Using git for patch management in Fedora

[Richard W.M. Jones]

Several packages are using git for patch management.  eg:

http://pkgs.fedoraproject.org/cgit/erlang.git/tree/erlang.spec#n46
http://pkgs.fedoraproject.org/cgit/libguestfs.git/tree/libguestfs.spec?h=f20#n22
http://pkgs.fedoraproject.org/cgit/qemu.git/tree/
http://pkgs.fedoraproject.org/cgit/ocaml.git/tree/ocaml.spec#n16

Some of these packages have invented home-brewed methods to generate
the Patch lines in the spec file, eg:

http://pkgs.fedoraproject.org/cgit/erlang.git/tree/otp-get-patches.sh
http://pkgs.fedoraproject.org/cgit/libguestfs.git/tree/copy-patches.sh?h=f20

More importantly, all are using random git repositories to store the
exploded tree.  This makes it difficult for co-maintainers and proven
packagers to fit in with the patch management chosen by the
maintainer.  Usually they won't have access to the git repository for
these patches, making it difficult to add patches and near impossible
to upgrade to a new version.

I think that git is an excellent way to manage patches, but we ought
to think about formalizing this process.  I think the goals should be:

(1) A git repository is used that co-maintainers and proven packagers
automatically have access to.

(2) A single method & script is used to update the patches in the spec file.

Although there is already a git repository satisfying (1) above,
namely dist-git, it isn't suitable for storing the exploded tree since
commits to the spec file would conflict with commits (patches) to the
tree.  So either a separate side repository which packages could
opt-in to, or perhaps a separate branch of the same git repo could be
used.  I think using a branch would require no additional
infrastructure.

For (2) I would suggest a lightweight technique where git-managed
patches are marked in the spec file using:

  ### GIT-MANAGED-PATCHES ###
  ### END-GIT-MANGED-PATCHES ###

and a simple script that replaces everything between those marks with
PatchXXXX lines.  The script could be adapted from copy-patches.sh
(see above).

To apply the patches, a standard RPM macro could be created:

  %prep
  %setup -q
  %{git_apply_patches}

which would expand to something like:

  git init
  git config user.email "%{name}-owner at fedoraproject.org"
  git config user.name "%{name}"
  git add .
  git commit -a -q -m "%{version} baseline"
  git am %{patches}

Thoughts on this?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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