`readline.set_history_length` results in unreadable history files

[encukou]

Hello,
Writing a history file after a call to readline.set_history_length results in an unreadable history file. For example, with cpython-3.12.4+20240713-x86_64-unknown-linux-gnu-install_only.tar.gz:

import readline
import os


readline.add_history("first line")
readline.add_history("second line")
readline.add_history("third line")

readline.set_history_length(2)
readline.write_history_file('historyfile.tmp')
try:
    readline.read_history_file('historyfile.tmp')
finally:
    os.remove('historyfile.tmp')

gives me a traceback on the read_history_file:

Traceback (most recent call last):
  File "/script.py", line 12, in <module>
    readline.read_history_file('historyfile.tmp')
OSError: [Errno 22] Invalid argument

This might be a conflict/incompatibility between editline and a history library that doesn't work with editline's _HiStOrY_V2_ format.

I've added a test to CPython that should fail in this case. I don't know what more I can do on the CPython side.

The same “Invalid argument” error happens when trying to read a history file produced with GNU readline. This can affect users switching from some earlier standalone builds.