nidus/internal/caldav/backend.go

package caldav

import (
	"bytes"
	"context"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"net/http/httptest"
	"path"
	"regexp"
	"strconv"
	"strings"
	"time"

	ical "github.com/emersion/go-ical"
	"github.com/emersion/go-webdav"
	"github.com/emersion/go-webdav/caldav"
	"github.com/yourusername/caldav-server/internal/auth"
	"github.com/yourusername/caldav-server/internal/config"
	"github.com/yourusername/caldav-server/internal/db"
	"github.com/yourusername/caldav-server/internal/icalfix"
	"github.com/yourusername/caldav-server/internal/icssub"
	"github.com/yourusername/caldav-server/internal/store"
)

// sharedNameSep separates the owner from the calendar name in the
// synthetic name used for calendars shared with another user, e.g.
// "alice~work" for alice's "work" calendar as seen by whoever it was
// shared with. It must not collide with characters allowed in real
// calendar names (validated wherever calendar names are taken as input).
const sharedNameSep = "~"

// Backend implements caldav.Backend using a filesystem store.
type Backend struct {
	cfg      *config.Config
	store    *store.Store
	dbase    *db.DB // may be nil if sharing is not configured
	logger   *slog.Logger
	icsCache *icssub.Cache
}

// NewBackend creates a CalDAV backend. dbase may be nil, in which case
// calendar sharing is disabled (only a user's own calendars are visible).
func NewBackend(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger) *Backend {
	return &Backend{cfg: cfg, store: st, dbase: dbase, logger: logger, icsCache: icssub.NewCache(icssub.DefaultTTL)}
}

// NewHandler returns an http.Handler for the /cal/ prefix.
//
// It wraps the go-webdav caldav.Handler with a small response-rewriting
// middleware that injects the non-standard (Apple/DAVx5) calendar-color
// property into PROPFIND responses for calendar collections, since
// go-webdav's caldav.Backend interface has no extension point for
// vendor-specific WebDAV properties.
func NewHandler(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger) http.Handler {
	b := NewBackend(cfg, st, dbase, logger)
	return &colorInjectingHandler{backend: b, next: &caldav.Handler{Backend: b}}
}

// -------- caldav.Backend interface --------

func (b *Backend) CurrentUserPrincipal(ctx context.Context) (string, error) {
	if auth.FromContext(ctx) == nil {
		return "", webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}
	// Must resolve to a path served by this same handler (i.e. under /cal/)
	// at exactly one path segment of depth, since go-webdav's caldav server
	// classifies resources purely by path depth relative to Handler.Prefix:
	// depth 1 = principal, depth 2 = home-set, depth 3 = calendar, depth 4 =
	// calendar object. A /principals/<user>/ path would never be reached
	// (nothing is mounted there) and would break discovery.
	return calPrincipalPath(), nil
}

func (b *Backend) CalendarHomeSetPath(ctx context.Context) (string, error) {
	if auth.FromContext(ctx) == nil {
		return "", webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}
	return calHomePath(), nil
}

func (b *Backend) ListCalendars(ctx context.Context) ([]caldav.Calendar, error) {
	p := auth.FromContext(ctx)
	if p == nil {
		return nil, webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}

	// The synthetic Birthdays calendar is always present, computed from
	// the requester's own contacts.
	names, err := b.dbase.ListCalendars(p.Username)
	if err != nil {
		return nil, fmt.Errorf("listing calendars: %w", err)
	}

	cals := []caldav.Calendar{b.birthdaysCalendarMeta(p.Username)}
	for _, cal := range names {
		if err := b.store.EnsureCollection(p.Username, "cal-"+cal.Name); err != nil {
			b.logger.Warn("ensuring calendar directory", "calendar", cal.Name, "error", err)
			continue
		}
		cals = append(cals, b.calendarMeta(p.Username, cal.Name, cal.Name))
	}

	// Also include the requester's own read-only ICS/webcal subscriptions.
	subs, err := b.dbase.ListICSSubscriptions(p.Username)
	if err != nil {
		b.logger.Warn("listing ics subscriptions", "error", err)
	}
	for _, sub := range subs {
		cals = append(cals, b.icsSubscriptionCalendarMeta(p.Username, sub))
	}

	// Also include any extra calendars that exist on disk but aren't registered
	disk, _ := b.store.ListCollections(p.Username)
	configured := make(map[string]bool)
	for _, cal := range names {
		configured["calendars/"+cal.Name] = true
	}
	for _, dir := range disk {
		if strings.HasPrefix(dir, "cal-") && !configured[dir] {
			name := strings.TrimPrefix(dir, "cal-")
			cals = append(cals, b.calendarMeta(p.Username, name, name))
		}
	}

	// Include calendars other users have shared with this one.
	if b.dbase != nil {
		shares, err := b.dbase.CalendarsSharedWith(p.Username)
		if err != nil {
			b.logger.Warn("listing shared calendars", "error", err)
		}
		for _, sh := range shares {
			if _, err := b.store.GetCollection(sh.Owner, "cal-"+sh.CalendarName); err != nil {
				continue // owner's calendar no longer exists
			}
			localName := sharedCalendarName(sh.Owner, sh.CalendarName)
			displayName := b.sharedDisplayName(p.Username, sh.Owner, sh.CalendarName)
			cals = append(cals, b.calendarMetaNamed(sh.Owner, sh.CalendarName, localName, displayName))
		}
	}

	return cals, nil
}

func (b *Backend) GetCalendar(ctx context.Context, calPath string) (*caldav.Calendar, error) {
	requester, localName, err := b.parseCalPath(ctx, calPath)
	if err != nil {
		return nil, err
	}
	if localName == birthdaysCalendarName {
		cal := b.birthdaysCalendarMeta(requester)
		return &cal, nil
	}
	if sub, err := b.dbase.GetICSSubscription(requester, localName); err == nil {
		cal := b.icsSubscriptionCalendarMeta(requester, sub)
		return &cal, nil
	}
	owner, realName, _, err := b.resolveCalendar(requester, localName, false)
	if err != nil {
		return nil, err
	}
	if _, err := b.store.GetCollection(owner, "cal-"+realName); err != nil {
		return nil, webdav.NewHTTPError(http.StatusNotFound, err)
	}
	displayName := realName
	if owner != requester {
		displayName = b.sharedDisplayName(requester, owner, realName)
	}
	cal := b.calendarMetaNamed(owner, realName, localName, displayName)
	return &cal, nil
}

func (b *Backend) GetCalendarObject(ctx context.Context, objPath string, req *caldav.CalendarCompRequest) (*caldav.CalendarObject, error) {
	requester, localName, objID, err := b.parseObjPath(ctx, objPath)
	if err != nil {
		return nil, err
	}
	if localName == birthdaysCalendarName {
		return b.birthdayCalendarObject(requester, objPath, objID)
	}
	if sub, err := b.dbase.GetICSSubscription(requester, localName); err == nil {
		return b.icsSubscriptionCalendarObject(localName, objID, sub)
	}
	owner, realName, _, err := b.resolveCalendar(requester, localName, false)
	if err != nil {
		return nil, err
	}

	data, err := b.store.GetObject(owner, "calendars/"+realName, objID)
	if err != nil {
		return nil, webdav.NewHTTPError(http.StatusNotFound, err)
	}

	return b.decodeObject(objPath, data)
}

func (b *Backend) ListCalendarObjects(ctx context.Context, calPath string, req *caldav.CalendarCompRequest) ([]caldav.CalendarObject, error) {
	requester, localName, err := b.parseCalPath(ctx, calPath)
	if err != nil {
		return nil, err
	}
	if localName == birthdaysCalendarName {
		return b.listBirthdayCalendarObjects(requester)
	}
	if sub, err := b.dbase.GetICSSubscription(requester, localName); err == nil {
		return b.listICSSubscriptionCalendarObjects(localName, sub)
	}
	owner, realName, _, err := b.resolveCalendar(requester, localName, false)
	if err != nil {
		return nil, err
	}

	ids, err := b.store.ListObjects(owner, "calendars/"+realName)
	if err != nil {
		return nil, err
	}

	var objs []caldav.CalendarObject
	for _, id := range ids {
		data, err := b.store.GetObject(owner, "cal-"+realName, id)
		if err != nil {
			continue
		}
		obj, err := b.decodeObject(calObjectPath(localName, id), data)
		if err != nil {
			b.logger.Warn("decoding calendar object", "id", id, "error", err)
			continue
		}
		objs = append(objs, *obj)
	}
	return objs, nil
}

func (b *Backend) QueryCalendarObjects(ctx context.Context, calPath string, query *caldav.CalendarQuery) ([]caldav.CalendarObject, error) {
	// List all and filter – sufficient for small collections.
	all, err := b.ListCalendarObjects(ctx, calPath, &query.CompRequest)
	if err != nil {
		return nil, err
	}
	return caldav.Filter(query, all)
}

func (b *Backend) CreateCalendar(ctx context.Context, calendar *caldav.Calendar) error {
	p := auth.FromContext(ctx)
	if p == nil {
		return webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}
	// Derive collection name from the trailing path segment. Calendars are
	// always created under the acting user's own namespace — sharing an
	// existing calendar is done via ShareCalendar, not by creating one
	// directly in someone else's name.
	name := path.Base(strings.TrimSuffix(calendar.Path, "/"))
	if err := b.dbase.CreateCalendar(p.Username, name); err != nil {
		if err == db.ErrReservedName {
			return webdav.NewHTTPError(http.StatusForbidden, err)
		}
		if err != db.ErrResourceExists {
			return fmt.Errorf("registering calendar: %w", err)
		}
	}
	return b.store.EnsureCollection(p.Username, "calendars/"+name)
}

func (b *Backend) DeleteCalendar(ctx context.Context, calPath string) error {
	requester, localName, err := b.parseCalPath(ctx, calPath)
	if err != nil {
		return err
	}
	if localName == birthdaysCalendarName {
		return webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("the birthdays calendar is read-only and computed automatically"))
	}
	// Unlike the birthdays calendar, an ICS subscription can be deleted:
	// that's how a user unsubscribes from it via CalDAV.
	if err := b.dbase.DeleteICSSubscription(requester, localName); err == nil {
		return nil
	} else if err != db.ErrResourceNotFound {
		return fmt.Errorf("unregistering ics subscription: %w", err)
	}
	owner, realName, _, err := b.resolveCalendar(requester, localName, true)
	if err != nil {
		return err
	}
	if err := b.dbase.DeleteCalendar(owner, realName); err != nil && err != db.ErrResourceNotFound {
		return fmt.Errorf("unregistering calendar: %w", err)
	}
	return b.store.DeleteCollection(owner, "calendars/"+realName)
}

func (b *Backend) PutCalendarObject(ctx context.Context, objPath string, calendar *ical.Calendar, opts *caldav.PutCalendarObjectOptions) (*caldav.CalendarObject, error) {
	requester, localName, objID, err := b.parseObjPath(ctx, objPath)
	if err != nil {
		return nil, err
	}
	if localName == birthdaysCalendarName {
		return nil, webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("the birthdays calendar is read-only and computed automatically"))
	}
	if _, err := b.dbase.GetICSSubscription(requester, localName); err == nil {
		return nil, webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("this calendar is a read-only ics/webcal subscription"))
	}
	owner, realName, _, err := b.resolveCalendar(requester, localName, true)
	if err != nil {
		return nil, err
	}

	var buf strings.Builder
	enc := ical.NewEncoder(&buf)
	if err := enc.Encode(calendar); err != nil {
		return nil, fmt.Errorf("encoding calendar: %w", err)
	}

	data := []byte(buf.String())
	if err := b.store.PutObject(owner, "calendars/"+realName, objID, data); err != nil {
		return nil, fmt.Errorf("storing calendar object: %w", err)
	}

	return b.decodeObject(objPath, data)
}

func (b *Backend) DeleteCalendarObject(ctx context.Context, objPath string) error {
	requester, localName, objID, err := b.parseObjPath(ctx, objPath)
	if err != nil {
		return err
	}
	if localName == birthdaysCalendarName {
		return webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("the birthdays calendar is read-only and computed automatically"))
	}
	if _, err := b.dbase.GetICSSubscription(requester, localName); err == nil {
		return webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("this calendar is a read-only ics/webcal subscription"))
	}
	owner, realName, _, err := b.resolveCalendar(requester, localName, true)
	if err != nil {
		return err
	}
	if err := b.store.DeleteObject(owner, "calendars/"+realName, objID); err != nil {
		return webdav.NewHTTPError(http.StatusNotFound, err)
	}
	return nil
}

// -------- helpers --------

// sharedCalendarName builds the synthetic local name a shared calendar is
// exposed under to the user it was shared with.
func sharedCalendarName(owner, calName string) string {
	return owner + sharedNameSep + calName
}

// resolveCalendar maps a local calendar name (as seen in a URL path by
// requester) to its real owner and on-disk name, checking permissions
// along the way. If localName contains sharedNameSep, it's treated as a
// reference to another user's calendar and looked up in the shares table;
// otherwise it's assumed to be one of requester's own calendars.
//
// If requireWrite is true, a share must grant PermWrite or this returns a
// 403 Forbidden error. Owners always have full access to their own
// calendars.
func (b *Backend) resolveCalendar(requester, localName string, requireWrite bool) (owner, realName string, perm db.Permission, err error) {
	if ownerName, calName, ok := strings.Cut(localName, sharedNameSep); ok {
		if b.dbase == nil {
			return "", "", "", webdav.NewHTTPError(http.StatusNotFound, fmt.Errorf("sharing not enabled"))
		}
		share, err := b.dbase.CalendarShareFor(ownerName, calName, requester)
		if err != nil {
			return "", "", "", webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("calendar not shared with you"))
		}
		if requireWrite && share.Permission != db.PermWrite {
			return "", "", "", webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("read-only share"))
		}
		return ownerName, calName, share.Permission, nil
	}
	return requester, localName, db.PermWrite, nil
}

func (b *Backend) calendarMeta(owner, realName, localName string) caldav.Calendar {
	return b.calendarMetaNamed(owner, realName, localName, realName)
}

// calendarMetaNamed is calendarMeta with an explicit displayName, used for
// a calendar shared with the requester whose name collides with one of
// their own calendars (see sharedDisplayName) — the Description still
// always names the real calendar, only the client-visible Name changes.
func (b *Backend) calendarMetaNamed(owner, realName, localName, displayName string) caldav.Calendar {
	desc := fmt.Sprintf("%s's %s calendar", b.dbase.DisplayName(owner), realName)
	return caldav.Calendar{
		Path: calHomePath() + localName + "/",
		// The displayed name is always the plain calendar name (never the
		// "owner~name" synthetic local name used internally to keep a
		// shared calendar's URL unique) — clients like DAVx5 show this as
		// the calendar's label, and "alice~work" looked confusing there.
		Name:                  displayName,
		Description:           desc,
		SupportedComponentSet: []string{"VEVENT", "VTODO", "VJOURNAL"},
		MaxResourceSize:       10 * 1024 * 1024, // 10 MiB
	}
}

// sharedDisplayName returns realName, suffixed with the owning user's
// name in parentheses (e.g. "Work (bob)") if requester has another
// calendar with the exact same display name — either one of their own,
// or another share from a different owner — so DAVx5 (and other clients)
// don't show two calendars with an identical, indistinguishable title.
// Only called for calendars shared with requester; requester's own
// calendars never need disambiguating against themselves.
func (b *Backend) sharedDisplayName(requester, owner, realName string) string {
	own, err := b.dbase.ListCalendars(requester)
	if err != nil {
		b.logger.Warn("listing own calendars for disambiguation", "error", err)
	}
	ownerLabel := b.dbase.DisplayName(owner)
	for _, c := range own {
		if c.Name == realName {
			return fmt.Sprintf("%s (%s)", realName, ownerLabel)
		}
	}

	shares, err := b.dbase.CalendarsSharedWith(requester)
	if err != nil {
		b.logger.Warn("listing shared calendars for disambiguation", "error", err)
	}
	for _, sh := range shares {
		if sh.Owner == owner && sh.CalendarName == realName {
			continue // the calendar itself, not a collision
		}
		if sh.CalendarName == realName {
			return fmt.Sprintf("%s (%s)", realName, ownerLabel)
		}
	}
	return realName
}

func (b *Backend) decodeObject(objPath string, data []byte) (*caldav.CalendarObject, error) {
	// Some clients (Outlook/Exchange, some Thunderbird/Lightning setups)
	// write Windows timezone names into TZID instead of IANA ones, which
	// go-ical can't resolve. A recurring event with such a TZID decodes
	// fine here but fails later, as a 500, the moment a CalDAV client
	// issues a time-range query that expands its recurrence — so fix it
	// up before decoding rather than only in QueryCalendarObjects.
	cal, err := ical.NewDecoder(bytes.NewReader(icalfix.NormalizeTimeZones(data))).Decode()
	if err != nil {
		return nil, fmt.Errorf("decoding ical: %w", err)
	}

	etag := fmt.Sprintf(`"%x"`, hashBytes(data))

	return &caldav.CalendarObject{
		Path:          objPath,
		ModTime:       time.Now(),
		ContentLength: int64(len(data)),
		ETag:          etag,
		Data:          cal,
	}, nil
}

func (b *Backend) parseCalPath(ctx context.Context, calPath string) (user, calName string, err error) {
	p := auth.FromContext(ctx)
	if p == nil {
		return "", "", webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}
	user = p.Username
	parts := strings.Split(strings.Trim(calPath, "/"), "/")
	// expected: cal/home/<calname>/
	if len(parts) < 3 {
		return "", "", webdav.NewHTTPError(http.StatusBadRequest, fmt.Errorf("invalid calendar path"))
	}
	calName = parts[2]
	return user, calName, nil
}

func (b *Backend) parseObjPath(ctx context.Context, objPath string) (user, calName, objID string, err error) {
	p := auth.FromContext(ctx)
	if p == nil {
		return "", "", "", webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}
	user = p.Username
	parts := strings.Split(strings.Trim(objPath, "/"), "/")
	// expected: cal/home/<calname>/<objid>
	if len(parts) < 4 {
		return "", "", "", webdav.NewHTTPError(http.StatusBadRequest, fmt.Errorf("invalid object path"))
	}
	calName = parts[2]
	objID = path.Base(objPath)
	return user, calName, objID, nil
}

// calPrincipalPath, calHomePath, and calObjectPath are the same for every
// user: authorization is resolved from the Basic Auth identity, not from
// the URL, so no username segment is needed in the path.
//
// The fixed "home" segment (in place of a username) is required, not
// cosmetic: go-webdav's caldav server classifies a request purely by how
// many path segments it has relative to the handler's mount point — 1
// segment is treated as the principal, 2 as the calendar-home-set, 3 as a
// calendar, 4 as a calendar object. Removing that segment entirely would
// make the home-set and calendar paths misclassified as principal/home-set
// respectively, breaking discovery (empty PROPFIND responses).
func calPrincipalPath() string {
	return "/cal/"
}

func calHomePath() string {
	return "/cal/home/"
}

func calObjectPath(calName, objID string) string {
	return fmt.Sprintf("/cal/home/%s/%s", calName, objID)
}

func hashBytes(data []byte) uint64 {
	var h uint64 = 14695981039346656037
	for _, b := range data {
		h ^= uint64(b)
		h *= 1099511628211
	}
	return h
}

// -------- calendar-color PROPFIND injection --------

// calCollectionPathRe matches a calendar collection's own path (e.g.
// "/cal/home/work/"), as opposed to a calendar object inside it (e.g.
// "/cal/home/work/abc123.ics") or the home-set/principal path.
var calCollectionPathRe = regexp.MustCompile(`^/cal/home/[^/]+/$`)

// colorInjectingHandler wraps a caldav.Handler and post-processes PROPFIND
// responses to add the non-standard `calendar-color` property (in Apple's
// "http://apple.com/ns/ical/" namespace) that DAVx5 and other clients read
// to color-code synced calendars. go-webdav's caldav.Backend interface has
// no extension point for vendor properties like this, so the response XML
// is rewritten after the fact instead.
type colorInjectingHandler struct {
	backend *Backend
	next    http.Handler
}

func (h *colorInjectingHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	if r.Method != "PROPFIND" || h.backend.dbase == nil {
		h.next.ServeHTTP(w, r)
		return
	}

	// Only bother rewriting if the client asked for calendar-color
	// specifically, or for all properties (propname/allprop requests, or
	// no body at all, are treated as "all properties" by most clients).
	var reqBody []byte
	if r.Body != nil {
		reqBody, _ = readAllAndReset(&r.Body)
	}
	wantsColor := len(reqBody) == 0 || bytes.Contains(reqBody, []byte("calendar-color")) || bytes.Contains(reqBody, []byte("allprop"))
	if !wantsColor {
		h.next.ServeHTTP(w, r)
		return
	}

	rec := httptest.NewRecorder()
	h.next.ServeHTTP(rec, r)

	for k, vs := range rec.Header() {
		for _, v := range vs {
			w.Header().Add(k, v)
		}
	}

	body := rec.Body.Bytes()
	if rec.Code == http.StatusMultiStatus && strings.Contains(rec.Header().Get("Content-Type"), "xml") {
		body = h.injectCalendarColors(r.Context(), body)
		w.Header().Set("Content-Length", strconv.Itoa(len(body)))
	}
	w.WriteHeader(rec.Code)
	_, _ = w.Write(body)
}

// responseBlockRe matches a single <response>...</response> element
// (non-greedy) inside a multistatus document, capturing its href.
var responseBlockRe = regexp.MustCompile(`(?s)<response[^>]*>.*?<href[^>]*>([^<]*)</href>.*?</response>`)

// propstatBlockRe matches a single <propstat>...</propstat> element
// within a response, non-greedily.
var propstatBlockRe = regexp.MustCompile(`(?s)<propstat[^>]*>.*?</propstat>`)

// okPropRe matches the first <prop xmlns="DAV:"> opening tag inside a
// 200 OK propstat block, used to find where to insert new property XML.
var okPropRe = regexp.MustCompile(`<prop xmlns="DAV:">`)

// injectCalendarColors scans a multistatus PROPFIND response body and, for
// each <response> whose href is a calendar collection with a color set,
// inserts a <calendar-color xmlns="http://apple.com/ns/ical/"> element
// into its first 200 OK <prop>. Since go-webdav's stock property map
// doesn't know this property, a client that explicitly asks for it (as
// DAVx5 does) gets back a 404 propstat for it — that bogus 404 entry is
// stripped first, since leaving both a 404 and our injected 200 for the
// same property name in one <response> is invalid multistatus and
// confuses clients (dav4jvm/DAVx5 was observed to keep showing no color
// when both were present).
func (h *colorInjectingHandler) injectCalendarColors(ctx context.Context, body []byte) []byte {
	p := auth.FromContext(ctx)
	if p == nil {
		return body
	}

	return responseBlockRe.ReplaceAllFunc(body, func(block []byte) []byte {
		m := responseBlockRe.FindSubmatch(block)
		if m == nil {
			return block
		}
		href := string(m[1])
		if !calCollectionPathRe.MatchString(href) {
			return block
		}
		localName := strings.TrimSuffix(strings.TrimPrefix(href, calHomePath()), "/")
		var color string
		var err error
		if localName == birthdaysCalendarName {
			color, err = h.backend.dbase.GetBirthdayCalendarColor(p.Username)
			if err != nil {
				color = ""
			}
			if color == "" {
				color = defaultBirthdayColor
			}
		} else if sub, serr := h.backend.dbase.GetICSSubscription(p.Username, localName); serr == nil {
			color = sub.Color
			if color == "" {
				color = defaultICSColor
			}
		} else {
			owner, realName, _, rerr := h.backend.resolveCalendar(p.Username, localName, false)
			if rerr != nil {
				return block
			}
			color, err = h.backend.dbase.GetCalendarColor(owner, realName)
			if err != nil || color == "" {
				return block
			}
		}

		// Drop any propstat block that only complains calendar-color is
		// unknown (a 404/not-found propstat containing a bare, empty
		// calendar-color element), from either namespace clients might
		// have queried it in.
		block = propstatBlockRe.ReplaceAllFunc(block, func(ps []byte) []byte {
			if bytes.Contains(ps, []byte("calendar-color")) && !bytes.Contains(ps, []byte("200 OK")) {
				return nil
			}
			return ps
		})

		idx := okPropRe.FindIndex(block)
		if idx == nil {
			return block
		}
		insertAt := idx[1]
		colorEl := []byte(fmt.Sprintf(`<calendar-color xmlns="http://apple.com/ns/ical/">%s</calendar-color>`, xmlEscapeColor(color)))
		out := make([]byte, 0, len(block)+len(colorEl))
		out = append(out, block[:insertAt]...)
		out = append(out, colorEl...)
		out = append(out, block[insertAt:]...)
		return out
	})
}

// xmlEscapeColor returns color formatted as an 8-digit ARGB/RGBA hex value
// (as Apple's calendar-color property expects), padding a plain 6-digit
// "#RRGGBB" (as produced by an HTML <input type="color">) with a fully
// opaque alpha channel.
func xmlEscapeColor(color string) string {
	if len(color) == 7 && color[0] == '#' {
		return color + "FF"
	}
	return color
}

// readAllAndReset reads body fully and replaces it with a fresh reader over
// the same bytes, so downstream handlers can still consume it.
func readAllAndReset(body *io.ReadCloser) ([]byte, error) {
	data, err := io.ReadAll(*body)
	if err != nil {
		return nil, err
	}
	*body = io.NopCloser(bytes.NewReader(data))
	return data, nil
}