Initialize a value. Lock must be held by caller.

def _init_value(self, key):
        """Initialize a value. Lock must be held by caller."""
        encoded = key.encode('utf-8')
        # Pad to be 8-byte aligned.
        padded = encoded + (b' ' * (8 - (len(encoded) + 4) % 8))
        value = struct.pack('i{0}sd'.format(len(padded)).encode(), len(encoded), padded, 0.0)
        while self._used + len(value) > self._capacity:
            self._capacity *= 2
            self._f.truncate(self._capacity)
            self._m = mmap.mmap(self._f.fileno(), self._capacity)
        self._m[self._used:self._used + len(value)] = value

        # Update how much space we've used.
        self._used += len(value)
        _pack_integer(self._m, 0, self._used)
        self._positions[key] = self._used - 8