bug#30994: bug#45857: 28.0.50; Not possible to set package-user-dir in early-init.el

[Stefan Monnier <monnier@iro.umontreal.ca>]

> Is there any place where all these complications are described, so
> that people (and we ourselves) could have a complete, comprehensive
> overview of this stuff and the various problems it needs to solve?  If
> not, how about taking this opportunity to describe that?  Because
> otherwise these incidents will keep happening.

Here's the story:

We have a few variables which are predefined in the dumped Emacs but
whose default value depends on information only available when Emacs is
executed, such as the user's $HOME.

We handle those using `custom-initialize-delayed`, so when the
variable's `defcustom` is procesed the variable isn't initialized yet,
and instead initialization happens in `startup.el` by calling
`custom-reevaluate-setting` on each one of those delayed variables.

In Emacs-26, we did that just before loading the `.emacs` file and
everybody was happy.  But the `early-init.el` file added to Emacs-27
introduced a difficulty there:

- Those vars's values might be needed in `early-init.el` so we should
  call `custom-reevaluate-setting` before we load `early-init.el`.

- The value of `blink-cursor-mode` depends on whether window-system is
  nil or not, which is only known after loading `early-init.el`, so we
  should call `custom-reevaluate-setting` after we load `early-init.el`.

Currently we solve this by calling `custom-reevaluate-setting` twice for
every one of those variables.  But it means that we silently override
any change made to those variables from `early-init.el`.

As noted, the only var whose default value needs to be (re)computed
after loading `early-init.el` is `blink-cursor-mode`.  In theory there
could be more, but I think we should aim to reduce rather than increase
those cases.  E.g. the current way `blink-cursor-mode` is defined means
that if I start `emacs -Q -nw` and later create a GUI frame, that frame
won't have a blinking cursor.  I think this is a bug (tho I can see
that some people may consider it a feature if they (like me) dislike
blinking cursors ;-0)


        Stefan