[tsayin4]

IOBase.readlines(hint) is documented to stop reading only when the total size of
the lines read so far exceeds hint, and Modules/_io/iobase.c implements exactly
that (line_length > hint - length). _pyio breaks on n >= hint, so it returns
one line fewer whenever the accumulated size is exactly hint:

>>> import io, _pyio
>>> open("x", "wb").write(b"a\nbb\nccc\n")
9
>>> io.open("x", "rb").readlines(2), _pyio.open("x", "rb").readlines(2)
([b'a\n', b'bb\n'], [b'a\n'])

The _pyio docstring itself says "exceeds", so only the comparison is wrong.

Tested: the new test was run against _io, unpatched _pyio and patched _pyio on
3.13 (the touched code is byte-identical on main); it fails on unpatched _pyio
(hint=2, 5 and 9) and passes for both implementations with the fix. I have not
run the full test_io suite locally (no local build yet), so CI is the check for
that.

• Issue: _pyio.IOBase.readlines(hint) stops one line early when the total size read equals *hint* #161272

[python-cla-bot]

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