-*-indented-text-*-

First, for the purposes of this policy, some terminology:

  <flavor> - a particular flavor of Emacs, for example:

    xemacs20
    xemacs21
    emacs21
    emacs22
    emacs23

  <emacs> - either a <flavor> or the generic value "emacs", i.e.:

    emacs
    xemacs20
    xemacs21
    emacs21
    emacs22
    emacs23

  add-on package - any package which wishes to use the emacsen-common
    infrastructure, usually in order to byte-compile itself for some
    subset of the currently installed emacs flavors.


1) emacsen-common

   Each flavor of emacs must depend on emacsen-common.  This package
   contains all of the Debian specific emacs flavor independent files
   like debian-rundir, and all the infrastructure implementing our
   mechanism for dealing with all the various emacsen.

2) Flavor indication

   Each emacs binary must set the variable debian-emacs-flavor to be
   the same as the name of the debian-package.  For example, the
   emacs20 package does this in startup.el:

     (defconst debian-emacs-flavor 'emacs23
       "A symbol representing the particular debian flavor of emacs running.
     Something like 'emacs20, 'xemacs20, etc.")
         (setq debian-emacs-flavor 'emacs20)

   For now, debian-emacs-flavor should be defined regardless of the
   use of -q or --no-site-file.

   [ NOTE: We may eventually consider moving debian-emacs-flavor to
     emacsen-common and have emacsen-common set it based on the
     argument to debian-startup.  That would minimize changes to
     startup.el, but it was attempted briefly in the past, too close
     to a freeze, and caused enough trouble that we backed off.]

3) Emacs startup strategy

  We have the following startup files/directories:

  emacsen-common: /etc/emacs/site-start.{el,d}
  xemacs20:       /etc/xemacs20/site-start.d
  xemacs21:       /etc/xemacs21/site-start.d
  emacs21:        /etc/emacs21/site-start.d
  emacs23:        /etc/emacs23/site-start.d

  /etc/emacs/site-start.el is a conffile, and is owned by
  emacsen-common.  This file should not be modified by *any* add-on
  packages, or by any emacsen package maintainer.  It is solely for
  the use of the local admin.  It should be empty unless the local
  admin modifies it.

  The load-path for a given emacs flavor must include /etc/<flavor>
  and /etc/emacs in that order (see section 9 below), and on startup
  each emacsen must call (debian-startup <flavor>) and then load
  "site-start".  This means that /etc/emacs/site-start.el will be
  loaded if it exists, unless there's an
  /etc/<flavor>/site-start.el{,c} or /etc/emacs/site-start.elc which
  will take precedence.  These site-start.el files are also the sole
  domain of the local system admin, and should be empty unless
  modified by the local admin.

  Given that the site-startup.el files are "off-limits" to the emacsen
  maintainers, the next question is, "Where should we run
  debian-startup?"  The safest possibility, and the one I had
  originally intended was to modify lisp/startup.el to do the right
  thing, and I had also intended that --no-site-file would disable all
  of the debian startup bits, including calling debian startup.  So
  here's what emacs21 had in startup.el:

    ;; Debian version of site-start.
    (if site-run-file
        (progn
          ;; Load all the debian package snippets.
          ;; It's in here because we want -q to kill it too.
          (if (load "debian-startup" t t nil)
              (debian-startup 'emacs21))
          ;; Now the normal site file...
          (load site-run-file t t nil)))

  This makes sure that debian's bits are setup as early as possible,
  and requires only minor modifications to the emacs source.  An
  alternate possibility might be to use lisp/site-init.el, but I'm not
  sure that would work right.  Feel free to contact me if you know
  better.

  debian-startup, among other things, calls debian-run-directories.
  debian-run-directories collects the union of all the file base names
  (i.e. without any .el or .elc extension, and without the directory
  component: i.e. /etc/xemacs/site-start.d/50foo.elc => 50foo), then
  temporarily augments the emacs load path to include
  /etc/<flavor>/site-start.d and /etc/emacs/site-start.d in that
  order, and then calls (load base-name) in alphabetical order.

  This result is that .elc files will take precedence over .el files
  in a given directory, and files in the <flavor> site-start.d
  directory will take precedence over those in the emacs common
  directory.

4) Code locations

  In addition to the /etc/<emacs>/site-start* directories, we have the
  following directories (their use will be described shortly):

  emacsen-common: /usr/share/emacs/site-lisp/
  xemacs20:       /usr/share/xemacs20/site-lisp/
  xemacs21:       /usr/share/xemacs21/site-lisp/
  emacs22:        /usr/share/emacs22/site-lisp/
  emacs23:        /usr/share/emacs23/site-lisp/

  These are treated as part of the normal emacsen load path with the
  <flavor> dir taking precedence over the emacs (common) dir.

  /usr/share/<flavor>/site-lisp should be used instead of the normal
  site-lisp directory for that flavor of emacs, and the package for a
  given flavor of emacs should not have the normal site-lisp
  directory.  For example, instead of the emacs23 package having
  /usr/share/emacs/23.2/site-lisp, it should only have
  /usr/share/emacs23/site-lisp.  This is important because it allows
  us to avoid having dangling directories for old versions across
  upgrades.  We could have chosen to keep a compatibility symlink, but
  that seemed likely to mask bugs in the debianized packages.

5) Packages with only marginal emacs relevance

   Generally, if a normal package just contains some emacs helper
   files, and does not need to perform any byte-compilation or other
   emacs dependent activities upon installation (for performance or
   other reasons), then it is not necessary to specify a dependency on
   emacsen or any flavor of emacs, and the package may just include
   files located in the standard emacs add-on directories.

6) Emacs add-on package support (there are examples later which make
   this much clearer)

  A) Each package may place a file named the same as the package into

       /usr/lib/emacsen-common/packages/install/
       /usr/lib/emacsen-common/packages/remove/

     and the package must call
  
       /usr/lib/emacsen-common/emacs-package-install foo

     in the postinst and 

       /usr/lib/emacsen-common/emacs-package-remove foo

     in the prerm.

     For now, "emacs-package-install foo" will effectively just result
     in a call to

       /usr/lib/emacsen-common/packages/install/foo

     for the common package emacs, and for each installed flavor.

     The arguments to these scripts will be the flavor being installed
     (or just the common name "emacs") followed by a list of other
     flavors already installed, potentially including the current one.

     So if emacs23 and xemacs21 were installed, installing foo would
     result in calls to:

       /usr/lib/emacsen-common/packages/install/foo emacs emacs23 xemacs21
       /usr/lib/emacsen-common/packages/install/foo emacs23 emacs23 xemacs21
       /usr/lib/emacsen-common/packages/install/foo xemacs21 emacs23 xemacs21

     emacs-package remove does the symmetric thing.

  B) Each emacsen main package must have a call to
     "/usr/lib/emacsen-common/emacs-install <flavor> in its postinst
     and a call to "/usr/lib/emacsen-common/emacs-remove <flavor> in
     its prerm.

     The emacs-install script (for now) just executes

       /usr/lib/emacsen-common/packages/install/<pkg> <flavor> <others>

     for each installed <pkg>, but might do more later.  Similarly,
     emacs-remove would just be equivalent to

       /usr/lib/emacsen-common/packages/remove/<pkg> <flavor> <others>

     for now.  Don't forget that the argument list to the add-on
     package install scripts will indicate both the flavor being
     installed, and the flavors already installed.  In this case,
     since we're actually installing a flavor, the first argument
     won't appear in the subsequent arguments.

  C) Each add-on package has the right to place files into the
     following directories:

       /etc/<emacs>/site-start.d
       /usr/share/<emacs>/site-lisp/<package-name>

  D) Each add-on package must declare relevant dependencies on other
     packages (including other add-on packages).  Note that add-on
     packages should not depend on emacsen-common directly, but rather
     on either the virtual package "emacsen" (see below), or some
     appropriate combination of flavors (i.e. Depends: emacs23 |
     emacs10).  In addition, any tools needed by the install/remove
     scripts must be listed as package dependencies.  For example,
     many add-on packages will probably use make in the install
     script, so they need to add "Depends: make" to their control
     file.  emacsen-common will make sure that the install/remove
     scripts are ordered to respect inter-add-on package dependencies.

  E) If an add-on package compiles any of its Emacs Lisp sources
     (which must be compiled to a subdirectory of
     /usr/share/<flavor>/site-lisp/<package-name> -- see section 4
     above) it must also create a symlink in the same directory as the
     compiled output that points to the corresponding source file.

     For example, if add-on package foo produces
     /usr/share/emacs23/site-lisp/foo/bar.elc, then it must also
     create a symlink /usr/share/emacs23/site-lisp/foo/bar.el pointing
     to the corresponding source file.  This ensures that Emacs will
     be able to locate the source code for the add-on package when
     using M-x find-function, etc.

7) Mandatory binary symlink

   Each emacsen main package must have a symlink
   /usr/bin/<package-name> to /usr/bin/<emacs-binary> so that when
   add-on package install/remove scripts are called, they can just use
   /usr/bin/$ARGV[0] to get the right binary for byte-compilation.

8) Virtual package

   Each emacsen main package will "Provides: emacsen".  It would be
   nice to use "emacs", but for historical reasons, and given the
   nature of the packaging system, we can't.

   Packages that just need to make sure some flavor of emacs is
   installed should just "Depends: emacsen".  If they depend on
   specific flavors of emacs, then they should list those dependencies
   explicitly instead.

9) Emacs lisp load path.

   At a minimum, each emacs must have the following directories in the
   given order in their load path:

     /etc/<flavor>
     /etc/emacs
     /usr/local/share/emacs/<upstream>/site-lisp
     /usr/local/share/emacs/site-lisp
     /usr/share/emacs/<flavor>/site-lisp
     /usr/share/emacs/site-lisp

   where <upstream> is the normal emacs upstream version number for
   the relevant flavor like 20.2 or 23.2.

   Emacs add-on packages may not modify load-path directly.  They must
   use (debian-pkg-add-load-path-item <path>).  This function will
   make sure that their additions end up in the right place -- before
   the emacs system directories, but after the /usr/local/
   directories.  Also, add-on packages will need to either check
   (fboundp 'debian-pkg-add-load-path-item) before calling this
   function, or add a dependency on emacsen-common (>= 1.4.14).

10) Usage of autoload instead of load in the site-start.d files.

   It's been suggested, and is probably a good idea that maintainers
   switch to using autoload rather than load when possible in their
   site-start.d files.

   For example, instead of (load "some-package"), you should use
   autoloads for all the top level, user visible functions.  Currently
   the calc package has a good example of this.



That's it.  I think this gives the add-on package maintainers the
flexibility they need to be able to DTRT, and I think the common case
won't be all that difficult.

Examples (I've been told that there may be some bugs in these
examples, but I haven't had time to check through them yet.  So don't
take them as gospel at the moment.  I suggest that you loook at
existing packages for examples of working code.  I will probably be
revamping these soon, but I don't want to do it now since there are
probably about to be some changes that will invalidate some of this):

1) xemacs21 and the add-on packages tm and auctex are already installed,
   and now someone installs emacs23.

   In its prerm, emacs23 would make this call:

     /usr/lib/emacsen-common/emacs-install emacs23

   which would result in calls to

     /usr/lib/emacsen-common/packages/install/auctex emacs xemacs21
     /usr/lib/emacsen-common/packages/install/auctex emacs23 xemacs21
     /usr/lib/emacsen-common/packages/emacs23/install/tm emacs xemacs21
     /usr/lib/emacsen-common/packages/emacs/install/tm emacs23 xemacs21
    
2) Now, given (1), assume that someone removes xemacs21.

   In its prerm, xemacs21 would make this call:

     /usr/lib/emacsen-common/emacs-remove xemacs21

   which would result in calls to

     /usr/lib/emacsen-common/packages/remove/auctex emacs emacs23 xemacs21
     /usr/lib/emacsen-common/packages/remove/auctex xemacs21 emacs23 xemacs21
     /usr/lib/emacsen-common/packages/remove/tm emacs emacs23 xemacs21
     /usr/lib/emacsen-common/packages/remove/tm xemacs21 emacs23 xemacs21

3) Now assume emacs23 and xemacs21 are installed, and that someone removes
   tm.

   The call to emacsen-package-remove in tm's prerm will result in
   the following calls:

     /usr/lib/emacsen-common/packages/remove/tm emacs emacs23 xemacs21
     /usr/lib/emacsen-common/packages/remove/tm emacs23 emacs23 xemacs21
     /usr/lib/emacsen-common/packages/remove/tm xemacs21 emacs23 xemacs21

   In the remove/tm file, tm is responsible for cleaning up any files
   it put into its allowed locations:

     /etc/<emacs>/site-start.d/
     /usr/share/<emacs>/site-lisp/tm

4) Finally, please see sample-package-install-foo and
   sample-package-remove-foo in /usr/share/doc/emacsen-common/.  These
   are sample install and remove scripts for a hypothetical package
   "foo" that only needs to byte compile a list of files for each
   flavor.
