath+"?"+query.Encode(), nil)
+	if c.cookie != nil {
+		req.AddCookie(c.cookie)
+	}
+	rec := httptest.NewRecorder()
+	return rec, h.svc.Complete(rec, req, s)
+}
+
+// consented is the query Google sends back after consent to c.
+func (h *harness) consented(c connect) url.Values {
+	q := c.consent.Query()
+	return url.Values{"state": {q.Get("state")}, "code": {h.google.Authorize(q.Get("code_challenge"))}}
+}
+
+func (h *harness) row(t *testing.T) (db.IntegrationsGoogle, bool) {
+	t.Helper()
+	row, err := db.New(h.st.Reader).GetIntegrationsGoogle(context.Background())
+	if errors.Is(err, sql.ErrNoRows) {
+		return db.IntegrationsGoogle{}, false
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	return row, true
+}
+
+func TestConnectRedirectsToConsent(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	c := h.connect(t, session)
+
+	if got, want := c.consent.Scheme+"://"+c.consent.Host+c.consent.Path, h.google.Endpoint().AuthURL; got != want {
+		t.Errorf("consent URL = %s, want %s", got, want)
+	}
+	q := c.consent.Query()
+	for name, want := range map[string]string{
+		"client_id":             gcaltest.ClientID,
+		"redirect_uri":          redirectURL,
+		"response_type":         "code",
+		"access_type":           "offline",
+		"prompt":                "consent",
+		"code_challenge_method": "S256",
+	} {
+		if got := q.Get(name); got != want {
+			t.Errorf("%s = %q, want %q", name, got, want)
+		}
+	}
+	if got := strings.Fields(q.Get("scope")); !slices.Equal(got, []string{
+		"https://www.googleapis.com/auth/calendar.events",
+		"https://www.googleapis.com/auth/calendar.calendarlist.readonly",
+	}) {
+		t.Errorf("scope = %q", q.Get("scope"))
+	}
+	if q.Get("state") == "" || q.Get("code_challenge") == "" {
+		t.Errorf("state %q, code_challenge %q: want both set", q.Get("state"), q.Get("code_challenge"))
+	}
+	if q.Get("client_secret") != "" || strings.Contains(c.consent.String(), gcaltest.ClientSecret) {
+		t.Error("consent URL carries the client secret")
+	}
+
+	ck := c.cookie
+	if ck.Name != "__Host-mealquest_google" || ck.Path != "/" || !ck.Secure || !ck.HttpOnly ||
+		ck.SameSite != http.SameSiteLaxMode || ck.MaxAge != 600 || ck.Domain != "" {
+		t.Errorf("connect cookie = %+v, want __Host-, Path=/, Secure, HttpOnly, Lax, 10 minutes", ck)
+	}
+	// Two connects never share a state.
+	if again := h.connect(t, session); again.consent.Query().Get("state") == q.Get("state") {
+		t.Error("two connects share a state")
+	}
+}
+
+func TestCompleteStoresSealedRefreshToken(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	c := h.connect(t, session)
+	rec, err := h.callback(c, session, h.consented(c))
+	if err != nil {
+		t.Fatalf("Complete: %v", err)
+	}
+
+	row, ok := h.row(t)
+	if !ok {
+		t.Fatal("no integration stored")
+	}
+	if bytes.Contains(row.RefreshTokenCt, []byte(gcaltest.RefreshToken)) {
+		t.Error("refresh token stored in plain text")
+	}
+	plain, err := h.box.Decrypt(row.RefreshTokenCt)
+	if err != nil || string(plain) != gcaltest.RefreshToken {
+		t.Errorf("stored token opens to %q, %v; want the refresh token", plain, err)
+	}
+	if want := "2026-10-07T18:30:00.000Z"; row.ConnectedAt != want || row.CreatedAt != want || row.UpdatedAt != want {
+		t.Errorf("connected_at %q created_at %q updated_at %q, want %s", row.ConnectedAt, row.CreatedAt, row.UpdatedAt, want)
+	}
+	if row.CalendarID.Valid || row.CalendarName.Valid || row.LastSyncError.Valid {
+		t.Errorf("new connection has calendar or sync error: %+v", row)
+	}
+	assertCookieCleared(t, rec)
+	assertNoSecret(t, "response", rec.Body.String()+headerText(rec.Header()))
+}
+
+func TestReconnectReplacesTokenAndKeepsCalendar(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	ctx := context.Background()
+	if _, err := db.New(h.st.Writer).UpsertIntegrationsGoogle(ctx, db.UpsertIntegrationsGoogleParams{
+		RefreshTokenCt: []byte("old sealed token"),
+		CalendarID:     sql.NullString{String: "family@group.calendar.google.com", Valid: true},
+		CalendarName:   sql.NullString{String: "Family", Valid: true},
+		ConnectedAt:    "2026-09-01T00:00:00.000Z",
+		LastSyncError:  sql.NullString{String: "token revoked", Valid: true},
+		CreatedAt:      "2026-09-01T00:00:00.000Z",
+		UpdatedAt:      "2026-09-01T00:00:00.000Z",
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	c := h.connect(t, session)
+	if _, err := h.callback(c, session, h.consented(c)); err != nil {
+		t.Fatalf("Complete: %v", err)
+	}
+	row, _ := h.row(t)
+	if plain, err := h.box.Decrypt(row.RefreshTokenCt); err != nil || string(plain) != gcaltest.RefreshToken {
+		t.Errorf("token after reconnect opens to %q, %v", plain, err)
+	}
+	if row.CalendarID.String != "family@group.calendar.google.com" || row.CalendarName.String != "Family" {
+		t.Errorf("calendar after reconnect = %v %v, want it kept", row.CalendarID, row.CalendarName)
+	}
+	if row.LastSyncError.Valid {
+		t.Errorf("last_sync_error = %q, want it cleared", row.LastSyncError.String)
+	}
+	if row.CreatedAt != "2026-09-01T00:00:00.000Z" || row.ConnectedAt != "2026-10-07T18:30:00.000Z" {
+		t.Errorf("created_at %q connected_at %q", row.CreatedAt, row.ConnectedAt)
+	}
+}
+
+func TestCompleteRefusesBadState(t *testing.T) {
+	t.Parallel()
+	for _, tc := range []struct {
+		name string
+		// edit changes the connect, the callback's session or its query.
+		edit func(t *testing.T, h *harness, c *connect, s *auth.Session, q url.Values)
+	}{
+		{"missing state", func(_ *testing.T, _ *harness, _ *connect, _ *auth.Session, q url.Values) { q.Del("state") }},
+		{"wrong state", func(_ *testing.T, _ *harness, _ *connect, _ *auth.Session, q url.Values) { q.Set("state", "forged") }},
+		{"no connect cookie", func(_ *testing.T, _ *harness, c *connect, _ *auth.Session, _ url.Values) { c.cookie = nil }},
+		{"tampered cookie", func(_ *testing.T, _ *harness, c *connect, _ *auth.Session, _ url.Values) {
+			c.cookie = &http.Cookie{Name: c.cookie.Name, Value: "x" + c.cookie.Value}
+		}},
+		{"another session", func(_ *testing.T, _ *harness, _ *connect, s *auth.Session, _ url.Values) { *s = other }},
+		{"expired", func(_ *testing.T, h *harness, _ *connect, _ *auth.Session, _ url.Values) {
+			h.clock = h.clock.Add(10 * time.Minute)
+		}},
+		{"state of another connect", func(t *testing.T, h *harness, _ *connect, _ *auth.Session, q url.Values) {
+			q.Set("state", h.connect(t, session).consent.Query().Get("state"))
+		}},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			h := newHarness(t)
+			c := h.connect(t, session)
+			s := session
+			q := h.consented(c)
+			tc.edit(t, h, &c, &s, q)
+
+			rec, err := h.callback(c, s, q)
+			if !errors.Is(err, gcal.ErrBadState) {
+				t.Fatalf("Complete error = %v, want ErrBadState", err)
+			}
+			if h.google.TokenCalls() != 0 {
+				t.Error("the code was exchanged despite the bad state")
+			}
+			if _, ok := h.row(t); ok {
+				t.Error("an integration was stored")
+			}
+			assertCookieCleared(t, rec)
+		})
+	}
+}
+
+// TestCompleteRefusesReplayedState presents a connect's cookie and state a
+// second time, with a second valid code from the same consent URL, as an
+// attacker holding a copy of the cookie could. The server refuses it,
+// whatever the browser did with the cleared cookie.
+func TestCompleteRefusesReplayedState(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	c := h.connect(t, session)
+	if _, err := h.callback(c, session, h.consented(c)); err != nil {
+		t.Fatalf("Complete: %v", err)
+	}
+	stored, _ := h.row(t)
+
+	_, err := h.callback(c, session, h.consented(c))
+	if !errors.Is(err, gcal.ErrBadState) {
+		t.Fatalf("replayed Complete error = %v, want ErrBadState", err)
+	}
+	if got := h.google.TokenCalls(); got != 1 {
+		t.Errorf("token endpoint called %d times, want once: the replayed code was exchanged", got)
+	}
+	if row, _ := h.row(t); !bytes.Equal(row.RefreshTokenCt, stored.RefreshTokenCt) {
+		t.Error("the replay replaced the stored token")
+	}
+}
+
+func TestCompleteRefusesMissingRefreshToken(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	h.google.OmitRefreshToken()
+	c := h.connect(t, session)
+	_, err := h.callback(c, session, h.consented(c))
+	if !errors.Is(err, gcal.ErrNoRefreshToken) {
+		t.Fatalf("Complete error = %v, want ErrNoRefreshToken", err)
+	}
+	if _, ok := h.row(t); ok {
+		t.Error("an integration was stored without a refresh token")
+	}
+	assertNoSecret(t, "error", err.Error())
+}
+
+func TestCompleteRefusesMissingScope(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	h.google.GrantScope("https://www.googleapis.com/auth/calendar.calendarlist.readonly")
+	c := h.connect(t, session)
+	_, err := h.callback(c, session, h.consented(c))
+	if !errors.Is(err, gcal.ErrScopes) {
+		t.Fatalf("Complete error = %v, want ErrScopes", err)
+	}
+	if _, ok := h.row(t); ok {
+		t.Error("an integration was stored without calendar.events")
+	}
+}
+
+func TestCompleteDenied(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	c := h.connect(t, session)
+	q := url.Values{"state": {c.consent.Query().Get("state")}, "error": {"access_denied"}}
+	if _, err := h.callback(c, session, q); !errors.Is(err, gcal.ErrDenied) {
+		t.Fatalf("Complete error = %v, want ErrDenied", err)
+	}
+	if h.google.TokenCalls() != 0 {
+		t.Error("token endpoint called after a denial")
+	}
+}
+
+func TestCompleteExchangeFailureIsRedacted(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	h.google.FailToken(http.StatusBadRequest)
+	c := h.connect(t, session)
+	_, err := h.callback(c, session, h.consented(c))
+	if err == nil {
+		t.Fatal("Complete succeeded despite the token endpoint failing")
+	}
+	if !strings.Contains(err.Error(), "HTTP 400") {
+		t.Errorf("error = %v, want the status", err)
+	}
+	assertNoSecret(t, "error", err.Error())
+	if _, ok := h.row(t); ok {
+		t.Error("an integration was stored")
+	}
+}
+
+func TestDisconnectRevokesAndClears(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	c := h.connect(t, session)
+	if _, err := h.callback(c, session, h.consented(c)); err != nil {
+		t.Fatal(err)
+	}
+	if err := h.svc.Disconnect(context.Background()); err != nil {
+		t.Fatalf("Disconnect: %v", err)
+	}
+	if got := h.google.Revoked(); !slices.Equal(got, []string{gcaltest.RefreshToken}) {
+		t.Errorf("revoked %q, want the refresh token", got)
+	}
+	if _, ok := h.row(t); ok {
+		t.Error("integration still stored after disconnect")
+	}
+	if err := h.svc.Disconnect(context.Background()); !errors.Is(err, gcal.ErrNotConnected) {
+		t.Errorf("second Disconnect = %v, want ErrNotConnected", err)
+	}
+}
+
+// TestDisconnectRacingReconnect drives a disconnect and a reconnect into
+// each other at Google, with distinct tokens. However they interleave, the
+// end state is consistent: disconnected, or connected with a token Google
+// has not revoked.
+func TestDisconnectRacingReconnect(t *testing.T) {
+	t.Parallel()
+	for _, tc := range []struct {
+		name string
+		// hold stops the request the other operation must not slip past.
+		hold func(*gcaltest.Google) (<-chan struct{}, func())
+		// first is held at Google while second runs.
+		first, second func(h *harness, c connect, q url.Values) error
+	}{
+		{
+			// The disconnect has read the stored token and is revoking it
+			// when the reconnect would store a new one.
+			name:   "reconnect during revoke",
+			hold:   (*gcaltest.Google).HoldRevoke,
+			first:  disconnect,
+			second: reconnect,
+		},
+		{
+			// The reconnect's new token is issued but not yet stored when
+			// the disconnect would revoke the old one.
+			name:   "disconnect during exchange",
+			hold:   (*gcaltest.Google).HoldToken,
+			first:  reconnect,
+			second: disconnect,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			h := newHarness(t)
+			first := h.connect(t, session)
+			if _, err := h.callback(first, session, h.consented(first)); err != nil {
+				t.Fatal(err)
+			}
+			c := h.connect(t, session)
+			q := h.consented(c)
+
+			arrived, release := tc.hold(h.google)
+			defer release()
+			firstDone := make(chan error, 1)
+			go func() { firstDone <- tc.first(h, c, q) }()
+			<-arrived
+			secondDone := make(chan error, 1)
+			go func() { secondDone <- tc.second(h, c, q) }()
+			// Let the second operation run as far as it can while the first
+			// is held at Google: to the end when nothing serializes them.
+			select {
+			case err := <-secondDone:
+				secondDone <- err
+			case <-time.After(500 * time.Millisecond):
+			}
+			release()
+			for _, done := range []chan error{firstDone, secondDone} {
+				if err := <-done; err != nil {
+					t.Logf("operation: %v", err)
+				}
+			}
+
+			row, ok := h.row(t)
+			if !ok {
+				return
+			}
+			plain, err := h.box.Decrypt(row.RefreshTokenCt)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !h.google.Live(string(plain)) {
+				t.Errorf("stored as connected with a revoked token (Google revoked %d tokens)", len(h.google.Revoked()))
+			}
+		})
+	}
+}
+
+func disconnect(h *harness, _ connect, _ url.Values) error {
+	return h.svc.Disconnect(context.Background())
+}
+
+func reconnect(h *harness, c connect, q url.Values) error {
+	_, err := h.callback(c, session, q)
+	return err
+}
+
+func TestDisconnectTokenAlreadyRevokedAtGoogle(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	c := h.connect(t, session)
+	if _, err := h.callback(c, session, h.consented(c)); err != nil {
+		t.Fatal(err)
+	}
+	h.google.AnswerRevoke(http.StatusBadRequest, `{"error":"invalid_token","error_description":"Token expired or revoked"}`)
+	if err := h.svc.Disconnect(context.Background()); err != nil {
+		t.Fatalf("Disconnect: %v", err)
+	}
+	if _, ok := h.row(t); ok {
+		t.Error("integration still stored")
+	}
+}
+
+func TestDisconnectRevokeFailureKeepsConnection(t *testing.T) {
+	t.Parallel()
+	for _, status := range []int{http.StatusBadRequest, http.StatusServiceUnavailable} {
+		h := newHarness(t)
+		c := h.connect(t, session)
+		if _, err := h.callback(c, session, h.consented(c)); err != nil {
+			t.Fatal(err)
+		}
+		h.google.AnswerRevoke(status, "")
+		err := h.svc.Disconnect(context.Background())
+		var revoke *gcal.RevokeError
+		if !errors.As(err, &revoke) {
+			t.Fatalf("HTTP %d: Disconnect error = %v, want *RevokeError", status, err)
+		}
+		assertNoSecret(t, "error", err.Error())
+		if _, ok := h.row(t); !ok {
+			t.Errorf("HTTP %d: connection cleared although Google did not revoke", status)
+		}
+	}
+}
+
+func TestDisconnectNotConnected(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	if err := h.svc.Disconnect(context.Background()); !errors.Is(err, gcal.ErrNotConnected) {
+		t.Fatalf("Disconnect = %v, want ErrNotConnected", err)
+	}
+	if len(h.google.Revoked()) != 0 {
+		t.Error("revoke called with nothing connected")
+	}
+}
+
+func TestNewServiceRequiresConfig(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	pub, _ := url.Parse(publicURL)
+	valid := gcal.Config{
+		ClientID: "id", ClientSecret: "secret", PublicURL: pub, SecretKey: []byte("k"),
+		Endpoint: gcal.GoogleEndpoint, RevokeURL: gcal.GoogleRevokeURL,
+	}
+	if _, err := gcal.NewService(h.st, h.box, valid); err != nil {
+		t.Fatalf("valid config: %v", err)
+	}
+	for name, edit := range map[string]func(*gcal.Config){
+		"client ID":     func(c *gcal.Config) { c.ClientID = "" },
+		"client secret": func(c *gcal.Config) { c.ClientSecret = "" },
+		"public URL":    func(c *gcal.Config) { c.PublicURL = nil },
+		"secret key":    func(c *gcal.Config) { c.SecretKey = nil },
+		"token URL":     func(c *gcal.Config) { c.Endpoint.TokenURL = "" },
+		"revoke URL":    func(c *gcal.Config) { c.RevokeURL = "" },
+	} {
+		cfg := valid
+		edit(&cfg)
+		if _, err := gcal.NewService(h.st, h.box, cfg); err == nil {
+			t.Errorf("NewService accepted a config without the %s", name)
+		}
+	}
+}
+
+func assertCookieCleared(t *testing.T, rec *httptest.ResponseRecorder) {
+	t.Helper()
+	for _, c := range rec.Result().Cookies() {
+		if c.Name == "__Host-mealquest_google" && c.MaxAge < 0 {
+			return
+		}
+	}
+	t.Error("connect cookie not cleared")
+}
+
+// assertNoSecret fails when text holds the refresh token, the access token
+// or the client secret.
+func assertNoSecret(t *testing.T, what, text string) {
+	t.Helper()
+	for _, secret := range []string{gcaltest.RefreshToken, gcaltest.AccessToken, gcaltest.ClientSecret} {
+		if strings.Contains(text, secret) {
+			t.Errorf("%s contains a secret: %s", what, text)
+		}
+	}
+}
+
+func headerText(h http.Header) string {
+	var b strings.Builder
+	for k, vs := range h {
+		b.WriteString(k + ": " + strings.Join(vs, ", ") + "\n")
+	}
+	return b.String()
+}

internal/gcal/export_test.go

@@ -0,0 +1,6 @@
+package gcal
+
+import "time"
+
+// SetClock replaces s's clock.
+func SetClock(s *Service, now func() time.Time) { s.now = now }

internal/gcal/gcaltest/gcaltest.go

@@ -0,0 +1,295 @@
+// Package gcaltest provides a fake Google OAuth server for tests of the
+// Google Calendar connection: a token endpoint that checks the client, the
+// redirect URI and PKCE and issues a distinct refresh token per exchange, and
+// a revoke endpoint that, like Google's, revokes the whole grant: every token
+// issued so far. It never touches the network beyond its own httptest
+// server.
+package gcaltest
+
+import (
+	"crypto/sha256"
+	"encoding/base64"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+
+	"golang.org/x/oauth2"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/gcal"
+)
+
+// Fixed credentials and tokens the fake issues and expects. The first
+// exchange issues RefreshToken; exchange n after it issues RefreshToken
+// followed by "-n", so every issued refresh token contains RefreshToken.
+const (
+	ClientID     = "mealquest-test.apps.googleusercontent.com"
+	ClientSecret = "google-client-s3cret-4f1c"
+	RefreshToken = "1//google-refresh-t0ken-9a8b7c"
+	AccessToken  = "ya29.google-access-t0ken-5d6e"
+)
+
+// Google is the fake. Its exported fields script the next responses; set
+// them before the request they affect.
+type Google struct {
+	Server *httptest.Server
+	// RedirectURL is the redirect URI the token endpoint requires.
+	RedirectURL string
+
+	mu sync.Mutex
+	// omitRefresh leaves refresh_token out of the token response.
+	omitRefresh bool
+	// scope is the token response's granted scopes.
+	scope string
+	// tokenStatus, when not 200, fails the token endpoint with a body that
+	// quotes the refresh token and client secret.
+	tokenStatus int
+	// revokeStatus and revokeBody answer the revoke endpoint; revokeBody
+	// quotes the token when revokeStatus is not 200 or 400.
+	revokeStatus int
+	revokeBody   string
+	codes        map[string]string // code -> PKCE challenge
+	tokenCalls   int
+	revoked      []string
+	// issued is every refresh token issued, in order; dead is those a
+	// revoke has invalidated.
+	issued []string
+	dead   map[string]bool
+	// tokenHold and revokeHold, when set, stop the next request to that
+	// endpoint.
+	tokenHold, revokeHold *hold
+}
+
+// hold stops one request: arrived is closed when it arrives, and it goes on
+// once release is closed.
+type hold struct {
+	arrived, release chan struct{}
+}
+
+// New starts a fake Google that requires redirectURL at its token endpoint.
+// It grants every scope in gcal.Scopes with a refresh token by default.
+func New(t *testing.T, redirectURL string) *Google {
+	t.Helper()
+	g := &Google{
+		RedirectURL:  redirectURL,
+		scope:        strings.Join(gcal.Scopes, " "),
+		tokenStatus:  http.StatusOK,
+		revokeStatus: http.StatusOK,
+		codes:        map[string]string{},
+		dead:         map[string]bool{},
+	}
+	mux := http.NewServeMux()
+	mux.HandleFunc("POST /token", g.token)
+	mux.HandleFunc("POST /revoke", g.revoke)
+	g.Server = httptest.NewServer(mux)
+	t.Cleanup(g.Server.Close)
+	return g
+}
+
+// Endpoint is the fake's authorization and token endpoint, with Google's
+// client authentication style.
+func (g *Google) Endpoint() oauth2.Endpoint {
+	return oauth2.Endpoint{
+		AuthURL:   g.Server.URL + "/auth",
+		TokenURL:  g.Server.URL + "/token",
+		AuthStyle: gcal.GoogleEndpoint.AuthStyle,
+	}
+}
+
+// RevokeURL is the fake's revocation endpoint.
+func (g *Google) RevokeURL() string { return g.Server.URL + "/revoke" }
+
+// Authorize plays the consent screen for an authorization request with PKCE
+// challenge and returns the code Google would send back.
+func (g *Google) Authorize(challenge string) string {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	code := fmt.Sprintf("code-%d", len(g.codes)+1)
+	g.codes[code] = challenge
+	return code
+}
+
+// OmitRefreshToken makes the token endpoint answer without a refresh token,
+// as Google does when it reuses an earlier grant.
+func (g *Google) OmitRefreshToken() {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.omitRefresh = true
+}
+
+// GrantScope sets the scopes the token endpoint reports as granted.
+func (g *Google) GrantScope(scope string) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.scope = scope
+}
+
+// FailToken makes the token endpoint answer status with a body that quotes
+// the refresh token and client secret.
+func (g *Google) FailToken(status int) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.tokenStatus = status
+}
+
+// AnswerRevoke makes the revoke endpoint answer status with body. A body of
+// "" with a status other than 200 quotes the token it was sent.
+func (g *Google) AnswerRevoke(status int, body string) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.revokeStatus, g.revokeBody = status, body
+}
+
+// TokenCalls is how many requests reached the token endpoint.
+func (g *Google) TokenCalls() int {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	return g.tokenCalls
+}
+
+// Revoked lists the tokens the revoke endpoint received, in order.
+func (g *Google) Revoked() []string {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	return append([]string(nil), g.revoked...)
+}
+
+// Live reports whether token was issued by the fake and no revoke has
+// invalidated it since.
+func (g *Google) Live(token string) bool {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	return slices.Contains(g.issued, token) && !g.dead[token]
+}
+
+// HoldToken stops the next token request after its refresh token is issued
+// and before it is answered, so a revoke can land in between. It returns a
+// channel closed when the request arrives and the function that lets it go
+// on.
+func (g *Google) HoldToken() (arrived <-chan struct{}, release func()) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.tokenHold = newHold()
+	return g.tokenHold.arrived, g.tokenHold.releaseFunc()
+}
+
+// HoldRevoke stops the next revoke request before Google acts on it. It
+// returns a channel closed when the request arrives and the function that
+// lets it go on.
+func (g *Google) HoldRevoke() (arrived <-chan struct{}, release func()) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.revokeHold = newHold()
+	return g.revokeHold.arrived, g.revokeHold.releaseFunc()
+}
+
+func newHold() *hold {
+	return &hold{arrived: make(chan struct{}), release: make(chan struct{})}
+}
+
+func (h *hold) releaseFunc() func() {
+	var once sync.Once
+	return func() { once.Do(func() { close(h.release) }) }
+}
+
+// wait stops the request at h, if any, without holding g.mu.
+func (g *Google) wait(h *hold) {
+	if h == nil {
+		return
+	}
+	g.mu.Unlock()
+	close(h.arrived)
+	<-h.release
+	g.mu.Lock()
+}
+
+func (g *Google) token(w http.ResponseWriter, r *http.Request) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.tokenCalls++
+	if err := r.ParseForm(); err != nil {
+		http.Error(w, "bad form", http.StatusBadRequest)
+		return
+	}
+	f := r.PostForm
+	challenge, known := g.codes[f.Get("code")]
+	delete(g.codes, f.Get("code"))
+	switch {
+	case f.Get("grant_type") != "authorization_code":
+		oauthError(w, http.StatusBadRequest, "unsupported_grant_type")
+		return
+	case f.Get("client_id") != ClientID || f.Get("client_secret") != ClientSecret:
+		oauthError(w, http.StatusUnauthorized, "invalid_client")
+		return
+	case !known, f.Get("redirect_uri") != g.RedirectURL, s256(f.Get("code_verifier")) != challenge:
+		oauthError(w, http.StatusBadRequest, "invalid_grant")
+		return
+	}
+	if g.tokenStatus != http.StatusOK {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(g.tokenStatus)
+		fmt.Fprintf(w, `{"error":"invalid_grant","error_description":"echo %s %s"}`, RefreshToken, ClientSecret)
+		return
+	}
+	refresh := ""
+	if !g.omitRefresh {
+		issue := RefreshToken
+		if n := len(g.issued); n > 0 {
+			issue = fmt.Sprintf("%s-%d", RefreshToken, n+1)
+		}
+		g.issued = append(g.issued, issue)
+		refresh = fmt.Sprintf(`"refresh_token":%q,`, issue)
+	}
+	h := g.tokenHold
+	g.tokenHold = nil
+	g.wait(h)
+	w.Header().Set("Content-Type", "application/json")
+	fmt.Fprintf(w, `{"access_token":%q,%s"expires_in":3599,"scope":%q,"token_type":"Bearer"}`,
+		AccessToken, refresh, g.scope)
+}
+
+func (g *Google) revoke(w http.ResponseWriter, r *http.Request) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	if r.Header.Get("Content-Type") != "application/x-www-form-urlencoded" {
+		http.Error(w, "want a form", http.StatusUnsupportedMediaType)
+		return
+	}
+	if err := r.ParseForm(); err != nil {
+		http.Error(w, "bad form", http.StatusBadRequest)
+		return
+	}
+	h := g.revokeHold
+	g.revokeHold = nil
+	g.wait(h)
+	token := r.PostForm.Get("token")
+	g.revoked = append(g.revoked, token)
+	if g.revokeStatus == http.StatusOK && slices.Contains(g.issued, token) {
+		// Google revokes the grant, not just the token: every token issued
+		// to it so far stops working.
+		for _, t := range g.issued {
+			g.dead[t] = true
+		}
+	}
+	body := g.revokeBody
+	if body == "" && g.revokeStatus != http.StatusOK {
+		body = "cannot revoke " + token
+	}
+	w.WriteHeader(g.revokeStatus)
+	fmt.Fprint(w, body)
+}
+
+func oauthError(w http.ResponseWriter, status int, code string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	fmt.Fprintf(w, `{"error":%q}`, code)
+}
+
+// s256 is the PKCE S256 challenge for verifier.
+func s256(verifier string) string {
+	sum := sha256.Sum256([]byte(verifier))
+	return base64.RawURLEncoding.EncodeToString(sum[:])
+}