Admin/components/README

clarified README;

Some notes on maintaining the Isabelle component repository at TUM
==================================================================

Quick reference
---------------

  $ install /home/isabelle/components/screwdriver-3.14.tar.gz
  $ install /home/isabelle/contrib/screwdriver-3.14/
  $ edit Admin/components/main: screwdriver-3.14
  $ isabelle components_checksum -u
  $ hg diff
  $ hg commit


Unique names
------------

Component names are globally unique over time and space: names of
published components are never re-used.  If some component needs to be
re-packaged, extra indices may be added to the official version number
like this:

  screwdriver-3.14    #default packaging/publishing, no index
  screwdriver-3.14-1  #another refinement of the same
  screwdriver-3.14-2  #yet another refinement of the same

There is no standard format for the structure of component names: they
are compared for equality only, without any guess at an ordering.

Components are registered in Admin/components/main (or similar) for
use of that particular Isabelle repository version, subject to regular
Mercurial history.  This allows to bisect Isabelle versions with full
record of the required components for testing.


Authentic archives
------------------

Isabelle components are managed as authentic .tar.gz archives in
/home/isabelle/components from where they are made publicly available
on http://isabelle.in.tum.de/components/.

Visibility on the HTTP server depends on local Unix file permission:
nonfree components should omit "read" mode for the Unix group/other;
regular components should be world-readable.

The file Admin/components/components.sha1 contains SHA1 identifiers
within the Isabelle repository, for integrity checking of the archives
that are exposed to the public file-system.  The components_checksum
tool helps to update these hash-keys wrt. the information within the
Isabelle repository.


Unpacked copy
-------------

A second unpacked copy is provided in /home/isabelle/contrib/.  This
allows users within the TUM network to activate arbitrary snapshots of
the repository with all standard components being available, without
extra copying or unpacking of the authentic archives.  Testing
services like "isatest" and "mira" do this routinely, and will break
accordingly if this is omitted.