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Hey Serhiy,
The implementation for your readline optimization seems less contentious (and less risky) than the remainder of the patch -- it could perhaps be easily split off into a separate patch, which may be far more easily committed.
I love the concept of this patch, although from my last reading (weeks ago), it's slightly scary that it relies on Py_REFCNT() to know whether to mutate a string or not. In principle this should never break, in practice, however, it is uncertain that there are no strange edge cases that aren't immediately obvious.
The _PyBytes_Resize doc is quite clear: "Only use this to build up a brand new bytes object; don’t use this if the bytes may already be known in other parts of the code"