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 or 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 manipulate individual repositories via non-monotonic actions, but this does not yet retrieve any changesets that have escaped into the public by accident. Only global operations like "pull" and "push" fall into this critical category. Note that "incoming" / "outgoing" allow 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 --------------------- Always use Mercurial version 1.0 or later, such as 1.0.1 or 1.0.2. The old 0.9.x versions do not work in a multi-user environment with shared file spaces. 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 should include at least an entry to identify yourself. For example, something like this in /home/wenzelm/.hgrc: [ui] username = wenzelm Of course, the user identity can be also configured in isabelle/.hg/hgrc on per-repository basis. Failing to specify the username correctly makes the system invent funny machine names that may persist indefinitely in the public 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 $HOME/.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 the adhoc command "hg serve -v", or the hgweb.cgi or hgwebdir.cgi scripts that are included in the Mercurial distribution. 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. Write access to the Isabelle repository requires an account at TUM, with properly configured ssh access to the local machines (e.g. macbroy20, atbroy100). 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@atbroy100//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@atbroy100//home/isabelle-repository/repos/isabelle Default paths for push and pull can be configure in isabelle/.hg/hgrc, for example: [paths] default = ssh://wenzelm@atbroy100//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@atbroy100//home/isabelle-repository/repos/isabelle Content discipline ------------------ Old-style centralized version control is occasionally compared to "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 RSS/Atom 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 headline 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. Building Isabelle from the repository version --------------------------------------------- Compared to a proper distribution or development snapshot, a repository version of Isabelle lacks textual version identifiers in some sources and scripts, and various components produced by Admin/build are missing. After applying that script with suitable arguments, the regular user instructions for building and running Isabelle from sources apply. Needless to say, the results from the build process must not be added to the repository! Makarius 30-Nov-2008