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 browsers, notably hgtk that is part of the TortoiseHg distribution and works for generic Python/GTk platforms. The alternative "view" utility helps to inspect the semantic content of 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, 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 configured 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 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 ----------------------------------------- Compared to a proper distribution or development snapshot, it is relatively hard to build from the raw repository version. Essential contributing components are missing and need to be reconstructed by running the Admin/build script by hand. Afterwards the main Isabelle system and logic images can be compiled via the toplevel ./build script. Note that the repository lacks some textual version identifiers in the sources and scripts; this implies some changed behavior when processing settings etc. There is no guarantee that the NEWS file is up to date at an arbitrary point in history.