I wrote a C-language program to reproduce this issue on Mac OS without Python.  It reproduces the issue in both increasing and decreasing order of initial read sizes, and it reliably reproduces it for each KB from 128KB to 1024KB ; the Python version reproduced the issue every 4K and only in decreasing order (probably something related to Python's memory management optimizations).

A new caveat: if the read buffer is allocated once before entering the read loop, then we don't get any EINVAL in the entire run; however, if the read buffer is allocated for each read request inside the loop, then we get EINVAL every other time in the range from 128KB and up.

I would like to file this issue with apple/Mac OS. What's the appropriate URL for this?