RE: always put Customizations in `custom-file', never in `user-init-file'

[Stephen J. Turnbull]

Drew Adams writes:

 > I'm not sure I follow completely. Could you run down such a scenario,
 > including when the error occurred? During which Emacs session: before,
 > during, or after migration, and when during the session?

No, I can't be precise because I'm not familiar with the Emacs startup
process.  In the XEmacs cases I recall, the error occured during
migration when the init file is being loaded but before the
customizations were initialized (or perhaps partially initialized).
Then the user would customize some options and custom-save after
return to top-level, which would omit any customizations that had not
been initialized.

 > Let's try to find a safe way to do it, then.

It's not terribly unsafe in terms of odds; thousands of people have
migrated init files with only a few having problems.  Thing is, a
large collection of Customizations is extremely costly to reproduce,
and will be a nagging annoyance for weeks or months.

So the most important thing is to keep a backup copy of the user init
file during migration.  It's also important to be aware that there are
potential pitfalls to using custom-save-all to write the new
customizations file, because it writes the whole database in one shot
based on current state, rather than merging changes into the on-disk
version.