bug#34502: Ask yourself, am I generating a NEW .overview file?

[積丹尼 Dan Jacobson]

nnml bug:
1. exit emacs
2. remove an .overview file. (Say it was corrupted. Or mom didn't like
it, whatever. State secrets, you know.)
3. start emacs and gnus. etc. All is OK.

1. exit emacs
2. remove an .overview file.
3. send yourself a mail
4. start emacs and gnus... upon entering the group,
the new overview file is generated only *containing only that new mail*.
The user is shocked to see all his other mail is "lost forever" there
is the *Summary* buffer.

Workaroud: first
"you should regenerate them by entering the server buffer using ‘^’, and then 
‘g’ on the
appropriate mail server (e.g., nnml) to cause regeneration."

So, bug: when gnus regenerates overview files on its own, if it finds a
new mail, it only uses that, not noticing all the existing mails.
It first needs to ask itself: "am I, ho-hum, appending to an existing .overview
file, or gosh-once-in-a-lifetime fresh generating it for the first time?" So it 
should look
first, if it finds no .overview file, it had better regenerate it
first before incorporating any new mail! Thanks!

What's worse about this bug is gnus will work great for days, months, with no
.overview file for that group. Until one fateful day, when the user
receives a new mail. And "KABAM", all his messages except that new mail
are suddenly gone. No ESC 99999 RET entering the group will retrieve
them back into the *Summary* buffer etc.

Sure, you could say it is his own "fault" for removing the .overview
file without asking grandma, and (seemingly) losing all his mails is
proper "punishment" for the crime, case closed.

But you could simply always check first: does the .overview file I am
about to cheerily append to exist in the first place?

Gnus v5.13
GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.4)
 of 2019-02-03, modified by Debian