return None

        try:

            # There shouldn't be any records where url is NULL, so running this

            # DELETE pulls double duty for us: it tells us whether or not we

            # can write to the database and it enforces database integrity.

            cursor.execute('DELETE FROM RTLinkCache WHERE url IS NULL')

        except sqlite3.OperationalError:

            cls.logger.debug("setup: error writing %s", cache_path, exc_info=True)

            can_write = False

        else:

            can_write = True

        if not (can_write or have_data):

            # If there's nothing to read and no way to write, sqlite provides

            # no benefit.

            cls.logger.debug("setup: not using %s: nothing to read or write", cache_path)

            return None

        elif not can_write:

            # Set up an in-memory database that we can write to, seeded with

            # the data available to read.

            try:

                cursor.close()

                db.close()

                db = sqlite3.connect(':memory:', isolation_level=None)

                cursor = db.cursor()

                cursor.execute('ATTACH DATABASE ? AS readsource',

                cursor.execute(cls.CREATE_TABLE_SQL)

                cursor.execute('INSERT INTO RTLinkCache SELECT * FROM readsource.RTLinkCache')

                cursor.execute('DETACH DATABASE readsource')

            except sqlite3.OperationalError as error:

                # We're back to the case of having nothing to read and no way

                # to write.

                cls.logger.debug("setup: error loading %s into memory", cache_path, exc_info=True)

                return None

            else:

                cls.logger.debug("setup: loaded %s into memory", cache_path)

        else:

            cls.logger.debug("setup: caching at %s", cache_path)

        cursor.close()

        db.commit()

        return db

    def __init__(self, cache_db: sqlite3.Connection) -> None:

        self._db = cache_db

        self._nourls: Set[TicketAttachmentIds] = set()

    def __iter__(self) -> Iterator[TicketAttachmentIds]:

        yield from self._db.execute('SELECT ticket_id, attachment_id FROM RTLinkCache')

        yield from self._nourls