Bug report

Bug description:

TextIOWrapper.__next__() stops updating the decoder snapshot that tell() relies on and disables tell() (OSError: telling position disabled by next() call), but flush() re-enables tell() without restoring the snapshot. tell() then computes the position from a stale snapshot and silently returns a wrong cookie.

import io
f = io.TextIOWrapper(io.BytesIO(b"a\n" + b"b" * 10000 + b"\nc\n"),
                     encoding="ascii")
f.readline()
next(f)
f.flush()
pos = f.tell()
print(pos)
f.seek(pos)
print(repr(f.read(3)))

This prints 3624 and 'bbb' instead of 10003 and 'c\n' (or the OSError that tell() raises without the flush() call); _pyio.TextIOWrapper returns 9768, also wrong. If the snapshot was replaced by an earlier seek(), tell() instead raises OSError: can't reconstruct logical file position (before gh-161053 it read past the snapshot).

gh-81217 asks for next() not to disable tell() at all.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-161055: Fix TextIOWrapper.tell() after next() and flush() #161056