bug#66993: [PATCH] project.el: avoid asking user about project-list-file lock

[Dmitry Gutov <dmitry@gutov.dev>]

On 18/11/2023 18:26, Eli Zaretskii wrote:
>> From:sbaugh@catern.com
>> Date: Sat, 18 Nov 2023 15:48:33 +0000 (UTC)
>> Cc: Spencer Baugh<sbaugh@janestreet.com>, Eli Zaretskii<eliz@gnu.org>,
>> 	66993@debbugs.gnu.org
>>
>>> What happens if savehist-mode is nil, though? Which it is by
>>> default. For users with this setup the project history will just
>>> disappear.
>> Indeed.
>>
>>> What kind of backward compatibility did you have in mind?
>> I was thinking about either keeping our code for saving to
>> project-list-file around in some obsoleted form, or using a subset of
>> the savehist code to save only project--list when it's not otherwise
>> enabled.
>>
>> But actually, maybe it's time that we just enable savehist by default.
> Even if we decide to do that (and I'm not at all sure we should), how
> would that solve the difficulty pointed out by Dmitry?  Even if
> savehist is ON by default, the user could turn it OFF, right?

I think the working hypothesis is that they are the same feature: if 
project--list is used for saving the previous inputs, it can be covered 
by savehist-mode.

And if used explicitly turns off savehist-mode (after it's been made the 
default), that can mean they're fine with not storing all of those 
histories, including the projects one.

AFAIK project-known-project-roots is also used by some "splash page" 
packages to list the known projects for quick visiting, but that's not 
very different from using file-name-history to show a list of "last 
visited files", which also some code does.

So I was primarily worried about migration -- for those who didn't 
enable savehist-mode yet (perhaps due to not being aware of it), but 
have existing saved project histories.