Re: Turning on savehist-mode by default

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 28 Nov 2023 15:07:46 -0600
> Cc: emacs-devel@gnu.org, public@thanosapollo.org
> From: Adam Porter <adam@alphapapa.net>
> 
> FWIW, IME, it may not be a good idea to enable savehist-mode by default. 
>   As an example, see this issue report on Ement.el's repository:
> 
> https://github.com/alphapapa/ement.el/issues/216
> 
> The gist is that, for users who enabled savehist, it was saving the 
> variable command-history, which included some complex commands whose 
> arguments (from interactive completion) included large data structures 
> (which are hidden from the user when called interactively).  This 
> resulted in savehist files hundreds of MB in size and very noticeable 
> pauses whenever savehist saved its data, from timers and upon Emacs exit.
> 
> The worst part was how long it took to discover the source of the 
> problem.  There were intermittent reports of weird pauses for months, 
> without any discernible reason, until a user finally narrowed it down to 
> using savehist.  Then we had to add this hacky workaround in Ement to 
> try to automatically avoid the problem.
> 
> Such cases may be rare (or not--I can't say), but when it happens, it 
> seems to be a bear to deal with, being very unintuitive.  For myself, 
> even having written a number of Emacs packages, I wasn't yet aware of 
> the command-history variable and how it saves arguments, and certainly 
> not the implications of using savehist as well.

savehist provides a customizable ability to limit the number of
history entries saved for each variable.

We could also add a new feature which limits the size of individual
entry: entries longer than that will not be saved.

Patches welcome.