Important notes on Mercurial repository access for Isabelle
===========================================================
Introduction
------------
Mercurial http://www.selenic.com/mercurial belongs to the current
generation of source code management systems that follow the so-called
paradigm of "distributed version control". This is a terrific name
for plain revision control without the legacy of CVS or SVN. See also
http://hginit.com/ for an introduction to the main ideas. The
Mercurial book http://hgbook.red-bean.com/ explains many more details.
Mercurial offers great flexibility in organizing the flow of changes,
both between individual developers and designated pull/push areas that
are shared with others. This additional power demands some additional
responsibility to maintain a certain development process that fits to
a particular project.
Regular Mercurial operations are strictly monotonic, where changeset
transactions are only added, but never deleted. There are special
tools to manipulate repositories via non-monotonic actions (such as
"rollback" or "strip"), but recovering from gross mistakes that have
escaped into the public can be hard and embarrassing. It is much
easier to inspect and amend changesets routinely before pushing.
The effect of the critical "pull" / "push" operations can be tested in
a dry run via "incoming" / "outgoing". The "fetch" extension includes
useful sanity checks beyond raw "pull" / "update" / "merge". Most
other operations are local to the user's repository clone. This gives
some freedom for experimentation without affecting anybody else.
Mercurial provides nice web presentation of incoming changes with a
digest of log entries; this also includes RSS/Atom news feeds. There
are add-on history browsers such as "hg view" and TortoiseHg. Unlike
the default web view, some of these tools help to inspect the semantic
content of non-trivial merge nodes.
Initial configuration
---------------------
The official 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 $HOME/.hgrc for per-user Mercurial configuration. The
initial configuration requires at least an entry to identify yourself
as follows:
[ui]
username = XXX
Isabelle contributors are free to choose either a short "login name"
(for accounts at TU Munich) or a "full name" -- with or without mail
address. It is important to stick to this choice once and for all.
The machine default that Mercurial produces for unspecified
[ui]username is not appropriate.
Such configuration can be given in $HOME/.hgrc (for each home
directory on each machine) or in .hg/hgrc (for each repository clone).
Here are some further examples for hgrc. This is how to provide
default options for common commands:
[defaults]
log = -l 10
This is how to configure some extension, which is called "graphlog"
and provides the "glog" command:
[extensions]
hgext.graphlog =
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 locally, and then use
regular mechanisms of Mercurial to report changes upstream, say via
mail to someone with write access to that file space. It is also
quite easy to publish changed clones again on the web, using the
ad-hoc command "hg serve -v", or the hgweb.cgi or hgwebdir.cgi scripts
that are included in the Mercurial distribution, and send a "pull
request" to someone else. There are also public hosting services for
Mercurial repositories.
The 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
pushes are also reviewed routinely in a post-hoc fashion; everybody
hooked on the main repository is called to keep an eye on incoming
changes. In any case, changesets need to be understandable, at the
time of writing and many years later.
Push access to the Isabelle repository requires an account at TUM,
with properly configured ssh to the local machines (e.g. macbroy20 ..
macbroy29). You also need to be a member of the "isabelle" Unix
group.
Sharing a locally modified clone then works as follows, using your
user name instead of "wenzelm":
hg out ssh://wenzelm@macbroy20//home/isabelle-repository/repos/isabelle
In fact, the "out" or "outgoing" command performs only a dry run: it
displays the changesets that would get published. An actual "push",
with a lasting effect on the Isabelle repository, works like this:
hg push ssh://wenzelm@macbroy20//home/isabelle-repository/repos/isabelle
Default paths for push and pull can be configured in
isabelle/.hg/hgrc, for example:
[paths]
default = ssh://wenzelm@macbroy20//home/isabelle-repository/repos/isabelle
Now "hg pull" or "hg push" will use that shared file space, unless a
different URL is specified explicitly.
When cloning a repository, the default path is set to the initial
source URL. So we could have cloned via that ssh URL in the first
place, to get exactly to the same point:
hg clone ssh://wenzelm@macbroy20//home/isabelle-repository/repos/isabelle
Simple merges
-------------
The main idea of Mercurial is to let individual users produce
independent branches of development first, but merge with others
frequently. The basic hg merge operation is more general than
required for the mode of operation with a shared pull/push area. The
"fetch" extension accommodates this case nicely, automating trivial
merges and requiring manual intervention for actual conflicts only.
The fetch extension can be configured via $HOME/.hgrc like this:
[extensions]
hgext.fetch =
[defaults]
fetch = -m "merged"
To keep merges semantically trivial and prevent genuine merge
conflicts or lost updates, it is essential to observe to following
general discipline wrt. the public Isabelle push area:
* Before editing, pull or fetch the latest version.
* Accumulate private commits for a maximum of 1-3 days.
* Start publishing again by pull or fetch, which normally produces
local merges.
* Test the merged result as usual and push back in real time.
Piling private changes and public merges longer than 0.5-2 hours is
apt to produce some mess when pushing eventually!
Content discipline
------------------
The following principles should be kept in mind when producing
changesets that are meant to be published at some point.
* The author of changes needs to 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 (and uniquely).
* The history of sources is an integral part of the sources
themselves. This means that private experiments and branches
should not be published by accident. Named branches are not
allowed on the public version. Note that old SVN-style branching
is replaced by regular repository clones in Mercurial.
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 people will have to inspect them routinely, to understand
why things have been done in a certain way at some point.
Authors of log entries should be aware that published changes are
actually read. The main point is not to announce novelties, but
to describe faithfully what has been done, and provide some clues
about the motivation behind it. The main recipient is someone who
needs to understand the change in the distant future to isolate
problems. Sometimes it is helpful to reference past changes via
changeset ids in the log entry.
* The standard changelog entry format of the Isabelle repository
admits several (vaguely related) items to be rolled into one
changeset. Each item is then described on a separate line that
may become longer as screen line and is terminated by punctuation.
These lines are roughly ordered by importance.
This format conforms to established Isabelle tradition. In
contrast, the default of Mercurial prefers a single headline
followed by a long body text. The reason for that is a somewhat
different development model of typical "distributed" projects,
where external changes pass through a hierarchy of reviewers and
maintainers, until they end up in some authoritative repository.
Isabelle changesets can be more spontaneous, growing from the
bottom-up.
The web style of http://isabelle.in.tum.de/repos/isabelle/
accommodates the Isabelle changelog format. Note that multiple
lines will sometimes display as a single paragraph in HTML, so
some terminating punctuation is required. Do not squeeze multiple
items on the same line in the original text!
Building a repository version of Isabelle
-----------------------------------------
The regular "isabelle build" tool allows to build session images as
usual, but this first requires to resolve add-on components first,
including the ML system. Some extra configuration is required to
approximate some of the system integration of official Isabelle
releases from a bare-bones repository snapshot. The special directory
Admin/ -- which is absent in official releases -- might provide some
further clues.
Here is a reasonably easy way to include important Isabelle components
on the spot:
(1) The bash script ISABELLE_HOME_USER/etc/settings is augmented by
some shell function invocations like this:
init_components "$HOME/.isabelle/contrib" "$ISABELLE_HOME/Admin/components/main"
init_components "$HOME/.isabelle/contrib" "$ISABELLE_HOME/Admin/components/optional"
This uses some central place "$HOME/.isabelle/contrib" to keep
component directories that are shared by all Isabelle versions.
(2) Missing components are resolved on the command line like this:
isabelle components -a
This will saturate the "$HOME/.isabelle/contrib" directory structure
according to $ISABELLE_COMPONENT_REPOSITORY.
Since the given component catalogs in $ISABELLE_HOME/Admin/components
are subject to the Mercurial history, it is possible to bisect over a
range of Isabelle versions while references to the contributing
components change accordingly.