bug#69517: [PATCH] Make gnus cache work with group names having '/'

[James Thomas via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii wrote:

>> Date: Sun, 03 Mar 2024 07:22:29 +0530
>> From:  James Thomas via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>>
>> Tags: patch
>>
>> Reproduction steps:
>>
>> - Setup Gnus with any group name having a slash ('/') such as
>>   "[Gmail]/Drafts" or an Atom feed (they usually have slashes) using the
>>   patch in bug#66188.
>>
>> - Press '*' on a message in the group.
>>
>> - Do (info "(gnus) Creating a Virtual Server")
>>
>> - Open the above from the Server buffer; RET on the new group fails.
>>
>> A patch is attached. I couldn't find the problematic commit or its
>> original branch (where it was a consolidated merge from) but
>> 'gnus-use-long-file-names' is apparently not meant for backends: it
>> can't even be customized with that 'not-cache' option. I think this is
>> the right way to solve it: the other lines removed in this patch are
>> even older, but they were never being called due to the above reason.
>
> Eric, could you please review the patch and install if it's okay?
>
> Thanks.

There's a small caveat after applying this:

Before this patch, cache entries would've effectively ignored
'nnmail-use-long-file-names' (nil by default) and used long names. After
this patch new entries would honor it, resulting in an extra directory
tree for the same group. But only the original one would be opened (due
to [[./lisp/gnus/nnmail.el::;; If this direc]]). To fix it, one would
have to copy all the files in from the original directory into the new
one (and retain the active file entry). Or of course, change the above
variable (but that could have other implications depending upon one's
configuration).

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