logging should cache LogRecord.message

Bug report

Bug description:

logging currently does not cache LogRecord.message:
https://github.com/python/cpython/blob/0dcbc8385322ff51f7fc3e586027d880275df4fa/Lib/logging/__init__.py#L391-L401
https://github.com/python/cpython/blob/0dcbc8385322ff51f7fc3e586027d880275df4fa/Lib/logging/__init__.py#L711

Since logging caches other string-formatting related results such as LogRecord.asctime and LogRecord.exc_text, I would like to suggest:

1. LogRecord.getMessage should check self.message first. If it is not computed yet, it should compute the message and set this attribute before returning.
2. Formatter shouldn't set record.message. It only needs to call LogRecord.getMessage to get the message.

How to reproduce

import logging

class A:
    def __repr__(self):
        print('repr called')
        return '<A>'

logger = logging.getLogger()
logger.addHandler(logging.StreamHandler())
logger.addHandler(logging.StreamHandler())
logger.error('%r', A())

Expected output

repr called
<A>
<A>

Actual output

repr called
<A>
repr called
<A>

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-134367

閱讀 Lib/logging/init.py 中所引用的 LogRecord 和 Formatter 位置，然後執行 reproduction 以觀察重複的 repr 呼叫。開始前查看連結的 PR gh-134367。完成的標準是：雙 handler 範例呼叫 repr 一次，同時兩個 handler 都產生訊息。