nidus/internal/caldav/backend.go

package caldav

import (
	"context"
	"fmt"
	"log/slog"
	"net/http"
	"path"
	"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/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
}

// 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}
}

// NewHandler returns an http.Handler for the /cal/ prefix.
func NewHandler(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger) http.Handler {
	b := NewBackend(cfg, st, dbase, logger)
	return &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"))
	}

	user, ok := b.cfg.Users[p.Username]
	if !ok {
		return nil, fmt.Errorf("user not found")
	}

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

	// Also include any extra calendars that exist on disk but aren't in config
	disk, _ := b.store.ListCollections(p.Username)
	configured := make(map[string]bool)
	for _, n := range user.Calendars {
		configured["cal-"+n] = 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)
			cals = append(cals, b.calendarMeta(sh.Owner, sh.CalendarName, localName))
		}
	}

	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
	}
	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)
	}
	cal := b.calendarMeta(owner, realName, localName)
	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
	}
	owner, realName, _, err := b.resolveCalendar(requester, localName, false)
	if err != nil {
		return nil, err
	}

	data, err := b.store.GetObject(owner, "cal-"+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
	}
	owner, realName, _, err := b.resolveCalendar(requester, localName, false)
	if err != nil {
		return nil, err
	}

	ids, err := b.store.ListObjects(owner, "cal-"+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, "/"))
	return b.store.EnsureCollection(p.Username, "cal-"+name)
}

func (b *Backend) DeleteCalendar(ctx context.Context, calPath string) error {
	requester, localName, err := b.parseCalPath(ctx, calPath)
	if err != nil {
		return err
	}
	owner, realName, _, err := b.resolveCalendar(requester, localName, true)
	if err != nil {
		return err
	}
	return b.store.DeleteCollection(owner, "cal-"+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
	}
	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, "cal-"+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
	}
	owner, realName, _, err := b.resolveCalendar(requester, localName, true)
	if err != nil {
		return err
	}
	if err := b.store.DeleteObject(owner, "cal-"+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 {
	desc := fmt.Sprintf("%s's %s calendar", owner, realName)
	return caldav.Calendar{
		Path:                  calHomePath() + localName + "/",
		Name:                  localName,
		Description:           desc,
		SupportedComponentSet: []string{"VEVENT", "VTODO", "VJOURNAL"},
		MaxResourceSize:       10 * 1024 * 1024, // 10 MiB
	}
}

func (b *Backend) decodeObject(objPath string, data []byte) (*caldav.CalendarObject, error) {
	cal, err := ical.NewDecoder(strings.NewReader(string(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
}