I don't think the proposed patch (file11012) makes it thread-safe, and I
believe you cannot get it thread-safe without using thread synchronization.

For example, if two threads simultaneously determine that there is still
space left (len(b) <= free), they might both write to self._write_buf
and self._write_end, and the last writer would win. (In case it isn't
clear how this might happen: thread 1 exhausts its Py_Ticker just after
checking the size, then thread 2 runs a full .write(), then thread 1
continues filling the buffer)