May 02, 2011

Fossil to Git export and mirroring

The biggest issue I have with fossil scm is that it's not git - there are just too many advanced tools which I got used to with git over time, which probably will never be implemented in fossil just because of it's "lean single binary" philosophy.
And things get even worse when you need to bridge git-fossil repos - common denominator here is git, so it's either constant "export-merge-import" cycle or some hacks, since fossil doesn't support incremental export to a git repo out of the box (but it does have support for full import/export), and git doesn't seem to have a plugin to track fossil remotes (yet?).
I thought of migrating away from fossil, but there's just no substitute (although quite a lot of attempts to implement that) for distributed issue tracking and documentation right in the same repository and plain easy to access format with a sensible web frontend for those who don't want to install/learn scm and clone the repo just to file a ticket.
None of git-based tools I've been able to find seem to meet this (seemingly) simple criterias, so dual-stack it is then.
Solution I came up with is real-time mirroring of all the changes in fossil repositories to a git.
It's quite a simple script, which is
  • watching fossil-path with inotify(7) for IN_MODIFY events (needs pyinotify for that)
  • checking for new revisions in fossil (source) repo against tip of a git
  • comparing these by timestamps, which are kept in perfect sync (by fossil-export as well)
  • exporting revisions from fossil as a full artifacts (blobs), importing these into git via git-fast-import

It's also capable to do oneshot updates (in which case it doesn't need anything but python-2.7, git and fossil), bootstrapping git mirrors as new fossil repos are created and catching-up with their sync on startup.

While the script uses quite a low-level (but standard and documented here and there) scm internals, it was actually very easy to write (~200 lines, mostly simple processing-generation code), because both scms in question are built upon principles of simple and robust design, which I deeply admire.

Resulting mirrors of fossil repos retain all the metadata like commit messages, timestamps and authors.
Limitation is that it only tracks one branch, specified at startup ("trunk", by default), and doesn't care about the tags at the moment, but I'll probably fix the latter when I'll do some tagging next time (hence will have a realworld test case).
It's also trivial to make the script do two-way synchronization, since fossil supports "fossil import --incremental" update right from git-fast-export, so it's just a simple pipe, which can be run w/o any special tools on demand.

Script itself.

fossil_echo --help:

usage: fossil_echo [-h] [-1] [-s] [-c] [-b BRANCH] [--dry-run] [-x EXCLUDE]
                      [-t STAT_INTERVAL] [--debug]
                      fossil_root git_root

Tool to keep fossil and git repositories in sync. Monitors fossil_root for
changes in *.fossil files (which are treated as source fossil repositories)
and pushes them to corresponding (according to basename) git repositories.
Also has --oneshot mode to do a one-time sync between specified repos.

positional arguments:
  fossil_root           Path to fossil repos.
  git_root              Path to git repos.

optional arguments:
  -h, --help            show this help message and exit
  -1, --oneshot         Treat fossil_root and git_root as repository paths and
                        try to sync them at once.
  -s, --initial-sync    Do an initial sync for every *.fossil repository found
                        in fossil_root at start.
  -c, --create          Dynamically create missing git repositories (bare)
                        inside git-root.
  -b BRANCH, --branch BRANCH
                        Branch to sync (must exist on both sides, default:
  --dry-run             Dump git updates (fast-import format) to stdout,
                        instead of feeding them to git. Cancels --create.
  -x EXCLUDE, --exclude EXCLUDE
                        Repository names to exclude from syncing (w/o .fossil
                        or .git suffix, can be specified multiple times).
  -t STAT_INTERVAL, --stat-interval STAT_INTERVAL
                        Interval between polling source repositories for
                        changes, if there's no inotify/kevent support
                        (default: 300s).
  --debug               Verbose operation mode.
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