Feature 16: Google Calendar Sync (#130)

Co-authored-by: orchestrator-agent <20+orchestrator-agent@code.cubecraftcreations.com>
2026-10-08 03:13:21 -04:00
parent 5ac5982c1f
commit 7003ff92ec
65 changed files with 8092 additions and 222 deletions
@@ -0,0 +1,749 @@
+// Package gcaltest provides a fake Google OAuth server and Calendar API for
+// tests of the Google Calendar connection: a token endpoint that checks the
+// client, the redirect URI and PKCE, issues a distinct refresh token per
+// exchange and trades a live refresh token for an access token; a revoke
+// endpoint that, like Google's, revokes the whole grant: every token issued
+// so far; a calendarList endpoint that requires a live grant's access
+// token; and events insert and update endpoints that, like Google's, keep
+// each calendar's events by id, answer an insert of an id the calendar
+// already has (even a cancelled event's) with HTTP 409, and refuse a
+// calendar not listed (404) or not writable (403 requiredAccessLevel). It
+// never touches the network beyond its own httptest server.
+package gcaltest
+
+import (
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"slices"
+	"strconv"
+	"strings"
+	"sync"
+	"testing"
+
+	"golang.org/x/oauth2"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/auth"
+	"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. Every
+// exchange and refresh issues AccessToken.
+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, revokeHold and calendarHold, when set, stop the next
+	// request to that endpoint.
+	tokenHold, revokeHold, calendarHold *hold
+	// calendars is what the calendarList endpoint lists, pageSize entries
+	// to a page (all of them when 0).
+	calendars []Calendar
+	pageSize  int
+	// calendarStatus, when not 200, fails the calendarList endpoint with a
+	// body that quotes the access token: every request when calendarFails
+	// is 0, else the next calendarFails requests.
+	calendarStatus int
+	calendarFails  int
+	calendarCalls  int
+	// rejectAccess makes the Calendar API answer HTTP 401 to every access
+	// token, while the grant stays live at the token endpoint.
+	rejectAccess bool
+	// events holds each calendar's events by id.
+	events map[string]map[string]Event
+	// inserts and updates count the events created and updated.
+	inserts, updates int
+	// eventWrites counts the requests that reached the events endpoints;
+	// failWrite, when not 0, fails the requests that make it that count up
+	// to failLast with failStatus and a body that quotes the access token.
+	eventWrites, failWrite, failLast, failStatus int
+	// eventHold, when set, stops the next events request.
+	eventHold *hold
+}
+
+// Event is an event as the fake keeps it: what MealQuest sent.
+type Event struct {
+	ID          string    `json:"id"`
+	Status      string    `json:"status"`
+	Summary     string    `json:"summary"`
+	Description string    `json:"description"`
+	Start       EventTime `json:"start"`
+	End         EventTime `json:"end"`
+}
+
+// EventTime is an event's start or end.
+type EventTime struct {
+	DateTime string `json:"dateTime"`
+	TimeZone string `json:"timeZone"`
+}
+
+// Calendar is one calendarList entry. The fake answers each with Google's
+// other fields too, so tests can check that nothing beyond id, summary,
+// primary and accessRole reaches the household.
+type Calendar struct {
+	ID              string
+	Summary         string
+	Primary         bool
+	AccessRole      string
+	Description     string
+	SummaryOverride string
+}
+
+// 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,
+		calendarStatus: http.StatusOK,
+		codes:          map[string]string{},
+		dead:           map[string]bool{},
+		events:         map[string]map[string]Event{},
+	}
+	mux := http.NewServeMux()
+	mux.HandleFunc("POST /token", g.token)
+	mux.HandleFunc("POST /revoke", g.revoke)
+	mux.HandleFunc("GET /calendar/v3/users/me/calendarList", g.calendarList)
+	mux.HandleFunc("POST /calendar/v3/calendars/{calendar}/events", g.insertEvent)
+	mux.HandleFunc("PUT /calendar/v3/calendars/{calendar}/events/{event}", g.updateEvent)
+	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" }
+
+// CalendarAPI is the fake's Calendar API base URL.
+func (g *Google) CalendarAPI() string { return g.Server.URL + "/calendar/v3" }
+
+// 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
+}
+
+// Connect connects svc to the fake as a household completing the consent
+// screen does. svc must use the fake's endpoints and redirect URL.
+func (g *Google) Connect(t *testing.T, svc *gcal.Service) {
+	t.Helper()
+	session := auth.Session{Subject: "gcaltest", CSRFToken: "gcaltest-session"}
+	started := httptest.NewRecorder()
+	raw, err := svc.Connect(started, session)
+	if err != nil {
+		t.Fatal(err)
+	}
+	consent, err := url.Parse(raw)
+	if err != nil {
+		t.Fatal(err)
+	}
+	q := url.Values{
+		"state": {consent.Query().Get("state")},
+		"code":  {g.Authorize(consent.Query().Get("code_challenge"))},
+	}
+	req := httptest.NewRequest(http.MethodGet, gcal.CallbackPath+"?"+q.Encode(), nil)
+	for _, c := range started.Result().Cookies() {
+		req.AddCookie(c)
+	}
+	if err := svc.Complete(httptest.NewRecorder(), req, session); err != nil {
+		t.Fatalf("connect: %v", err)
+	}
+}
+
+// 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
+}
+
+// RevokeGrant revokes every token issued so far, as the household removing
+// MealQuest's access in their Google account does.
+func (g *Google) RevokeGrant() {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	for _, t := range g.issued {
+		g.dead[t] = true
+	}
+}
+
+// SetCalendars makes the calendarList endpoint list cals, pageSize to a
+// page (all on one page when 0).
+func (g *Google) SetCalendars(pageSize int, cals ...Calendar) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.calendars, g.pageSize = slices.Clone(cals), pageSize
+}
+
+// FailCalendarList makes the calendarList endpoint answer status with a
+// body that quotes the access token.
+func (g *Google) FailCalendarList(status int) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.calendarStatus, g.calendarFails = status, 0
+}
+
+// FailCalendarListNext makes the next n calendarList requests answer status
+// as FailCalendarList does, and later ones answer normally.
+func (g *Google) FailCalendarListNext(n, status int) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.calendarStatus, g.calendarFails = status, n
+}
+
+// RejectAccessToken makes the Calendar API answer HTTP 401 to every access
+// token while refreshing the grant still succeeds, as Google does when it
+// refuses the access token it just issued.
+func (g *Google) RejectAccessToken() {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.rejectAccess = true
+}
+
+// AcceptAccessToken undoes RejectAccessToken, as Google does for the grant
+// of a later connection.
+func (g *Google) AcceptAccessToken() {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.rejectAccess = false
+}
+
+// CalendarCalls is how many requests reached the calendarList endpoint.
+func (g *Google) CalendarCalls() int {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	return g.calendarCalls
+}
+
+// HoldCalendarList stops the next calendarList request before it is
+// answered. It returns a channel closed when the request arrives and the
+// function that lets it go on.
+func (g *Google) HoldCalendarList() (arrived <-chan struct{}, release func()) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.calendarHold = newHold()
+	return g.calendarHold.arrived, g.calendarHold.releaseFunc()
+}
+
+// Events returns the events of calendar, by id.
+func (g *Google) Events(calendar string) map[string]Event {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	out := map[string]Event{}
+	for id, e := range g.events[calendar] {
+		out[id] = e
+	}
+	return out
+}
+
+// EventWrites returns how many events were inserted and how many updated.
+func (g *Google) EventWrites() (inserts, updates int) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	return g.inserts, g.updates
+}
+
+// ResetEventWrites zeroes the insert and update counts.
+func (g *Google) ResetEventWrites() {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.inserts, g.updates = 0, 0
+}
+
+// CancelEvent marks event id of calendar cancelled, as deleting it in
+// Google Calendar does: Google keeps the id.
+func (g *Google) CancelEvent(calendar, id string) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	e := g.events[calendar][id]
+	e.Status = "cancelled"
+	g.events[calendar][id] = e
+}
+
+// FailEventWrite makes the nth request to the events endpoints from now
+// (1 for the next) answer status with a body that quotes the access token.
+func (g *Google) FailEventWrite(n, status int) {
+	g.FailEventWrites(n, 1, status)
+}
+
+// FailEventWrites makes count requests to the events endpoints in a row,
+// from the nth from now (1 for the next), answer status with a body that
+// quotes the access token.
+func (g *Google) FailEventWrites(n, count, status int) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.failWrite, g.failLast, g.failStatus = g.eventWrites+n, g.eventWrites+n+count-1, status
+}
+
+// EventRequests is how many requests reached the events endpoints.
+func (g *Google) EventRequests() int {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	return g.eventWrites
+}
+
+// HoldEvents stops the next request to the events endpoints before it is
+// answered. It returns a channel closed when the request arrives and the
+// function that lets it go on.
+func (g *Google) HoldEvents() (arrived <-chan struct{}, release func()) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.eventHold = newHold()
+	return g.eventHold.arrived, g.eventHold.releaseFunc()
+}
+
+// TokenCalls is how many requests, exchanges and refreshes, 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
+	if f.Get("grant_type") == "refresh_token" {
+		g.refresh(w, f)
+		return
+	}
+	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)
+}
+
+// refresh answers a refresh_token grant: an access token for a live refresh
+// token, invalid_grant quoting the token for any other.
+func (g *Google) refresh(w http.ResponseWriter, f url.Values) {
+	token := f.Get("refresh_token")
+	switch {
+	case f.Get("client_id") != ClientID || f.Get("client_secret") != ClientSecret:
+		oauthError(w, http.StatusUnauthorized, "invalid_client")
+		return
+	case !slices.Contains(g.issued, token) || g.dead[token]:
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusBadRequest)
+		fmt.Fprintf(w, `{"error":"invalid_grant","error_description":"Token has been expired or revoked: %s"}`, token)
+		return
+	case g.tokenStatus != http.StatusOK:
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(g.tokenStatus)
+		fmt.Fprintf(w, `{"error":"server_error","error_description":"echo %s %s"}`, token, ClientSecret)
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	fmt.Fprintf(w, `{"access_token":%q,"expires_in":3599,"scope":%q,"token_type":"Bearer"}`, AccessToken, g.scope)
+}
+
+// calendarList answers like Google's calendarList.list: the scripted
+// calendars, a page at a time, to a live grant's access token.
+func (g *Google) calendarList(w http.ResponseWriter, r *http.Request) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	g.calendarCalls++
+	h := g.calendarHold
+	g.calendarHold = nil
+	g.wait(h)
+	auth := r.Header.Get("Authorization")
+	if auth != "Bearer "+AccessToken || !g.grantLive() || g.rejectAccess {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusUnauthorized)
+		fmt.Fprintf(w, `{"error":{"code":401,"message":"Invalid Credentials: %s"}}`, auth)
+		return
+	}
+	if status := g.calendarStatus; status != http.StatusOK {
+		if g.calendarFails > 0 {
+			g.calendarFails--
+			if g.calendarFails == 0 {
+				g.calendarStatus = http.StatusOK
+			}
+		}
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(status)
+		fmt.Fprintf(w, `{"error":{"code":%d,"message":"echo %s"}}`, status, auth)
+		return
+	}
+	start := 0
+	if pt := r.URL.Query().Get("pageToken"); pt != "" {
+		n, err := strconv.Atoi(strings.TrimPrefix(pt, "page-"))
+		if err != nil || n <= 0 || n > len(g.calendars) {
+			http.Error(w, "bad pageToken", http.StatusBadRequest)
+			return
+		}
+		start = n
+	}
+	end := len(g.calendars)
+	if g.pageSize > 0 {
+		end = min(start+g.pageSize, end)
+	}
+	items := []map[string]any{}
+	for _, c := range g.calendars[start:end] {
+		item := map[string]any{
+			"kind":            "calendar#calendarListEntry",
+			"etag":            `"1700000000000000"`,
+			"id":              c.ID,
+			"summary":         c.Summary,
+			"description":     c.Description,
+			"timeZone":        "America/New_York",
+			"colorId":         "14",
+			"backgroundColor": "#9fe1e7",
+			"foregroundColor": "#000000",
+			"selected":        true,
+			"accessRole":      c.AccessRole,
+			"defaultReminders": []map[string]any{
+				{"method": "popup", "minutes": 30},
+			},
+			"conferenceProperties": map[string]any{"allowedConferenceSolutionTypes": []string{"hangoutsMeet"}},
+		}
+		if c.Primary {
+			item["primary"] = true
+		}
+		if c.SummaryOverride != "" {
+			item["summaryOverride"] = c.SummaryOverride
+		}
+		items = append(items, item)
+	}
+	page := map[string]any{"kind": "calendar#calendarList", "etag": `"p33k"`, "items": items}
+	if end < len(g.calendars) {
+		page["nextPageToken"] = fmt.Sprintf("page-%d", end)
+	} else {
+		page["nextSyncToken"] = "sync-token"
+	}
+	w.Header().Set("Content-Type", "application/json")
+	if err := json.NewEncoder(w).Encode(page); err != nil {
+		panic(err)
+	}
+}
+
+// insertEvent answers like Google's events.insert with a client-chosen id.
+func (g *Google) insertEvent(w http.ResponseWriter, r *http.Request) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	cal, ok := g.eventCalendar(w, r)
+	if !ok {
+		return
+	}
+	var e Event
+	if err := json.NewDecoder(r.Body).Decode(&e); err != nil || e.ID == "" {
+		http.Error(w, "bad event", http.StatusBadRequest)
+		return
+	}
+	if _, exists := g.events[cal][e.ID]; exists {
+		apiError(w, http.StatusConflict, "duplicate", "The requested identifier already exists.")
+		return
+	}
+	if g.events[cal] == nil {
+		g.events[cal] = map[string]Event{}
+	}
+	g.events[cal][e.ID] = e
+	g.inserts++
+	writeEvent(w, e)
+}
+
+// updateEvent answers like Google's events.update.
+func (g *Google) updateEvent(w http.ResponseWriter, r *http.Request) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	cal, ok := g.eventCalendar(w, r)
+	if !ok {
+		return
+	}
+	id := r.PathValue("event")
+	if _, exists := g.events[cal][id]; !exists {
+		apiError(w, http.StatusNotFound, "notFound", "Not Found")
+		return
+	}
+	var e Event
+	if err := json.NewDecoder(r.Body).Decode(&e); err != nil || e.ID != id {
+		http.Error(w, "bad event", http.StatusBadRequest)
+		return
+	}
+	g.events[cal][id] = e
+	g.updates++
+	writeEvent(w, e)
+}
+
+// eventCalendar runs the checks every events request shares: the hold, a
+// live grant's access token, a scripted failure, and a listed, writable
+// calendar, which it returns. When a check fails it answers and returns
+// false. The caller holds g.mu.
+func (g *Google) eventCalendar(w http.ResponseWriter, r *http.Request) (string, bool) {
+	g.eventWrites++
+	h := g.eventHold
+	g.eventHold = nil
+	g.wait(h)
+	auth := r.Header.Get("Authorization")
+	if auth != "Bearer "+AccessToken || !g.grantLive() || g.rejectAccess {
+		apiError(w, http.StatusUnauthorized, "authError", "Invalid Credentials: "+auth)
+		return "", false
+	}
+	if g.failWrite != 0 && g.eventWrites >= g.failWrite && g.eventWrites <= g.failLast {
+		apiError(w, g.failStatus, "backendError", "echo "+auth)
+		return "", false
+	}
+	cal := r.PathValue("calendar")
+	i := slices.IndexFunc(g.calendars, func(c Calendar) bool { return c.ID == cal })
+	switch {
+	case i < 0:
+		apiError(w, http.StatusNotFound, "notFound", "Not Found")
+		return "", false
+	case g.calendars[i].AccessRole != "writer" && g.calendars[i].AccessRole != "owner":
+		apiError(w, http.StatusForbidden, "requiredAccessLevel", "You need to have writer access to this calendar.")
+		return "", false
+	}
+	return cal, true
+}
+
+// writeEvent answers with e as Google does, with fields of its own.
+func writeEvent(w http.ResponseWriter, e Event) {
+	w.Header().Set("Content-Type", "application/json")
+	if err := json.NewEncoder(w).Encode(map[string]any{
+		"kind": "calendar#event", "etag": `"3300000000000000"`, "id": e.ID, "status": e.Status,
+		"htmlLink": "https://www.google.com/calendar/event?eid=" + e.ID,
+		"summary":  e.Summary, "description": e.Description, "start": e.Start, "end": e.End,
+		"iCalUID": e.ID + "@google.com", "sequence": 0,
+	}); err != nil {
+		panic(err)
+	}
+}
+
+// apiError answers a Calendar API error with reason and message.
+func apiError(w http.ResponseWriter, status int, reason, message string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if err := json.NewEncoder(w).Encode(map[string]any{"error": map[string]any{
+		"code": status, "message": message,
+		"errors": []map[string]any{{"domain": "global", "reason": reason, "message": message}},
+	}}); err != nil {
+		panic(err)
+	}
+}
+
+// grantLive reports whether any refresh token issued is still live. The
+// caller holds g.mu.
+func (g *Google) grantLive() bool {
+	for _, t := range g.issued {
+		if !g.dead[t] {
+			return true
+		}
+	}
+	return false
+}
+
+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[:])
+}