[asdf-devel] summary proposal for COMMON_LISP_PATH and system search changes

[Gary King]

Take 2.

Thanks for all the comments and ideas; and is a new summary.


# Proposal Summary

These are independent but make the most sense if taken as a
whole.

  * Specify new defaults for `*central-registry*`,

  * use an environment variable named `CL_SYSTEMS_PATH` to
    override ASDF's defaults,

  * use `*central-registry*` to find and load an ASDF
    configuration file (if present) and

  * modify the default behavior of
    `sysdef-central-registry-search` to search deeply.

##  default value for *central-registry*

CL_SYSTEMS_PATH environment variable with syntax like

     path[:path]* (on *nix)

or

     path[;path]* (on Windows)

I.e., path designators separated by an OS dependent marker
(colons or semi-colons). This is used by ASDF to help build
the default value for `*central-registry*`. Each `path` will
be interpreted as follows:

* a `!` will be replaced by the default value (see below) so
   that it is easy to extend the path rather than just
   replacing it.

* empty ones will be removed from the list,

* since these represent _directories_, trailing slashes will
   be added if necessary,

* if the last component is a `*` or `**` then sub-directories
   of this path will be searched recursively either one level
   deep (if `*`) or all the way down (if `**`).

If not specified, `*central-registry*` defaults to (on
*nix-like systems):

     (list
       *default-pathname-defaults*
       (merge-pathnames
         (make-pathname :directory '(:relative ".common-lisp"))
                        (user-homedir-pathname)))
       (merge-pathnames
         (make-pathname :directory '(:relative ".local" "share"
"common-lisp"))
                        (user-homedir-pathname)))
       (make-pathname :directory '(:absolute "usr" "local" "share"
"common-lisp"))
       (make-pathname :directory '(:absolute "usr" "share" "common-
lisp"))
       )


(On a windows system, the last two would be replaced by
c:\common-lisp and c:\windows\common-lisp).

Note that this list would be automatically filtered for any
entries for which the moral equivalent of `(ignore-errors
(truename x))` fails.

## modification for `sysdef-central-registry-search`

Currently `sysdef-central-registry-search` searches in the
top-level of each directory in `*central-registry*` to find
systems. If we use the above, then a more useful approach
would be to search in the top-level and in a sub-directory
named `systems`. An even more useful approach would to search
in all sub-directories of each entry.

## configuration file

When ASDF starts, it will:

  1. setup `*central-registry*` using its own defaults and the
`CL_SYSTEMS_PATH` environment variable.

  2. look for and load the *first* file named
`asdf-config.lisp` in each of these paths traversed in order
(so that user settings take precedence over system ones).
Note that the ASDF package will exist when this file is read.

I.e., I'm suggesting "if there are any user settings, then
ignore the system settings" one rather than the "user
settings overwrite system settings".