Git-aware Darcs: a tutorial

0. What is Darcs-git

Darcs-git is a branch of Darcs that can work with Git repositories.

Darcs-git is deliberately Darcs, not Git. All commands either work in the same way on Git repositories as on Darcs repositories, or they fail. If you're a Darcs user, you'll like darcs-git. If you're a Git user, you'll probably find it infuriating.

On the other hand, Darcs-git uses stock Git repositories; a Darcs command either works as-is on a Git repository, or fails.

1. What you can expect

The following should work reasonably well on Git repositories:

  darcs changes
  darcs whatsnew
  darcs pull
  darcs send
  darcs record

The following commands work, but have serious performance problems:

  darcs diff
  darcs changes with a file argument

The following commands should in principle work but haven't been tested:

  darcs add
  darcs remove
  darcs dist
  darcs trackdown

The following commands don't work because I'm lazy::

  darcs push
  darcs unrecord
  darcs unpull
  darcs amend-record
  darcs annotate
  darcs rollback

The following commands only work on native Darcs repositories, either because they don't make sense on Git repositories, or because there are perfectly good native Git tools to perform their function:

  darcs initialize
  darcs get/put
  darcs check
  darcs repair
  darcs optimize
  darcs mv
  darcs replace
  darcs resolve
  darcs tag
  darcs setpref

Remote Git repositories are not supported.

2. A tutorial

(0) Build darcs-git

  $ darcs get --partial http://www.pps.jussieu.fr/~jch/software/repos/darcs-git
  $ cd darcs-git
  $ make darcs
  $ make Context.hs
  $ make darcs
  $ cp darcs ~/bin/

(1) Get a copy of the Linux Git repository:

  $ cd /usr/local/src
  $ mkdir linux-2.6
  $ cd linux-2.6
  $ rsync -r rsync://rsync.kernel.org/pub/linux/kernel/people/torvalds/linux-2.6.git .git
  $ curl http://rsync.kernel.org/pub/linux/kernel/people/torvalds/linux-2.6.git/HEAD > .git/HEAD

We still need to bring the cache and working directory into a state that Darcs will be happy with. While this could in principle be done with Darcs itself, it will be faster to do it with Git:

  $ read-tree `cat .git/HEAD`
  $ checkout-cache -a
  $ update-cache --refresh

(2) Check what the friendly Linux hackers have been up to:

  $ darcs changes | more
  $ darcs changes -s | more

(3) Create a local clone of the Linux repository:

  $ cd ..
  $ mkdir linux-2.6-local
  $ mkdir linux-2.6-local/.git
  $ ln -s `pwd`/linux-2.6/.git/objects linux-2.6-local/.git
  $ cp linux-2.6/.git/HEAD linux-2.6-local/.git
  $ cd linux-2.6-local
  $ read-tree `cat .git/HEAD`
  $ checkout-cache -a
  $ update-cache --refresh

(4) Commit some work

First, check that Darcs is happy with the new repository.

  $ darcs whatsnew

This should take a few seconds at most; if it takes minutes instead, try running ``update-cache --refresh''.

Okay, let's add myself to the list of Linux maintainers.

  $ echo 'P:    Juliusz Chroboczek' >> MAINTAINERS

Let's see if Darcs agrees.

  $ darcs whatsnew -s
  $ darcs whatsnew

Everything looks fine, let's record (commit) this patch.

  $ darcs record -a
  $ darcs changes | more
  $ darcs changes -s | more

(5) Send it upstream

If Linus were using Darcs, we could just send him a Darcs patch, which is a patch-like data structure that contains just enough context information to allow Darcs to perform a history-sensitive merge:

  $ darcs send ../linux-2.6

However, until Linus switches to Darcs, we're stuck with old-fashioned patches.

  $ darcs diff -u --patch='.' | mail bill@microsoft.com

Unfortunately, until I've spent some time optimising ``darcs diff'', the above won't terminate on a repository the size of Linux'.

3. Caveats

There is little input validation. In particular, if you enter an e-mail address that doesn't end in ``>'', Darcs will write a commit that neither Git nor Darcs itself will be able to parse.

Darcs never updates the Git cache. If you perform many commits using Darcs, you'll need to manually run ``update-cache --refresh''.

Darcs treats Git merges by reverse-engineering a Darcs merge (thanks to David Roundy for outlining how that can be done). In practice, this means that Darcs will collapse as soon as it sees a nontrivial Git merge.

Juliusz Chroboczek wrote:

How is it, performance-wise?

	-hpa

It depends very much on the command, but in general it's no faster than plain Darcs right now, except for accessing the working directory when you have an up-to-date cache. The nice thing, of course, is that since Darcs-git works on pure Git repositories, you can always switch to using plain Git commands when an operation is likely to be too slow, as I've done in the example above.

(Editor's note: ``Patch'' is Darcs-speak for what BK calls a Changeset.)

Commands that only touch the working dir and the top of the history -- darcs what, darcs record, and so on -- are relatively fast; unlike the native Git equivalents, they stat every file in your tree (I'll be fixing this RSN), but they don't touch the contents of unchanged files. Darcs whatsnew on a full kernel takes 4s on my machine.

Commands that read history (darcs changes, darcs send, darcs push) are very fast until they encounter a merge, as they need to read the full contents of one of the two branches (just the changed files, of course). In practice, that means that doing |darcs whatsnew| gets me the last 90 patches in Linux' history in a few seconds, then stalls. I've tried to track the problem down, but after an afternoon of work I had gotten no further. (I've explained the issue in excruciating detail on Darcs-Devel, if you're interested.)

Commands that push patches around scale rather badly with the amount of files touched by a patch. In practice, this means that you'll be able to push around all patches in a kernel tree in a reasonable time, except for the initial commit (220MB of touched files), which you can only push using the native Git tools.

And then there is what I consider as the actual showstopper: darcs diff is completely brain-damaged (very much so in plain Darcs, even more so in Darcs-git). It needs a complete rewrite.

                                        Juliusz