Warp/server/internal/roomserver/visibility.go

package roomserver

import (
	"context"
	"fmt"

	"github.com/dariuszjaniak/warp/server/internal/matrix/event"
	"github.com/dariuszjaniak/warp/server/internal/matrix/roomversion"
)

// VisibleRange is the span of a room's event stream a user may read.
//
// It is a range rather than a yes or no because membership is not a yes or
// no either. A user who has left a room keeps what they saw while they were
// in it and sees nothing after their leave; a user who joined a room whose
// history visibility is `joined` sees nothing from before their join. Both
// are the same question — where does this user's window start and end — and
// answering it once per room is what lets a timeline read be a single
// bounded query rather than a filter applied to every event in it.
type VisibleRange struct {
	// Allowed is false when the user may see nothing of the room at all.
	Allowed bool

	// From is the first position the user may see, exclusive. Zero means
	// the start of the room.
	From int64

	// To is the last position the user may see, inclusive. Zero means the
	// top of the stream, and goes on meaning that as the stream grows.
	To int64

	// Membership is the user's current membership, empty for a user who was
	// never in the room.
	Membership string

	// HistoryVisibility is the room's current history visibility, which the
	// bounds above were derived from.
	HistoryVisibility string
}

// Contains reports whether a position is inside the range.
func (v VisibleRange) Contains(pos int64) bool {
	switch {
	case !v.Allowed:
		return false
	case pos <= v.From:
		return false
	case v.To != 0 && pos > v.To:
		return false
	}
	return true
}

// Clamp narrows a caller's window to what the range allows, reporting
// whether anything is left of it.
//
// The bounds are exclusive at the bottom and inclusive at the top, the same
// as a storage.TimelineRequest, so the result can be handed straight to
// one.
func (v VisibleRange) Clamp(from, to int64) (int64, int64, bool) {
	if !v.Allowed {
		return 0, 0, false
	}
	from = max(from, v.From)
	if v.To != 0 && (to == 0 || to > v.To) {
		to = v.To
	}
	if to != 0 && from >= to {
		return 0, 0, false
	}
	return from, to, true
}

// VisibleRange works out what a user may read of a room.
//
// The rules are the spec's history visibility:
//
//   - world_readable: everything, to anybody, member or not.
//   - shared: everything, to anybody who is or has been a member.
//   - invited: from the user's invite onwards.
//   - joined: from the user's join onwards.
//
// A user who has left or was banned is bounded at their own membership
// event in every case, which is what stops a kicked user's client from
// going on streaming the room. An invitee is told enough to decide — the
// stripped state that travels with the invite — and no timeline.
//
// The approximation here is the room's *current* history visibility. The
// spec judges each event by the visibility in force when it was sent, so a
// room that was once world_readable and is now `joined` hides history here
// that the spec would show, and the reverse shows history it would hide.
// Doing it exactly means resolving the history visibility at every event in
// the window — a state read per event — and belongs with the state storage
// work of PLAN.md open question 1. Until then this follows the setting the
// room's members last agreed on, which is the one they would expect to
// apply.
func (r *API) VisibleRange(ctx context.Context, roomID, userID string) (VisibleRange, error) {
	room, rules, err := r.roomAndRules(ctx, roomID)
	if err != nil {
		return VisibleRange{}, err
	}
	summary, err := r.store.RoomSummary(ctx, room.ID)
	if err != nil {
		return VisibleRange{}, err
	}
	return r.visibleRange(ctx, roomID, rules, userID, summary.HistoryVisibility)
}

// visibleRange is VisibleRange with the room's history visibility already
// in hand, for the callers — a sync over many rooms — that have it.
func (r *API) visibleRange(ctx context.Context, roomID string, rules roomversion.Rules, userID, historyVisibility string) (VisibleRange, error) {
	out := VisibleRange{HistoryVisibility: historyVisibility}

	changes, err := r.membershipHistory(ctx, roomID, rules, userID)
	if err != nil {
		return VisibleRange{}, err
	}
	if len(changes) > 0 {
		out.Membership = changes[len(changes)-1].membership
	}

	worldReadable := historyVisibility == HistoryVisibilityWorldReadable
	switch out.Membership {
	case event.MembershipJoin:
		out.Allowed = true
	case event.MembershipLeave, event.MembershipBan:
		out.Allowed = true
		// Bounded at their own membership event: what they saw stays
		// theirs, what happened afterwards does not.
		out.To = changes[len(changes)-1].position
	default:
		// Invited, knocking, or never a member. Only a world-readable room
		// has anything to show them.
		out.Allowed = worldReadable
	}

	// World-readable removes the floor — they could read the room as a
	// stranger, so there is nothing to gain by hiding it from them as a
	// member — but not the ceiling: a leaver's *membership* view still ends
	// where they left, and the stranger's view is what /messages on a
	// world-readable room gives them anyway.
	if !worldReadable && out.Allowed {
		switch historyVisibility {
		case HistoryVisibilityInvited, HistoryVisibilityJoined:
			out.From = spanStart(changes, historyVisibility)
		}
	}
	return out, nil
}

// membershipChange is one of a user's membership events in a room.
type membershipChange struct {
	membership string
	position   int64
}

// membershipHistory reads a user's membership events in a room, oldest
// first.
func (r *API) membershipHistory(ctx context.Context, roomID string, rules roomversion.Rules, userID string) ([]membershipChange, error) {
	stored, err := r.store.UserMembershipEvents(ctx, roomID, userID)
	if err != nil {
		return nil, err
	}
	out := make([]membershipChange, 0, len(stored))
	for _, s := range stored {
		e, err := parseEvent(s, rules)
		if err != nil {
			return nil, err
		}
		membership, err := e.Membership()
		if err != nil {
			// A member event with no membership could not have been
			// accepted into the room, so this is a corrupted row rather
			// than something a client sent. Skipping it would widen the
			// window silently.
			return nil, fmt.Errorf("member event %s of %s has no membership: %w", s.EventID, roomID, err)
		}
		out = append(out, membershipChange{membership: membership, position: s.StreamOrdering})
	}
	return out, nil
}

// spanStart is the floor of the user's current membership span, exclusive.
//
// For `joined` visibility that is the join that began the span they are in
// — or, for a user who has left, the one that began the span they left
// from. For `invited` it is the invite that preceded that join, because an
// invitee is entitled to the room from the moment they were asked.
//
// A user who joined, left and rejoined therefore sees their latest span and
// not the earlier ones, which is what the spec's per-event rule works out
// to for everything after their last join.
func spanStart(changes []membershipChange, historyVisibility string) int64 {
	// Walk back to the membership event that opened the current span.
	start := len(changes) - 1
	for start > 0 && changes[start].membership != event.MembershipJoin {
		start--
	}
	for start > 0 && changes[start-1].membership == event.MembershipJoin {
		// Consecutive joins are profile changes, not rejoins; the span
		// began at the first of them.
		start--
	}
	if historyVisibility == HistoryVisibilityInvited &&
		start > 0 && changes[start-1].membership == event.MembershipInvite {
		start--
	}
	// Exclusive, so one below the event itself: the membership event is the
	// first thing the user sees, which is how a client renders "you joined".
	return changes[start].position - 1
}

// requireReadable refuses a caller who may see nothing of a room, and
// returns the range for one who may.
func (r *API) requireReadable(ctx context.Context, roomID, userID string) (VisibleRange, error) {
	visible, err := r.VisibleRange(ctx, roomID, userID)
	if err != nil {
		return VisibleRange{}, err
	}
	return visible, visible.refusal(roomID, userID)
}

// refusal turns a range that allows nothing into the error a client is
// given for it.
func (v VisibleRange) refusal(roomID, userID string) error {
	if v.Allowed {
		return nil
	}
	if v.Membership == event.MembershipInvite {
		return fmt.Errorf("%w: %s has been invited to %s but has not joined it",
			ErrForbidden, userID, roomID)
	}
	return fmt.Errorf("%w: %s is not in %s", ErrForbidden, userID, roomID)
}