Updating Elisp files while Emacs is running

[Philipp Stephani]

Hi,

OS-level package managers such as Debian's dpkg generally replace files
unconditionally when upgrading. Typically this is not a problem because
running binaries will continue to use the in-memory image, and daemons can
get restarted or sent a signal without user interaction. However, for Emacs
the situation seems different: Emacs not only needs its (dumped) binary,
but also relies on Elisp files. For Elisp files already loaded when the
upgrade is running this typically doesn't cause problems, but when Elisp
files aren't already loaded, replacing them causes problems because the
directories containing them are versioned (see e.g. lispdir in configure.ac),
so that when the version changes (even for minor changes, such as updating
from the recent pretest-1 to pretest-2), the files are no longer at the
expected location, and libraries can't be loaded any more, either manually
or via autoloads. Similar things happen for system-wide ELPA packages (in
the package-directory-list directories), because the names of these
directories are also versioned. How could this be changed so that
unattended upgrades changing the Emacs version are possible while Emacs is
running? Could the Elisp file directories get names that don't include the
version number (maybe hidden behind a configure option)? Is it possible to
install system-wide ELPA packages into directories that don't contain
version numbers? Or is there another way, e.g. using a signal to
reinitialize the load-path and the package system? I'm aware these
suggestions would still cause issues if e.g. Elisp files get moved around
or the byte code format changes, but that seems to be comparatively rare.

Thanks,
Philipp