This issue remembers me #3618 (opened 2 years ago): I proposed to use RLock instead of Lock, but RLock was implemented in Python and were too slow. Today, we have RLock implemented in C and it may be possible to use them. Would it solve this issue?

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There are at least two deadlocks, both in _bufferedwriter_flush_unlocked():
 - call to _bufferedwriter_raw_write()
 - call to PyErr_CheckSignals()

> The lock is precisely there so that the buffered object doesn't 
> have to be MT-safe or reentrant. It doesn't seem reasonable
> to attempt to restore the file to a "stable" state in the middle
> of an inner routine.

Oh, so release the lock around the calls to _bufferedwriter_raw_write() (aorund PyObject_CallMethodObjArgs() in _bufferedwriter_raw_write()) and PyErr_CheckSignals() is not a good idea? Or is it just complex because the buffer object have to be in a consistent state?

> (in a more sophisticated version, we could store pending writes 
> so that they get committed at the end of the currently 
> executing write)

If the pending write fails, who gets the error?