dendrite/roomserver/storage/shared/storage.go

package shared

import (
	"context"
	"database/sql"
	"encoding/json"
	"fmt"

	"github.com/matrix-org/dendrite/internal/caching"
	"github.com/matrix-org/dendrite/internal/sqlutil"
	"github.com/matrix-org/dendrite/roomserver/api"
	"github.com/matrix-org/dendrite/roomserver/storage/tables"
	"github.com/matrix-org/dendrite/roomserver/types"
	"github.com/matrix-org/gomatrixserverlib"
	"github.com/tidwall/gjson"
)

// Ideally, when we have both events we should redact the event JSON and forget about the redaction, but we currently
// don't because the redaction code is brand new. When we are more certain that redactions don't misbehave or are
// vulnerable to attacks from remote servers (e.g a server bypassing event auth rules shouldn't redact our data)
// then we should flip this to true. This will mean redactions /actually delete information irretrievably/ which
// will be necessary for compliance with the law. Note that downstream components (syncapi) WILL delete information
// in their database on receipt of a redaction. Also note that we still modify the event JSON to set the field
// unsigned.redacted_because - we just don't clear out the content fields yet.
//
// If this hasn't been done by 09/2020 this should be flipped to true.
const redactionsArePermanent = false

type Database struct {
	DB                  *sql.DB
	Cache               caching.RoomServerCaches
	Writer              sqlutil.Writer
	EventsTable         tables.Events
	EventJSONTable      tables.EventJSON
	EventTypesTable     tables.EventTypes
	EventStateKeysTable tables.EventStateKeys
	RoomsTable          tables.Rooms
	TransactionsTable   tables.Transactions
	StateSnapshotTable  tables.StateSnapshot
	StateBlockTable     tables.StateBlock
	RoomAliasesTable    tables.RoomAliases
	PrevEventsTable     tables.PreviousEvents
	InvitesTable        tables.Invites
	MembershipTable     tables.Membership
	PublishedTable      tables.Published
	RedactionsTable     tables.Redactions
}

func (d *Database) SupportsConcurrentRoomInputs() bool {
	return true
}

func (d *Database) EventTypeNIDs(
	ctx context.Context, eventTypes []string,
) (map[string]types.EventTypeNID, error) {
	result := make(map[string]types.EventTypeNID)
	remaining := []string{}
	for _, eventType := range eventTypes {
		if nid, ok := d.Cache.GetRoomServerEventTypeNID(eventType); ok {
			result[eventType] = nid
		} else {
			remaining = append(remaining, eventType)
		}
	}
	if len(remaining) > 0 {
		nids, err := d.EventTypesTable.BulkSelectEventTypeNID(ctx, remaining)
		if err != nil {
			return nil, err
		}
		for eventType, nid := range nids {
			result[eventType] = nid
			d.Cache.StoreRoomServerEventTypeNID(eventType, nid)
		}
	}
	return result, nil
}

func (d *Database) EventStateKeys(
	ctx context.Context, eventStateKeyNIDs []types.EventStateKeyNID,
) (map[types.EventStateKeyNID]string, error) {
	return d.EventStateKeysTable.BulkSelectEventStateKey(ctx, eventStateKeyNIDs)
}

func (d *Database) EventStateKeyNIDs(
	ctx context.Context, eventStateKeys []string,
) (map[string]types.EventStateKeyNID, error) {
	result := make(map[string]types.EventStateKeyNID)
	remaining := []string{}
	for _, eventStateKey := range eventStateKeys {
		if nid, ok := d.Cache.GetRoomServerStateKeyNID(eventStateKey); ok {
			result[eventStateKey] = nid
		} else {
			remaining = append(remaining, eventStateKey)
		}
	}
	if len(remaining) > 0 {
		nids, err := d.EventStateKeysTable.BulkSelectEventStateKeyNID(ctx, remaining)
		if err != nil {
			return nil, err
		}
		for eventStateKey, nid := range nids {
			result[eventStateKey] = nid
			d.Cache.StoreRoomServerStateKeyNID(eventStateKey, nid)
		}
	}
	return result, nil
}

func (d *Database) StateEntriesForEventIDs(
	ctx context.Context, eventIDs []string,
) ([]types.StateEntry, error) {
	return d.EventsTable.BulkSelectStateEventByID(ctx, eventIDs)
}

func (d *Database) StateEntriesForTuples(
	ctx context.Context,
	stateBlockNIDs []types.StateBlockNID,
	stateKeyTuples []types.StateKeyTuple,
) ([]types.StateEntryList, error) {
	return d.StateBlockTable.BulkSelectFilteredStateBlockEntries(
		ctx, stateBlockNIDs, stateKeyTuples,
	)
}

func (d *Database) AddState(
	ctx context.Context,
	roomNID types.RoomNID,
	stateBlockNIDs []types.StateBlockNID,
	state []types.StateEntry,
) (stateNID types.StateSnapshotNID, err error) {
	err = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
		if len(state) > 0 {
			var stateBlockNID types.StateBlockNID
			stateBlockNID, err = d.StateBlockTable.BulkInsertStateData(ctx, txn, state)
			if err != nil {
				return fmt.Errorf("d.StateBlockTable.BulkInsertStateData: %w", err)
			}
			stateBlockNIDs = append(stateBlockNIDs[:len(stateBlockNIDs):len(stateBlockNIDs)], stateBlockNID)
		}
		stateNID, err = d.StateSnapshotTable.InsertState(ctx, txn, roomNID, stateBlockNIDs)
		if err != nil {
			return fmt.Errorf("d.StateSnapshotTable.InsertState: %w", err)
		}
		return nil
	})
	if err != nil {
		return 0, fmt.Errorf("d.Writer.Do: %w", err)
	}
	return
}

func (d *Database) EventNIDs(
	ctx context.Context, eventIDs []string,
) (map[string]types.EventNID, error) {
	return d.EventsTable.BulkSelectEventNID(ctx, eventIDs)
}

func (d *Database) SetState(
	ctx context.Context, eventNID types.EventNID, stateNID types.StateSnapshotNID,
) error {
	return d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
		return d.EventsTable.UpdateEventState(ctx, txn, eventNID, stateNID)
	})
}

func (d *Database) StateAtEventIDs(
	ctx context.Context, eventIDs []string,
) ([]types.StateAtEvent, error) {
	return d.EventsTable.BulkSelectStateAtEventByID(ctx, eventIDs)
}

func (d *Database) SnapshotNIDFromEventID(
	ctx context.Context, eventID string,
) (types.StateSnapshotNID, error) {
	_, stateNID, err := d.EventsTable.SelectEvent(ctx, nil, eventID)
	return stateNID, err
}

func (d *Database) EventIDs(
	ctx context.Context, eventNIDs []types.EventNID,
) (map[types.EventNID]string, error) {
	return d.EventsTable.BulkSelectEventID(ctx, eventNIDs)
}

func (d *Database) EventsFromIDs(ctx context.Context, eventIDs []string) ([]types.Event, error) {
	nidMap, err := d.EventNIDs(ctx, eventIDs)
	if err != nil {
		return nil, err
	}

	var nids []types.EventNID
	for _, nid := range nidMap {
		nids = append(nids, nid)
	}

	return d.Events(ctx, nids)
}

func (d *Database) RoomNID(ctx context.Context, roomID string) (types.RoomNID, error) {
	if nid, ok := d.Cache.GetRoomServerRoomNID(roomID); ok {
		return nid, nil
	}
	roomNID, err := d.RoomsTable.SelectRoomNID(ctx, nil, roomID)
	if err == sql.ErrNoRows {
		return 0, nil
	}
	d.Cache.StoreRoomServerRoomNID(roomID, roomNID)
	return roomNID, err
}

func (d *Database) RoomNIDExcludingStubs(ctx context.Context, roomID string) (roomNID types.RoomNID, err error) {
	roomNID, err = d.RoomNID(ctx, roomID)
	if err != nil {
		return
	}
	latestEvents, _, err := d.RoomsTable.SelectLatestEventNIDs(ctx, nil, roomNID)
	if err != nil {
		return
	}
	if len(latestEvents) == 0 {
		roomNID = 0
		return
	}
	return
}

func (d *Database) LatestEventIDs(
	ctx context.Context, roomNID types.RoomNID,
) (references []gomatrixserverlib.EventReference, currentStateSnapshotNID types.StateSnapshotNID, depth int64, err error) {
	var eventNIDs []types.EventNID
	eventNIDs, currentStateSnapshotNID, err = d.RoomsTable.SelectLatestEventNIDs(ctx, nil, roomNID)
	if err != nil {
		return
	}
	references, err = d.EventsTable.BulkSelectEventReference(ctx, nil, eventNIDs)
	if err != nil {
		return
	}
	depth, err = d.EventsTable.SelectMaxEventDepth(ctx, nil, eventNIDs)
	if err != nil {
		return
	}
	return
}

func (d *Database) StateBlockNIDs(
	ctx context.Context, stateNIDs []types.StateSnapshotNID,
) ([]types.StateBlockNIDList, error) {
	return d.StateSnapshotTable.BulkSelectStateBlockNIDs(ctx, stateNIDs)
}

func (d *Database) StateEntries(
	ctx context.Context, stateBlockNIDs []types.StateBlockNID,
) ([]types.StateEntryList, error) {
	return d.StateBlockTable.BulkSelectStateBlockEntries(ctx, stateBlockNIDs)
}

func (d *Database) GetRoomVersionForRoom(
	ctx context.Context, roomID string,
) (gomatrixserverlib.RoomVersion, error) {
	if roomVersion, ok := d.Cache.GetRoomVersion(roomID); ok {
		return roomVersion, nil
	}
	return d.RoomsTable.SelectRoomVersionForRoomID(
		ctx, nil, roomID,
	)
}

func (d *Database) GetRoomVersionForRoomNID(
	ctx context.Context, roomNID types.RoomNID,
) (gomatrixserverlib.RoomVersion, error) {
	if roomID, ok := d.Cache.GetRoomServerRoomID(roomNID); ok {
		if roomVersion, ok := d.Cache.GetRoomVersion(roomID); ok {
			return roomVersion, nil
		}
	}
	return d.RoomsTable.SelectRoomVersionForRoomNID(
		ctx, roomNID,
	)
}

func (d *Database) SetRoomAlias(ctx context.Context, alias string, roomID string, creatorUserID string) error {
	return d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
		return d.RoomAliasesTable.InsertRoomAlias(ctx, txn, alias, roomID, creatorUserID)
	})
}

func (d *Database) GetRoomIDForAlias(ctx context.Context, alias string) (string, error) {
	return d.RoomAliasesTable.SelectRoomIDFromAlias(ctx, alias)
}

func (d *Database) GetAliasesForRoomID(ctx context.Context, roomID string) ([]string, error) {
	return d.RoomAliasesTable.SelectAliasesFromRoomID(ctx, roomID)
}

func (d *Database) GetCreatorIDForAlias(
	ctx context.Context, alias string,
) (string, error) {
	return d.RoomAliasesTable.SelectCreatorIDFromAlias(ctx, alias)
}

func (d *Database) RemoveRoomAlias(ctx context.Context, alias string) error {
	return d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
		return d.RoomAliasesTable.DeleteRoomAlias(ctx, txn, alias)
	})
}

func (d *Database) GetMembership(
	ctx context.Context, roomNID types.RoomNID, requestSenderUserID string,
) (membershipEventNID types.EventNID, stillInRoom bool, err error) {
	var requestSenderUserNID types.EventStateKeyNID
	err = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
		requestSenderUserNID, err = d.assignStateKeyNID(ctx, txn, requestSenderUserID)
		return err
	})
	if err != nil {
		return 0, false, fmt.Errorf("d.assignStateKeyNID: %w", err)
	}

	senderMembershipEventNID, senderMembership, err :=
		d.MembershipTable.SelectMembershipFromRoomAndTarget(
			ctx, roomNID, requestSenderUserNID,
		)
	if err == sql.ErrNoRows {
		// The user has never been a member of that room
		return 0, false, nil
	} else if err != nil {
		return
	}

	return senderMembershipEventNID, senderMembership == tables.MembershipStateJoin, nil
}

func (d *Database) GetMembershipEventNIDsForRoom(
	ctx context.Context, roomNID types.RoomNID, joinOnly bool, localOnly bool,
) ([]types.EventNID, error) {
	if joinOnly {
		return d.MembershipTable.SelectMembershipsFromRoomAndMembership(
			ctx, roomNID, tables.MembershipStateJoin, localOnly,
		)
	}

	return d.MembershipTable.SelectMembershipsFromRoom(ctx, roomNID, localOnly)
}

func (d *Database) GetInvitesForUser(
	ctx context.Context,
	roomNID types.RoomNID,
	targetUserNID types.EventStateKeyNID,
) (senderUserIDs []types.EventStateKeyNID, eventIDs []string, err error) {
	return d.InvitesTable.SelectInviteActiveForUserInRoom(ctx, targetUserNID, roomNID)
}

func (d *Database) Events(
	ctx context.Context, eventNIDs []types.EventNID,
) ([]types.Event, error) {
	eventJSONs, err := d.EventJSONTable.BulkSelectEventJSON(ctx, eventNIDs)
	if err != nil {
		return nil, err
	}
	results := make([]types.Event, len(eventJSONs))
	for i, eventJSON := range eventJSONs {
		var roomNID types.RoomNID
		var roomVersion gomatrixserverlib.RoomVersion
		result := &results[i]
		result.EventNID = eventJSON.EventNID
		roomNID, err = d.EventsTable.SelectRoomNIDForEventNID(ctx, eventJSON.EventNID)
		if err != nil {
			return nil, err
		}
		roomVersion, err = d.RoomsTable.SelectRoomVersionForRoomNID(ctx, roomNID)
		if err != nil {
			return nil, err
		}
		result.Event, err = gomatrixserverlib.NewEventFromTrustedJSON(
			eventJSON.EventJSON, false, roomVersion,
		)
		if err != nil {
			return nil, err
		}
	}
	if !redactionsArePermanent {
		d.applyRedactions(results)
	}
	return results, nil
}

func (d *Database) GetTransactionEventID(
	ctx context.Context, transactionID string,
	sessionID int64, userID string,
) (string, error) {
	eventID, err := d.TransactionsTable.SelectTransactionEventID(ctx, transactionID, sessionID, userID)
	if err == sql.ErrNoRows {
		return "", nil
	}
	return eventID, err
}

func (d *Database) MembershipUpdater(
	ctx context.Context, roomID, targetUserID string,
	targetLocal bool, roomVersion gomatrixserverlib.RoomVersion,
) (*MembershipUpdater, error) {
	txn, err := d.DB.Begin()
	if err != nil {
		return nil, err
	}
	var updater *MembershipUpdater
	_ = d.Writer.Do(d.DB, txn, func(txn *sql.Tx) error {
		updater, err = NewMembershipUpdater(ctx, d, txn, roomID, targetUserID, targetLocal, roomVersion)
		return nil
	})
	return updater, err
}

func (d *Database) GetLatestEventsForUpdate(
	ctx context.Context, roomNID types.RoomNID,
) (*LatestEventsUpdater, error) {
	txn, err := d.DB.Begin()
	if err != nil {
		return nil, err
	}
	var updater *LatestEventsUpdater
	_ = d.Writer.Do(d.DB, txn, func(txn *sql.Tx) error {
		updater, err = NewLatestEventsUpdater(ctx, d, txn, roomNID)
		return nil
	})
	return updater, err
}

// nolint:gocyclo
func (d *Database) StoreEvent(
	ctx context.Context, event gomatrixserverlib.Event,
	txnAndSessionID *api.TransactionID, authEventNIDs []types.EventNID,
) (types.RoomNID, types.StateAtEvent, *gomatrixserverlib.Event, string, error) {
	var (
		roomNID          types.RoomNID
		eventTypeNID     types.EventTypeNID
		eventStateKeyNID types.EventStateKeyNID
		eventNID         types.EventNID
		stateNID         types.StateSnapshotNID
		redactionEvent   *gomatrixserverlib.Event
		redactedEventID  string
		err              error
	)

	err = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
		if txnAndSessionID != nil {
			if err = d.TransactionsTable.InsertTransaction(
				ctx, txn, txnAndSessionID.TransactionID,
				txnAndSessionID.SessionID, event.Sender(), event.EventID(),
			); err != nil {
				return fmt.Errorf("d.TransactionsTable.InsertTransaction: %w", err)
			}
		}

		// TODO: Here we should aim to have two different code paths for new rooms
		// vs existing ones.

		// Get the default room version. If the client doesn't supply a room_version
		// then we will use our configured default to create the room.
		// https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-createroom
		// Note that the below logic depends on the m.room.create event being the
		// first event that is persisted to the database when creating or joining a
		// room.
		var roomVersion gomatrixserverlib.RoomVersion
		if roomVersion, err = extractRoomVersionFromCreateEvent(event); err != nil {
			return fmt.Errorf("extractRoomVersionFromCreateEvent: %w", err)
		}

		if roomNID, err = d.assignRoomNID(ctx, txn, event.RoomID(), roomVersion); err != nil {
			return fmt.Errorf("d.assignRoomNID: %w", err)
		}

		if eventTypeNID, err = d.assignEventTypeNID(ctx, txn, event.Type()); err != nil {
			return fmt.Errorf("d.assignEventTypeNID: %w", err)
		}

		eventStateKey := event.StateKey()
		// Assigned a numeric ID for the state_key if there is one present.
		// Otherwise set the numeric ID for the state_key to 0.
		if eventStateKey != nil {
			if eventStateKeyNID, err = d.assignStateKeyNID(ctx, txn, *eventStateKey); err != nil {
				return fmt.Errorf("d.assignStateKeyNID: %w", err)
			}
		}

		if eventNID, stateNID, err = d.EventsTable.InsertEvent(
			ctx,
			txn,
			roomNID,
			eventTypeNID,
			eventStateKeyNID,
			event.EventID(),
			event.EventReference().EventSHA256,
			authEventNIDs,
			event.Depth(),
		); err != nil {
			if err == sql.ErrNoRows {
				// We've already inserted the event so select the numeric event ID
				eventNID, stateNID, err = d.EventsTable.SelectEvent(ctx, txn, event.EventID())
			}
			if err != nil {
				return err
			}
		}

		if err = d.EventJSONTable.InsertEventJSON(ctx, txn, eventNID, event.JSON()); err != nil {
			return fmt.Errorf("d.EventJSONTable.InsertEventJSON: %w", err)
		}
		redactionEvent, redactedEventID, err = d.handleRedactions(ctx, txn, eventNID, event)
		return nil
	})
	if err != nil {
		return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("d.Writer.Do: %w", err)
	}

	return roomNID, types.StateAtEvent{
		BeforeStateSnapshotNID: stateNID,
		StateEntry: types.StateEntry{
			StateKeyTuple: types.StateKeyTuple{
				EventTypeNID:     eventTypeNID,
				EventStateKeyNID: eventStateKeyNID,
			},
			EventNID: eventNID,
		},
	}, redactionEvent, redactedEventID, nil
}

func (d *Database) PublishRoom(ctx context.Context, roomID string, publish bool) error {
	return d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
		return d.PublishedTable.UpsertRoomPublished(ctx, txn, roomID, publish)
	})
}

func (d *Database) GetPublishedRooms(ctx context.Context) ([]string, error) {
	return d.PublishedTable.SelectAllPublishedRooms(ctx, true)
}

func (d *Database) assignRoomNID(
	ctx context.Context, txn *sql.Tx,
	roomID string, roomVersion gomatrixserverlib.RoomVersion,
) (types.RoomNID, error) {
	if roomNID, ok := d.Cache.GetRoomServerRoomNID(roomID); ok {
		return roomNID, nil
	}
	// Check if we already have a numeric ID in the database.
	roomNID, err := d.RoomsTable.SelectRoomNID(ctx, txn, roomID)
	if err == sql.ErrNoRows {
		// We don't have a numeric ID so insert one into the database.
		roomNID, err = d.RoomsTable.InsertRoomNID(ctx, txn, roomID, roomVersion)
		if err == sql.ErrNoRows {
			// We raced with another insert so run the select again.
			roomNID, err = d.RoomsTable.SelectRoomNID(ctx, txn, roomID)
		}
	}
	if err == nil {
		d.Cache.StoreRoomServerRoomNID(roomID, roomNID)
	}
	return roomNID, err
}

func (d *Database) assignEventTypeNID(
	ctx context.Context, txn *sql.Tx, eventType string,
) (types.EventTypeNID, error) {
	if eventTypeNID, ok := d.Cache.GetRoomServerEventTypeNID(eventType); ok {
		return eventTypeNID, nil
	}
	// Check if we already have a numeric ID in the database.
	eventTypeNID, err := d.EventTypesTable.SelectEventTypeNID(ctx, txn, eventType)
	if err == sql.ErrNoRows {
		// We don't have a numeric ID so insert one into the database.
		eventTypeNID, err = d.EventTypesTable.InsertEventTypeNID(ctx, txn, eventType)
		if err == sql.ErrNoRows {
			// We raced with another insert so run the select again.
			eventTypeNID, err = d.EventTypesTable.SelectEventTypeNID(ctx, txn, eventType)
		}
	}
	if err == nil {
		d.Cache.StoreRoomServerEventTypeNID(eventType, eventTypeNID)
	}
	return eventTypeNID, err
}

func (d *Database) assignStateKeyNID(
	ctx context.Context, txn *sql.Tx, eventStateKey string,
) (types.EventStateKeyNID, error) {
	if eventStateKeyNID, ok := d.Cache.GetRoomServerStateKeyNID(eventStateKey); ok {
		return eventStateKeyNID, nil
	}
	// Check if we already have a numeric ID in the database.
	eventStateKeyNID, err := d.EventStateKeysTable.SelectEventStateKeyNID(ctx, txn, eventStateKey)
	if err == sql.ErrNoRows {
		// We don't have a numeric ID so insert one into the database.
		eventStateKeyNID, err = d.EventStateKeysTable.InsertEventStateKeyNID(ctx, txn, eventStateKey)
		if err == sql.ErrNoRows {
			// We raced with another insert so run the select again.
			eventStateKeyNID, err = d.EventStateKeysTable.SelectEventStateKeyNID(ctx, txn, eventStateKey)
		}
	}
	if err == nil {
		d.Cache.StoreRoomServerStateKeyNID(eventStateKey, eventStateKeyNID)
	}
	return eventStateKeyNID, err
}

func extractRoomVersionFromCreateEvent(event gomatrixserverlib.Event) (
	gomatrixserverlib.RoomVersion, error,
) {
	var err error
	var roomVersion gomatrixserverlib.RoomVersion
	// Look for m.room.create events.
	if event.Type() != gomatrixserverlib.MRoomCreate {
		return gomatrixserverlib.RoomVersion(""), nil
	}
	roomVersion = gomatrixserverlib.RoomVersionV1
	var createContent gomatrixserverlib.CreateContent
	// The m.room.create event contains an optional "room_version" key in
	// the event content, so we need to unmarshal that first.
	if err = json.Unmarshal(event.Content(), &createContent); err != nil {
		return gomatrixserverlib.RoomVersion(""), err
	}
	// A room version was specified in the event content?
	if createContent.RoomVersion != nil {
		roomVersion = gomatrixserverlib.RoomVersion(*createContent.RoomVersion)
	}
	return roomVersion, err
}

// handleRedactions manages the redacted status of events. There's two cases to consider in order to comply with the spec:
// "servers should not apply or send redactions to clients until both the redaction event and original event have been seen, and are valid."
// https://matrix.org/docs/spec/rooms/v3#authorization-rules-for-events
// These cases are:
//  - This is a redaction event, redact the event it references if we know about it.
//  - This is a normal event which may have been previously redacted.
// In the first case, check if we have the referenced event then apply the redaction, else store it
// in the redactions table with validated=FALSE. In the second case, check if there is a redaction for it:
// if there is then apply the redactions and set validated=TRUE.
//
// When an event is redacted, the redacted event JSON is modified to add an `unsigned.redacted_because` field. We use this field
// when loading events to determine whether to apply redactions. This keeps the hot-path of reading events quick as we don't need
// to cross-reference with other tables when loading.
//
// Returns the redaction event and the event ID of the redacted event if this call resulted in a redaction.
func (d *Database) handleRedactions(
	ctx context.Context, txn *sql.Tx, eventNID types.EventNID, event gomatrixserverlib.Event,
) (*gomatrixserverlib.Event, string, error) {
	var err error
	isRedactionEvent := event.Type() == gomatrixserverlib.MRoomRedaction && event.StateKey() == nil
	if isRedactionEvent {
		// an event which redacts itself should be ignored
		if event.EventID() == event.Redacts() {
			return nil, "", nil
		}

		err = d.RedactionsTable.InsertRedaction(ctx, txn, tables.RedactionInfo{
			Validated:        false,
			RedactionEventID: event.EventID(),
			RedactsEventID:   event.Redacts(),
		})
		if err != nil {
			return nil, "", err
		}
	}

	redactionEvent, redactedEvent, validated, err := d.loadRedactionPair(ctx, txn, eventNID, event)
	if err != nil {
		return nil, "", err
	}
	if validated || redactedEvent == nil || redactionEvent == nil {
		// we've seen this redaction before or there is nothing to redact
		return nil, "", nil
	}
	if redactedEvent.RoomID() != redactionEvent.RoomID() {
		// redactions across rooms aren't allowed
		return nil, "", nil
	}

	// mark the event as redacted
	err = redactedEvent.SetUnsignedField("redacted_because", redactionEvent)
	if err != nil {
		return nil, "", err
	}
	if redactionsArePermanent {
		redactedEvent.Event = redactedEvent.Redact()
	}
	// overwrite the eventJSON table
	err = d.EventJSONTable.InsertEventJSON(ctx, txn, redactedEvent.EventNID, redactedEvent.JSON())
	if err != nil {
		return nil, "", err
	}

	return &redactionEvent.Event, redactedEvent.EventID(), d.RedactionsTable.MarkRedactionValidated(ctx, txn, redactionEvent.EventID(), true)
}

// loadRedactionPair returns both the redaction event and the redacted event, else nil.
func (d *Database) loadRedactionPair(
	ctx context.Context, txn *sql.Tx, eventNID types.EventNID, event gomatrixserverlib.Event,
) (*types.Event, *types.Event, bool, error) {
	var redactionEvent, redactedEvent *types.Event
	var info *tables.RedactionInfo
	var err error
	isRedactionEvent := event.Type() == gomatrixserverlib.MRoomRedaction && event.StateKey() == nil

	var eventBeingRedacted string
	if isRedactionEvent {
		eventBeingRedacted = event.Redacts()
		redactionEvent = &types.Event{
			EventNID: eventNID,
			Event:    event,
		}
	} else {
		eventBeingRedacted = event.EventID() // maybe, we'll see if we have info
		redactedEvent = &types.Event{
			EventNID: eventNID,
			Event:    event,
		}
	}

	info, err = d.RedactionsTable.SelectRedactionInfoByEventBeingRedacted(ctx, txn, eventBeingRedacted)
	if err != nil {
		return nil, nil, false, err
	}
	if info == nil {
		// this event hasn't been redacted or we don't have the redaction for it yet
		return nil, nil, false, nil
	}

	if isRedactionEvent {
		redactedEvent = d.loadEvent(ctx, info.RedactsEventID)
	} else {
		redactionEvent = d.loadEvent(ctx, info.RedactionEventID)
	}

	return redactionEvent, redactedEvent, info.Validated, nil
}

// applyRedactions will redact events that have an `unsigned.redacted_because` field.
func (d *Database) applyRedactions(events []types.Event) {
	for i := range events {
		if result := gjson.GetBytes(events[i].Unsigned(), "redacted_because"); result.Exists() {
			events[i].Event = events[i].Redact()
		}
	}
}

// loadEvent loads a single event or returns nil on any problems/missing event
func (d *Database) loadEvent(ctx context.Context, eventID string) *types.Event {
	nids, err := d.EventNIDs(ctx, []string{eventID})
	if err != nil {
		return nil
	}
	if len(nids) == 0 {
		return nil
	}
	evs, err := d.Events(ctx, []types.EventNID{nids[eventID]})
	if err != nil {
		return nil
	}
	if len(evs) != 1 {
		return nil
	}
	return &evs[0]
}