README_REPOSITORY

Important notes on Mercurial repository access for Isabelle.

Important notes on Mercurial repository access for Isabelle
===========================================================

Preamble
--------

Mercurial http://www.selenic.com/mercurial belongs to a new generation
of source code management systems, following the paradigm of
"distributed version control".  Compared to the old centralized model
of CVS/SVN, this gives considerable more power and freedom in
organizing the flow of changes, both between individual developers and
designated pull/push areas that are shared with others.

More power for the user also means more responsibility!  Due to its
decentralized nature, changesets that have been published once,
e.g. via push to a shared repository that is visible on the net,
cannot be easily retracted from the public again.  Regular Mercurial
operations are strictly monotonic, where changset transactions are
only added, but never deleted.  There are special tools to "repair"
individual repositories via non-monotonic actions, but this does not
yet retrieve unwanted changesets that have escaped into the public.

Only global operations like pull/push, unbundle/bundle etc. fall into
this critical category; incoming/outgoing or in/out may help to
inspect changesets before exchanging them globally.  Anything else in
Mercurial is local to the user's repository clone (including
commit/update/merge etc.) and is in fact much simpler and safer to use
than the corresponding operations of CVS or SVN.


Initial configuration
=====================

Never use any Mercurial version before 1.0!  At the moment, versions
1.0, 1.0.1 and 1.0.2 all work equally well.


The public pull area of the Isabelle repository can be cloned like
this:

  hg clone http://isabelle.in.tum.de/repos/isabelle

This will create a local directory "isabelle", unless an alternative
name is specified.  The full repository meta-data and history of
changes is in isabelle/.hg; local configuration for this clone can be
added to isabelle/.hg/hgrc, but note that hgrc files are never copied
by another clone operation!

There is also ~/.hgrc for per-user Mercurial configuration.  The
initial configuration should include at least an entry to identify
yourself.  For example, something like this in /home/wenzelm/.hgrc:

  [ui]
  username = wenzelm

Failing to configure the username correctly makes the system invent
funny machine names that may persist eternally in the flow of
changesets.

In principle, user names can be chosen freely, but for longterm
committers of the Isabelle repository the obvious choice is to keep
with the old CVS naming scheme.


There are other useful configuration to go into ~/.hgrc, e.g. defaults
for common commands:

  [defaults]
  log = -l 10

The next example shows how to install some Mercurial extension:

  [extensions]
  hgext.graphlog =

Now the additional glog command will be available.


See also the fine documentation for further details, especially the
book http://hgbook.red-bean.com/


Shared pull/push access
=======================

The entry point http://isabelle.in.tum.de/repos/isabelle is world
readable, both via plain web browsing and the hg client as described
above.  Anybody can produce a clone, change it arbitrarily, and then
use regular mechanisms of Mercurial to report changes "upstream", say
via e-mail to someone with write access to that file space.  It is
also quite easy to publish changed clones again on the web, using hg
serve, or the hgweb.cgi or hgwebdir.cgi scripts that are included in
the Mercurial distribution.

This downstream/upstream mode of operation is quite common in the
distributed version control community, and works well for occasional
changes produced by anybody out there.  Of course, upstream
maintainers need to review and moderate changes being proposed, before
pushing anything onto the official Isabelle repository at TUM.


Direct pull/push access requires an account at TUM, with properly
configured ssh access to the local machines (e.g. macbroy20,
atbroy100).

The simple wrapper script /home/isabelle/mercurial/bin/hg provides a
uniform view on the different Linux installations on the local
network.  Thus it is advisable to add that directory to the shell PATH
of the account at TUM:

  PATH="/home/isabelle/mercurial/bin:$PATH"

Now a clone of the shared push/pull area can be produced like this,
using your user name instead of "wenzelm":

  hg clone ssh://wenzelm@atbroy100//home/isabelle-repository/repos/isabelle

In fact, the only difference to the previous clone of
http://isabelle.in.tum.de/repos/isabelle will be a different default
pull/push path in isabelle/.hg/hgrc:

  [paths]
  default = ssh://wenzelm@atbroy100//home/isabelle-repository/repos/isabelle

This means an earlier pull-only clone can be changed into a pull/push
version by editing this single line of the internal repository
configuration.


Content discipline
==================

Old-style centralized version control is occasionally compared with a
"library where everybody scribbles into the books".  Or seen the other
way round, the centralized model discourages individual
experimentation (with local branches etc.), because everything is
forced to happen on a shared file space.  With Mercurial, arbitrary
variations on local clones are no problem, but care is required again
when publishing changes eventually.

The following principles should be kept in mind when producing
changesets that might become public at some point.

  * The author of changes should be properly identified, using
    ui/username configuration as described above.

    While Mercurial also provides means for signed changesets, we want
    to keep things simple and trust that users specify their identity
    correctly.

  * The history of sources is an integral part of the sources
    themselves.  This means that private experiments and branches
    should not be published, unless they are really meant to become
    universally available.

    Note that exchanging local experiments with some other users can
    be done directly on peer-to-peer basis, without affecting the
    central pull/push area.

  * Log messages are an integral part of the history of sources.
    Other users will have to look there eventually, to understand why
    things have been done in a certain way at some point.

    Mercurial provides nice web presentation of incoming changes with
    a digest of log entries; this also includes Atom/RSS news feeds.
    Users should be aware that others will actually read what is
    written into log messages.

    The usual changelog presentation style for the Isabelle repository
    admits log entries that consist of several lines, but without the
    special head line that is used in Mercurial projects elsewhere.
    Since some display styles strip newlines from text, it is
    advisable to separate lines via punctuation, and not rely on
    two-dimensional presentation too much.


Makarius 29-Nov-2008