Git usage
=========
:toc:
Cyril Brulebois <kibi@debian.org>


[NOTE]
The old documentation is still available on the wiki:
https://wiki.debian.org/XStrikeForce/git-usage[wiki.debian.org/XSF/git-usage];
this documentation is just a draft for now.


Getting started
---------------

Upstream repositories are hosted on
https://cgit.freedesktop.org/[git.freedesktop.org]

Debian repositories are hosted on
https://salsa.debian.org/[salsa.debian.org] under the `xorg-team`
group. Each repository is about a single Debian source package.

We have two types of repositories:

* *regular* packages: 1 upstream repository → 1 Debian repository → 1
   Debian source package (non-native).
* *bundle* packages: multiple upstream repositories → 1 Debian
   repository → 1 Debian source package (native).

A local git repository can have several remotes. In the context of
Debian packaging, one usually starts by cloning the Debian repository,
so `origin` will likely point to `salsa.debian.org`. One can use
`upstream` to point to `anongit.freedesktop.org`. The following
documentation assumes this convention.

The following bits in `~/.gitconfig` will make it possible to fetch
updates using `https` (anonymously), and to push updates
through `ssh` without having to fiddle with the remote’s URL (in other
words: using `https://salsa.debian.org` everywhere):

----
[url "ssh://git@salsa.debian.org"]
    pushInsteadOf = "https://salsa.debian.org"
----

To get the repository from `salsa.debian.org` one can run `debcheckout
$package` (or `debcheckout $package $package.git`), which will use the
`Vcs-Git` fields in the APT cache to pick the appropriate git
location. To add the `upstream` remote (using the info stored in
`debian/watch`), one can use `xsf-remote-add-upstream` script from the
https://salsa.debian.org/xorg-team/debian/xsf-tools[xsf-tools]
repository.

**TODO:** There will be more information about how to deal with the
many repositories maintained by the X Strike Force in a later chapter.

The usual workflow is to keep the target suite in `debian/changelog`
to `UNRELEASED` until the upload happens, the last commit before a
commit being only `dch -r`. To achieve that, and to avoid noise since
those packages are comaintained, it’s advised to set the following
variable in `~/.devscripts`:

----
DEBCHANGE_RELEASE_HEURISTIC=changelog
----


Regular packages
----------------

For most packages (exceptions include `xorg-server`), development is
linear, and happens in a `master` branch. That `master` branch is
pushed in the Debian repository as `upstream-$suite`
(e.g. `upstream-unstable`), depending on the target suite. Usually,
`upstream-unstable` tracks `upstream/master`.

The packaging is kept in `debian-$suite` branches, branched from
`upstream-$suite`. When cloning a Debian repository, the default
branch is `debian-unstable`.

To create the initial packaging from the `upstream-unstable` branch,
just run `git checkout -b debian-unstable`, add packaging files
(`changelog`, `control`, `copyright`, `rules` etc. under `debian/`),
and that’s it.

Here’s how to merge from upstream (`$foo` being a tag or
`upstream/master`):

----
    git checkout upstream-unstable
    git merge $foo
    git log $foo > ChangeLog
    dch -v $debianrevision
    git commit -am 'Bump changelogs.'
----

`$debianrevision` is usually `$foo` with `-1` appended (first upload),
and sometimes prepended with a epoch (for example `2:`). Passing
`$foo-1` is usually a good rule of thumb, since `dch` will complain if
the epoch is missing (given the specified version string wouldn’t be
newer than the current one).

When development isn’t linear
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For packages like `xorg-server` and `libx11`, there are stable
branches which receive updates for a while. Trying to switch from
`1.10.2` to `1.11.0` might trigger a lot of conflicts. But in the end
what matters is the changes between `upstream-$suite` and
`debian-$suite`. Here’s an example, supposing `upstream-unstable` and
`debian-unstable` are pointing to the “old” branches, and supposing
the new branch is `upstream/master`:

----
git checkout -b debian-unstable-new upstream/master
git merge -s ours upstream-unstable
git merge debian-unstable
git branch -d debian-unstable
git branch -m debian-unstable
----

Subtitles:

* Create a `debian-unstable-new` branch starting with the upstream
  `master` branch, and switch to it.
* “Merge” the old `upstream-unstable` branch, actually keeping only
  the new upstream branch.
* Merge the old packaging on top of it.
* Remove the old branch (so that the name can be reused).
* Rename the current `debian-unstable-new` branch into
  `debian-unstable`.

Since the tip of the new `debian-unstable` branch is a descendant of
the tip of the old `debian-unstable` one, it can be pushed normally.

Since old `upstream-unstable` and new `upstream-unstable` diverged,
this branch has to be pushed with a `-f` to force the update (it’s not
a fast-forward).


Bundle packages
---------------

One bundle package is a Debian native package, with just a (Debian)
tarball, instead of an upstream tarball plus a Debian diff.

There is no upstream branches here, only `debian-$suite`.

The repository contains a `debian/` directory for the packaging, and
one directory per upstream source. Merging a new upstream release
means updating the contents of the relevant directory with the
contents of the new upstream tarball. Fetching new tarballs is
automated through a specific target: `make -f debian/rules
get-tarballs`

To perform an update, first run `dch -i` to create a new changelog
entry if the previous commit was an upload (the new entry targets the
`UNRELEASED` suite, see “Foreword”).

Assuming `get-tarballs` made `foo-bar.tar.gz` appear in the top-level
directory, here’s how to update (trailing slashes are not needed, just
there to clarify we’re working on directories):

----
git rm -r foo/
tar xf foo-bar.tar.gz
mv foo-bar/ foo/
git add foo/
dch "foo bar"
debcommit -a
----

Using the `xsf-remote-add-upstream` script will create several
`upstream-$foo` remotes, using info stored in `debian/watch*`. This
helps browsing the history of a given repository (rather than having
to look at a big fat diff with autogenerated files in the middle).


Upgrade checklist
-----------------

[NOTE]
Since it’s likely for a reader of this page to be on her way to update
a package, here’s a tiny upgrade checklist.

Basic checks include looking into what happened to those files since
the last packaging update:

* `COPYING`: Update `debian/copyright` accordingly.
* `configure.ac` (or `configure.in`): Update (build-)dependencies
  accordingly.

About xorg macros (they show up very often in the diff), they’re
shipped in the `xutils-dev` package, which contains a file to help map
macro versions and package versions:
`/usr/share/doc/xutils-dev/versions`

Some packages might have more specific instructions. That’s the case
for at least `xorg-server`. See its `debian/README.source`, below the
generic “how to use quilt” blurb.
