Performance issue: mailbox.mbox slow because it does a tell/lseek for every line

Bug report

Bug description:

Parsing a large mbox file with mailbox.mbox is surprisingly slow and I did some profiling and debugging. For a large file (my testcase, 3,000 mails, 26,000,000 lines), somewhere around half the time is spent calling tell(), once per line in PartialFile. As documented in issue #107158, tell() here uses lseek which is a system call rather than just returning a constant.

# Slow code, assuming a large my_mail.mbox
import mailbox
box = mailbox.mbox("my_mail.mbox", create=False)
for message in box:
    pass

Replacing mailbox.mbox with a simple mmapped file that scans for b"\nFrom " and calls BytesParser(policy=policy.default).parsebytes(...) directly gives the same results in my testing (I'm sure it's not as powerful). In my tests that cuts the parsing overhead from 11 seconds to 1, and total parse time from >20 to 9.

CPython versions tested on:

3.12

Operating systems tested on:

Windows

Linked PRs

• gh-157543

从 mailbox.mbox 的迭代路径及其 PartialFile 实现开始，重点关注此处所述的逐行 tell() 和 lseek() 行为。先查看链接的 PR gh-157543，然后对解析大型 mbox 文件进行基准测试，并确认在不改变解析结果的情况下性能问题已得到解决。