upgradeToExclusiveLock

    fn upgradeToExclusiveLock(self: *Manifest) error{CacheCheckFailed}!bool {
        if (self.have_exclusive_lock) return false;
        assert(self.manifest_file != null);
        const io = self.cache.io;

        if (std.process.can_spawn or !builtin.single_threaded) {
            const manifest_file = self.manifest_file.?;
            // Here we intentionally have a period where the lock is released, in case there are
            // other processes holding a shared lock.
            manifest_file.unlock(io);
            manifest_file.lock(io, .exclusive) catch |err| {
                self.diagnostic = .{ .manifest_lock = err };
                return error.CacheCheckFailed;
            };
        }
        self.have_exclusive_lock = true;
        return true;
    }