bug#66742: 30.0.50; transient-mark-mode is not enabled after re-dumping Emacs

[Ihor Radchenko <yantar92@posteo.net>]

Eli Zaretskii <eliz@gnu.org> writes:

>> This is a surprise to me!
>> Is it documented anywhere?
>
> What do you want to document?

I think we misunderstand each other.

>> I thought that we should expect the value to
>> be evaluated unless explicitly wrapped into (eval-when-compile ...).
>
> At what time do you think defcustom's value is evaluated?

Load time, even for pdumped libraries.

>> Also, I just looked up defvar instances across Emacs and I am seeing
>> multiple cases when the value is calculated dynamically, assuming
>> loading time:
>
> Of course, it's loading time.  But AFAIU you were talking about
> evaluation during dumping, i.e. when Emacs is built.

No. It is not what I am talking about.
In this bug report, I am talking about re-dumping the already built
Emacs with the aim of adding extra arbitrary libraries. According to
manual page for dump-emacs-portable,

     If you want to use this function in an Emacs that was already
     dumped, you must run Emacs with the ‘-batch’ option.

This implies that re-dumping is one of the intended use-cases.

> ... This is only
> relevant for defcustom's in packages that are preloaded, because those
> packages are loaded at build time, not at run time.  So for those
> packages, and only for those packages, any defcustom whose default
> value is not a constant, should be re-evaluated at startup, to make
> sure the value is suitable for the system on which Emacs runs.

I can see your point, although I wish it were not so tricky.
In any case, even within a scope of this bug report, the reproducer I
provided does not load any extra packages. So, at least, there is some
bug with preloaded packages.

Maybe, the caveats with runtime/re-dump time evaluation should be
documented.

> ...
> This is not relevant, since packages that aren't preloaded will be
> loaded at run time, and therefore the values will be evaluated in the
> correct context.  If you are talking about these, then I don't
> understand what prompted you to raise this issue to begin with, and in
> a thread that discusses dumping.

... but ideally, I wish that we would not have to patch the existing
libraries in order to preload them. Otherwise, `dump-emacs-portable'
function is not very useful in practice (except for building Emacs) and
will most likely lead to subtle bugs if attempted by users.

-- 
Ihor Radchenko // yantar92,
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