dgit-maint-debrebase

NAME

dgit - tutorial for package maintainers, using a workflow centered around git-debrebase(1)

INTRODUCTION

This document describes elements of a workflow for maintaining a non-native Debian package using dgit. We maintain the Debian delta as a series of git commits on our master branch. We use git-debrebase(1) to shuffle our branch such that this series of git commits appears at the end of the branch. All the public git history is fast-forwarding, i.e., we do not rewrite and force-push. Some advantages of this workflow:

•

Manipulate the delta queue using the full power of git-rebase(1), instead of relying on quilt(1), and without having to switch back and forth between patches-applied and patches-unapplied branches when committing changes and trying to build, as with gbp-pq(1).

•

If you are using 3.0 (quilt), provide your delta queue as a properly separated series of quilt patches in the source package that you upload to the archive (unlike with dgit-maint-merge(7)).

•

Avoid the git tree being dirtied by the application or unapplication of patches, as they are always applied.

•

Benefit from dgit's safety catches. In particular, ensure that your upload always matches exactly your git HEAD.

•

Provide your full git history in a standard format on dgit-repos, where it can benefit downstream dgit users, such as people using dgit to do an NMU (see dgit-nmu-simple(7) and dgit-user(7)).

•

Minimise the amount you need to know about 3.0 (quilt) in order to maintain Debian source packages which use that format.

This workflow is appropriate for packages where the Debian delta contains multiple pieces which interact, or which you don't expect to be able to upstream soon. For packages with simple and/or short-lived Debian deltas, use of git-debrebase(1) introduces unneeded complexity. For such packages, consider the workflow described in dgit-maint-merge(7).

INITIAL DEBIANISATION

This section explains how to start using this workflow with a new package. It should be skipped when converting an existing package to this workflow.

When upstream tags releases in git

Suppose that the latest stable upstream release is 1.2.2, and this has been tagged '1.2.2' by upstream.

    % git clone -oupstream https://some.upstream/foo.git
    % cd foo
    % git verify-tag 1.2.2
    % git reset --hard 1.2.2
    % git branch --unset-upstream

The final command detaches your master branch from the upstream remote, so that git doesn't try to push anything there, or merge unreleased upstream commits. To maintain a copy of your packaging branch on salsa.debian.org in addition to dgit-repos, you can do something like this:

    % git remote add -f origin salsa.debian.org:Debian/foo.git
    % git push --follow-tags -u origin master

Now go ahead and Debianise your package. Make commits on the master branch, adding things in the debian/ directory, or patching the upstream source. For technical reasons, it is essential that your first commit introduces the debian/ directory containing at least one file, and does nothing else. In other words, make a commit introducing debian/ before patching the upstream source. Finally, you need an orig tarball:

    % git deborig

See git-deborig(1) if this fails. This tarball is ephemeral and easily regenerated, so we don't commit it anywhere (e.g. with tools like pristine-tar(1)). Comparing upstream's tarball releases Using untagged upstream commits

When upstream releases only tarballs

Because we want to work in git, we need a virtual upstream branch with virtual release tags. gbp-import-orig(1) can manage this for us. To begin

    % mkdir foo
    % cd foo
    % git init
    % git checkout -b upstream
    % gbp import-orig \
        --upstream-branch=upstream --debian-branch=master \
        --upstream-tag='upstream/%(version)s' \
        --sign-tags --no-pristine-tar \
        ../foo_1.2.2.orig.tar.xz
    % git branch -f upstream

This should leave you on the master branch. Next, our upstream branch cannot be pushed to dgit-repos, but since we will need it whenever we import a new upstream version, we must push it somewhere. The usual choice is salsa.debian.org:

    % git remote add -f origin salsa.debian.org:Debian/foo.git
    % git push --follow-tags -u origin master upstream

You are now ready to proceed as above, making commits to the debian/ directory and to the upstream source. As above, for technical reasons, it is essential that your first commit introduces the debian/ directory containing at least one file, and does nothing else. In other words, make a commit introducing debian/ before patching the upstream source. A convenient way to ensure this requirement is satisfied is to start by creating debian/gbp.conf:

    [DEFAULT]
    upstream-branch = upstream
    debian-branch = master
    upstream-tag = upstream/%(version)s
    sign-tags = True
    pristine-tar = False
    pristine-tar-commit = False
    [import-orig]
    merge = False

and commit that:

    % git add debian/gbp.conf && git commit -m "create gbp.conf"

Note that we couldn't create debian/gbp.conf before now for the same technical reasons which require our first commit to introduce debian/ without patching the upstream source. That's why we had to pass a lot of options to our first call to gbp-import-orig(1).

CONVERTING AN EXISTING PACKAGE

This section explains how to convert an existing Debian package to this workflow. It should be skipped when debianising a new package. If you have an existing git history that you have pushed to an ordinary git server like salsa.debian.org, we start with that. If you don't already have it locally, you'll need to clone it, and obtain the corresponding orig.tar from the archive:

    % git clone salsa.debian.org:Debian/foo
    % cd foo
    % dgit setup-new-tree
    % origtargz

If you don't have any existing git history, or you have history only on the special dgit-repos server, we start with dgit clone:

    % dgit clone foo
    % cd foo

Then we make new upstream tags available:

    % git remote add -f upstream https://some.upstream/foo.git

We now use a git debrebase convert-from-* command to convert your existing history to the git-debrebase(5) data model. Which command you should use depends on some facts about your repository:

(A) There is no delta queue.

If there do not exist any Debian patches, use

(B) There is a delta queue, and patches are unapplied.

This is the standard git-buildpackage(1) workflow: there are Debian patches, but the upstream source is committed to git without those patches applied. Use

(C) There is a delta queue, and patches are applied.

Use

Finally, you need to ensure that your git HEAD is dgit-compatible, i.e., it is exactly what you would get if you deleted .git, invoked dpkg-buildpackage -S, and then unpacked the resultant source package. To achieve this, you might need to delete debian/source/local-options. One way to have dgit check your progress is to run dgit build-source.

GIT CONFIGURATION

git-debrebase(1) does not yet support using git merge to merge divergent branches of development (see "OTHER MERGES" in git-debrebase(5)). You should configure git such that git pull does not try to merge:

    % git config --local pull.rebase true

Now when you pull work from other Debian contributors, git will rebase your work on top of theirs. If you use this clone for upstream development in addition to Debian packaging work, you may not want to set this global setting. Instead, see the branch.autoSetupRebase and branch.<name>.rebase settings in git-config(5).

IMPORTING NEW UPSTREAM RELEASES

There are two steps: obtaining git refs that correspond to the new release, and importing that release using git-debrebase(1).

Obtaining the release

When upstream tags releases in git

    % git fetch --tags upstream

If you want to package an untagged upstream commit (because upstream does not tag releases or because you want to package an upstream development snapshot), see "Using untagged upstream commits" above. When upstream releases only tarballs You will need the debian/gbp.conf from "When upstream releases only tarballs", above. You will also need your upstream branch. Above, we pushed this to salsa.debian.org. You will need to clone or fetch from there, instead of relying on dgit clone/dgit fetch alone. Then, either

    % gbp import-orig ../foo_1.2.3.orig.tar.xz

or if you have a working watch file

    % gbp import-orig --uscan

Importing the release

    % git debrebase new-upstream 1.2.3

This invocation of git-debrebase(1) involves a git rebase. You may need to resolve conflicts if the Debian delta queue does not apply cleanly to the new upstream source. If all went well, you can now review the merge of the new upstream release:

    git diff debian/1.2.2-1..HEAD -- . ':!debian'

Also, diff with --name-status and --diff-filter=ADR to see just the list of added or removed files, which is useful to determine whether there are any new or deleted files that may need accounting for in your copyright file. If you obtained a tarball from upstream, you are ready to try a build. If you merged a git tag from upstream, you will first need to generate a tarball:

    % git deborig

EDITING THE DEBIAN PACKAGING

Just make commits on master that change the contents of debian/.

EDITING THE DELTA QUEUE

Adding new patches

Adding new patches is straightforward: just make commits touching only files outside of the debian/ directory. You can also use tools like git-revert(1), git-am(1) and git-cherry-pick(1).

Editing patches: starting a debrebase

git-debrebase(1) is a wrapper around git-rebase(1) which allows us to edit, re-order and delete patches. Run

    % git debrebase -i

to start an interactive rebase. You can edit, re-order and delete commits just as you would during git rebase -i.

Editing patches: finishing a debrebase

After completing the git rebase, your branch will not be a fast-forward of the git HEAD you had before the rebase. This means that we cannot push the branch anywhere. If you are ready to upload, dgit push-source (or dgit push-built) will take care of fixing this up for you. If you are not yet ready to upload, and want to push your branch to a git remote such as salsa.debian.org,

    % git debrebase conclude

Note that each time you conclude a debrebase you introduce a pseudomerge into your git history, which may make it harder to read. Try to do all of the editing of the delta queue that you think will be needed for this editing session in a single debrebase, so that there is a single debrebase stitch.

BUILDING AND UPLOADING

You can use dpkg-buildpackage(1) for test builds. When you are ready to build for an upload, use dgit sbuild, dgit pbuilder or dgit cowbuilder. Upload with dgit push-source or dgit push-built. Remember to pass --new if the package is new in the target suite. In some cases where you used git debrebase convert-from-gbp since the last upload, it is not possible for dgit to make your history fast-forwarding from the history on dgit-repos. In such cases you will have to pass --overwrite to dgit. git-debrebase will normally tell you if this will be needed. If you want to upload with git-debpush(1), for the first upload you should pass the --quilt=linear quilt mode option (see git-debpush(1)). Right before uploading, if you did not just already do so, you might want to have git-debrebase(1) shuffle your branch such that the Debian delta queue appears right at the tip of the branch you will push:

    % git debrebase
    % dgit push-source

Note that this will introduce a new pseudomerge. After dgit pushing, be sure to git push to salsa.debian.org, if you're using that.

HANDLING DFSG-NON-FREE MATERIAL

Illegal material

Here we explain how to handle material that is merely DFSG-non-free. Material which is legally dangerous (for example, files which are actually illegal) cannot be handled this way. If you encounter possibly-legally-dangerous material in the upstream source code you should seek advice. It is often best not to make a fuss on a public mailing list (at least, not at first). Instead, email your archive administrators. For Debian that is
To: [email protected], [email protected]

DFSG-non-free: When upstream tags releases in git

Our approach is to maintain a DFSG-clean upstream branch, and create tags on this branch for each release that we want to import. We then import those tags per "Importing the release", above. In the case of a new package, we base our initial Debianisation on our first DFSG-clean tag. For the first upstream release that requires DFSG filtering:

    % git checkout -b upstream-dfsg 1.2.3
    % git rm evil.bin
    % git commit -m "upstream version 1.2.3 DFSG-cleaned"
    % git tag -s 1.2.3+dfsg
    % git checkout master

Now either proceed with "Importing the release" on the 1.2.3+dfsg tag, or in the case of a new package,

    % git branch --unset-upstream
    % git reset --hard 1.2.3+dfsg

and proceed with "INITIAL DEBIANISATION". For subsequent releases (whether or not they require additional filtering):

    % git checkout upstream-dfsg
    % git merge 1.2.4
    % git rm further-evil.bin # if needed
    % git commit -m "upstream version 1.2.4 DFSG-cleaned" # if needed
    % git tag -s 1.2.4+dfsg
    % git checkout master
    % # proceed with "Importing the release" on 1.2.4+dfsg tag

Our upstream-dfsg branch cannot be pushed to dgit-repos, but since we will need it whenever we import a new upstream version, we must push it somewhere. Assuming that you have already set up an origin remote per the above,

    % git push --follow-tags -u origin master upstream-dfsg

DFSG-non-free: When upstream releases only tarballs

The easiest way to handle this is to add a Files-Excluded field to debian/copyright, and a uversionmangle setting in debian/watch. See uscan(1). Alternatively, see the --filter option detailed in gbp-import-orig(1).

INCORPORATING NMUS

In the simplest case,

    % dgit fetch
    % git merge --ff-only dgit/dgit/sid

If that fails, because your branch and the NMUers' work represent divergent branches of development, you have a number of options. Here we describe the two simplest. Note that you should not try to resolve the divergent branches by editing files in debian/patches. Changes there would either cause trouble, or be overwritten by git-debrebase(1).

Rebasing your work onto the NMU

    % git rebase dgit/dgit/sid

If the NMUer added new commits modifying the upstream source, you will probably want to debrebase before your next upload to tidy those up. For example, the NMUer might have used git-revert(1) to unapply one of your patches. A debrebase can be used to strip both the patch and the reversion from the delta queue.

Manually applying the debdiff

If you cannot rebase because you have already pushed to salsa.debian.org, say, you can manually apply the NMU debdiff, commit and debrebase. The next dgit push will require --overwrite.

HINTS AND TIPS

Minimising pseudomerges

Above we noted that each time you conclude a debrebase, you introduce a pseudomerge into your git history, which may make it harder to read. A simple convention you can use to minimise the number of pseudomerges is to git debrebase conclude only right before you upload or push to salsa.debian.org. It is possible, though much less convenient, to reduce the number of pseudomerges yet further. We debrebase only (i) when importing a new release, and (ii) right before uploading. Instead of editing the existing delta queue, you append fixup commits (and reversions of commits) that alter the upstream source to the required state. You can push and pull to and from salsa.debian.org during this. Just before uploading, you debrebase, once, to tidy everything up.

The debian/patches directory

In this workflow, debian/patches is purely an output of git-debrebase(1). You should not make changes there. They will either cause trouble, or be ignored and overwritten by git-debrebase(1). debian/patches will often be out-of-date because git-debrebase(1) will only regenerate it when it needs to. So you should not rely on the information in that directory. When preparing patches to forward upstream, you should use git-format-patch(1) on git commits, rather than sending files from debian/patches.

Upstream branches

In this workflow, we specify upstream tags rather than any branches. Except when (i) upstream releases only tarballs, (ii) we require DFSG filtering, or (iii) you also happen to be involved in upstream development, we do not maintain any local branch corresponding to upstream, except temporary branches used to prepare patches for forwarding, and the like. The idea here is that from Debian's point of view, upstream releases are immutable points in history. We want to make sure that we are basing our Debian package on a properly identified upstream version, rather than some arbitrary commit on some branch. Tags are more useful for this. Upstream's branches remain available as the git remote tracking branches for your upstream remote, e.g. remotes/upstream/master.

The first ever dgit push

If this is the first ever dgit push of the package, consider passing --deliberately-not-fast-forward instead of --overwrite. This avoids introducing a new origin commit into your git history. (This origin commit would represent the most recent non-dgit upload of the package, but this should already be represented in your git history.)

Inspecting the history

The git history made by git-debrebase can seem complicated. Here are some suggestions for helpful invocations of gitk and git. They can be adapted for other tools like tig(1), git-log(1), magit, etc. History of package in Debian, disregarding history from upstream:

    % gitk --first-parent

History of the packaging, excluding the delta queue:

    % gitk :/debian :!/debian/patches

Just the delta queue (i.e. Debian's changes to upstream):

    % gitk --first-parent -- :/ :!/debian

Full history including old versions of the delta queue:

    % gitk --date-order

Complete diff since the last upload:

    % git diff dgit/dgit/sid..HEAD -- :/ :!/debian/patches

Interdiff of delta queue since last upload, if you really want it:

    % git debrebase make-patches
    % git diff dgit/dgit/sid..HEAD -- debian/patches

And of course there is:

    % git debrebase status

Alternative ways to start a debrebase

Above we started an interactive debrebase by invoking git-debrebase(1) like this:

    % git debrebase -i

It is also possible to perform a non-interactive rebase, like this:

    % git debrebase -- [git-rebase options...]

A third alternative is to have git-debrebase(1) shuffle all the Debian changes to the end of your branch, and then manipulate them yourself using git-rebase(1) directly. For example,

    % git debrebase
    % git rebase -i HEAD~5      # there are 4 Debian patches

If you take this approach, you should be very careful not to start the rebase too early, including before the most recent pseudomerge. git-rebase without a base argument will often start the rebase too early, and should be avoided. Run git-debrebase instead. See also "ILLEGAL OPERATIONS" in git-debrebase(5).

SEE ALSO

dgit(1), dgit(7), git-debrebase(1), git-debrebase(5)

AUTHOR

This tutorial was written and is maintained by Sean Whitton <[email protected]>. It contains contributions from other dgit contributors too - see the dgit copyright file.