Re: Turning on savehist-mode by default

[Adam Porter]

In addition to what Po said, I'd like to gently reiterate what I said 
earlier in this thread: Given my experience with savehist causing 
unexpected and hard-to-debug performance problems[0], I'd guess that 
there are more such cases in the wild waiting to be triggered.  If, e.g. 
Emacs 30.1 enabled it by default, I can imagine a number of users 
suddenly encountering weird pauses, and they'd probably blame GC 
initially[1].

As well, I have some concerns about savehist's having the potential to 
cause weird bugs in other libraries: The savehist-save function seems to 
comment out individual elements of savehist-minibuffer-history-variables 
that it determines are unreadable.  That's understandable from its 
perspective, but what effect will that have on libraries that may not be 
expecting for their data structures to have certain parts disappear 
after restarting Emacs?  I can just imagine the bug reports from users 
showing apparently corrupted or elided data structures, and having no 
clue as to what is mutating them, because the code isn't within the 
library having a bug reported against it.

IMHO, a feature like this is indeed expected and overdue in Emacs, but I 
don't think savehist is a good way to enable it, given its current 
implementation.  ISTM that something built on top of the new 
multisession library would be more appropriate and (at least, have the 
potential to be) more robust.

If savehist is to be used for this, I would at least recommend taking a 
release cycle to publicly ask users to enable it by default and report 
the results, and wait for at least a few hundred positive reports before 
enabling it by default.  Otherwise, I would expect not-insignificant 
fallout, IMHO.

Thanks,
Adam

0: https://github.com/alphapapa/ement.el/issues/216

1: Which may, in fact, be part of the problem, considering how 
savehist-save works: it prints a Lisp object to a buffer, then tries to 
read it to determine whether it's readable--doesn't that generate 
garbage?  Now consider doing that every 1-5 minutes and upon exit.