(PITCH)

[archive as vcs]

The Debian archive is a version control system.

  Clone (checkout) <=> apt-get source
  Commit+push <=> upload

But:

  No sensible branching
  history browsing is appalling 
  Terrible interaction with other VCSs

Solution

  Well, we might replace it.  But the archive is various other things
  besides an appallingly bad VCS.  So instead, let's build something
  to deal with the problems.

  What that really means is that we need a better gateway.

[manpage]

dgit is a tool which lets you treat the archive as if it were a git
server.

It provides uniform operation for all packages:

You can clone any package, work on it, build it, and upload it.

You don't need to know maintainer's workflow.  It doesn't matter
whether the maintainer uses dgit, other git tools, quilt, CVS or SCCS.

With dgit you do all direct source code management in git.  As a dgit
user you do not interact with the archive directly.

dgit is particularly useful for NMUers: you can prepare an RC bugfix,
with full support from git, withut needing to know anything about the
package's usual VCS arrangements.

[demo]

dgit also has great potential for downstreams - that is, derivatives
and users who want to modify a package.  Having used dgit clone or
fetch, you can merge into your downstream branch.  (There are some
issues with this right now for non-DDs, which I'm going to discuss
later.)

As a maintainer you can choose, if you like, to use the dgit git
history as your primary working history.  Any fast forwarding
patches-applied git workflow works with dgit.  In particular, you can
have the full upstream git history in the ancestry if your dgit git
history.


(PRINCIPLES OF OPERATION)

[ .dsc, dpkg-source -x, git checkout, identity ]

This is Debian so you want to know how it works.  So let me run
through dgit's principles of operation.  The data model is as follows:

A dgit-generated upload's .dsc contains a git commit hash.
This specifies a commit whose tree is identical to the results of
dpkg-source -x on the .dsc.

But the actual git history is not stored in the archive.  It is
obtained via the git protocol from an actual git server.  (Currently
this is on alioth but it's going to move.)

The only other constraint on the git commit named in the .dsc is that
the successive dgit uploads must have a fast-forwarding history.
Specifically, each upload made with dgit must have as an ancestor the
current state of that package in the archive.

[ synthetic commit example ]

Non-dgit uploads don't have a (suitable) git commit hash.  But
dgit clone needs to produce a suitable git commit.  It does this by
inventing (in a deterministic way) a commit corresponding to the state
of the archive.  If necessary, it also generates a synthetic merge
commit to tie the invented commit into the previous dgit history.

(QUILTY WORKFLOW)

At the moment, dgit doesn't attempt to do anything clever with `3.0
(quilt)' source packages.

The synthetic git history generated from non-dgit uploads does not
represent the quilt patch stack.  And conversely, dgit push involves
dpkg-source commit, to make the git tree be the same as dpkg-source
would extract.  So dgit has to make some patches, and currently it
makes single synthetic patch whose description contains some info from
git log.

Overall this means that currently when you work on a quilty package in
dgit, you don't interact with the quilt patch stack.  This is less
than ideal and I intend to improve this, perhaps by having dgit use
git-dpm as a bidirectional gateway between `3.0 (quilt)' and git.

This will generate a rebasing-style git branch.  After a patch series
has been edited with rebase, dgit push will have to generate a `fake
merge' commit to make the resulting history fast-forwarding.  (This is
a well-understood git manipulation.)

(ACCESS PROBLEMS)

[table]



  works well for DDs
  works badly for everyone else :-/
    serious problem, needs escalating

  access limitations

    ssh cocca psql -c SELECT BLAH

      ftpmaster have a plan to help with this but has not been implemented

    alioth collab-maint

      push by signed tag

      need to abandon alioth because of collab-maint security issues


xxx adoption