=head1 NAME git-debrebase - git data model for Debian packaging =head1 INTRODUCTION git-debrebase is a tool for representing in git, and manpulating, Debian packages based on upstream source code. The Debian packaging has a fast forwarding history. The delta queue (changes to upstream files) is represented as a series of individual git commits, which can worked on with rebase, and also shared. =head2 DISCUSSION git-debrebase is designed to work well with dgit. git-debrebase can also be used in workflows without source packages, for example to work on Debian-format packages outside or alongside Debian. git-debrebase itself is not very suitable for use by Debian derivatives, to work on packages inherited from Debian, because it assumes that you want to throw away any packaging provided by your upstream. However, use of git-debrebase in Debian does not make anything harder for derivatives, and it can make some things easier. When using gitk on branches managed by git-debrebase, B, B and B (or B) produce more useful output than the default. =head1 TERMINOLOGY =over =item Pseudomerge A merge which does not actually merge the trees; instead, it is constructed by taking the tree from one of the parents (ignoring the contents of the other parents). These are used to make a rewritten history fast forward from a previous tip, so that it can be pushed and pulled normally. Manual construction of pseudomerges can be done with C but is not normally needed when using git-debrebase. =item Packaging files Files in the source tree within B, excluding anything in B. =item Upstream The version of the package without Debian's packaging. Typically provided by the actual upstream project, and sometimes tracked by Debian contributors in a branch C. Upstream contains upstream files, but some upstreams also contain packaging files in B. Any such non-upstream files found in upstream are thrown away by git-debrebase each time a new upstream version is incorporated. =item Upstream files Files in the source tree outside B. These may include unmodified source from upstream, but also files which have been modified or created for Debian. =item Delta queue Debian's changes to upstream files: a series of git commits. =item Quilt patches Files in B generated for the benefit of dpkg-source's 3.0 (quilt) .dsc source package format. Not used, often deleted, and regenerated when needed (such as when uploading to Debian), by git-debrebase. =item Interchange branch; breakwater; stitched; laundered See L. =item Anchor; Packaging See L. =item ffq-prev; debrebase-last See L. =back =head1 DIAGRAM ------/--A!----/--B3!--%--/--> interchange view / / / with debian/ directory % % % entire delta queue applied / / / 3.0 (quilt) has debian/patches / / 3* "master" on Debian git servers / / / 2* 2* 2 / / / 1 1 1 breakwater branch, merging baseline / / / unmodified upstream code ---@-----@--A----@--B--C plus debian/ (but no debian/patches) / / / no ref refers to this: we --#-----#-------#-----> upstream reconstruct its identity by inspecting interchange branch Key: 1,2,3 commits touching upstream files only A,B,C commits touching debian/ only B3 mixed commit (eg made by an NMUer) # upstream releases -@- anchor merge, takes contents of debian/ from the / previous `breakwater' commit and rest from upstream -/- pseudomerge; contents are identical to / parent lower on diagram. % dgit- or git-debrebase- generated commit of debian/patches. `3.0 (quilt)' only; generally dropped by git-debrebase. * Maintainer's HEAD was here while they were editing, before they said they were done, at which point their tools made -/- (and maybe %) to convert to the fast-forwarding interchange branch. ! NMUer's HEAD was here when they said `dgit push'. Rebase branch launderer turns each ! into an equivalent *. =head1 BRANCHES AND BRANCH STATES - OVERVIEW git-debrebase has one primary branch, the B. This branch is found on Debian contributor's workstations (typically, a maintainer would call it B), in the Debian dgit git server as the suite branch (B) and on other git servers which support Debian work (eg B on salsa). The interchange branch is fast-forwarding (by virtue of pseudomerges, where necessary). It is possible to have multiple different interchange branches for the same package, stored as different local and remote git branches. However, divergence should be avoided where possible - see L. A suitable interchange branch can be used directly with dgit. In this case each dgit archive suite branch is a separate interchange branch. Within the ancestry of the interchange branch, there is another important, implicit branch, the B. The breakwater contains unmodified upstream source, but with Debian's packaging superimposed (replacing any C directory that may be in the upstream commits). The breakwater does not contain any representation of the delta queue (not even debian/patches). The part of the breakwater processed by git-debrebase is the part since the most reecent B, which is usually a special merge generated by git-debrebase. When working, locally, the user's branch can be in a rebasing state, known as B. While a branch is unstitched, it is not in interchange format. The previous interchange branch tip is recorded, so that the previous history and the user's work can later be stitched into the fast-forwarding interchange form. An unstitched branch may be in B state, which means it has a more particular special form convenient for manipulating the delta queue. =head1 BRANCH CONTENTS - DETAILED SPECIFICATION It is most convenient to describe the B branch first. A breakwater is B, but is not usually named by a ref. It contains B (ancestors first): =over =item Anchor An B commit, which is usually a special two-parent merge: The first parent contains the most recent version, at that point, of the Debian packaging (in debian/); it also often contains upstream files, but they are to be ignored. Often the first parent is a previous breakwater tip. The second parent is an upstream source commit. It may sometimes contain a debian/ subdirectory, but if so that is to be ignored. The second parent's upstream files are identical to the anchor's. Anchor merges always contain C<[git-debrebase anchor: ...]> as a line in the commit message. Alternatively, an anchor may be a single-parent commit which introduces the C directory and makes no other changes: ie, the start of Debian packaging. =item Packaging Zero or more single-parent commits containing only packaging changes. (And no quilt patch changes.) =back The B branch state is B. A laundered branch is based on a breakwater but also contains, additionally, B the breakwater, a representation of the delta queue: =over =item Delta queue commits Zero or more single-parent commits contaioning only changes to upstream files. =back The merely B (ie, unstitched but unlaundered) branch state is also B. It has the same contents as the laundered state, except that it may contain, additionally, in B: =over =item Linear commits to the source Further commit(s) containing changes to to upstream files and/or to packaging, possibly mixed within a single commit. (But not quilt patch changes.) =item Quilt patch addition for `3.0 (quilt)' Commit(s) which add patches to B, and add those patches to the end of B. These are only necessary when working with packages in C<.dsc 3.0 (quilt)> format. For git-debrebase they are purely an output; they are deleted when branches are laundered. git-debrebase takes care to make a proper patch series out of the delta queue, so that any resulting source packages are nice. =back Finally, an B branch is B. It has the same contents as an unlaundered branch state, but may (and usually will) additionally contain (in some order, possibly intermixed with the extra commits which may be found on an unstitched unlaundered branch): =over =item Pseudomerge to make fast forward A pseudomerge making the branch fast forward from previous history. The contributing parent is itself in interchange format. Normally the overwritten parent is a previous tip of an interchange branch, but this is not necessary as the overwritten parent is not examined. If the two parents have identical trees, the one with the later commit date (or, if the commit dates are the same, the first parent) is treated as the contributing parent. =item dgit dsc import pseudomerge Debian .dsc source package import(s) made by dgit (during dgit fetch of a package most recently uploaded to Debian without dgit, or during dgit import-dsc). git-debrebase requires that each such import is in the fast-forwarding format produced by dgit: a two-parent pseudomerge, whose contributing parent is in the non-fast-forwarding dgit dsc import format (not described further here), and whose overwritten parent is the previous interchange tip (eg, the previous tip of the dgit suite branch). =back =head1 STITCHING, PSEUDO-MERGES, FFQ RECORD Whenever the branch C is unstitched, the previous head is recorded in the git ref C. Unstiched branches are not fast forward from the published interchange branches [1]. So before a branch can be pushed, the right pseudomerge must be reestablished. This is the stitch operation, which consumes the ffq-prev ref. When the user has an unstitched branch, they may rewrite it freely, from the breakwater tip onwards. Such a git rebase is the default operation for git-debrebase. Rebases should not go back before the breakwater tip, and certainly not before the most recent anchor. Unstitched branches must not be pushed to interchange branch refs (by the use of C or equivalent). It is OK to share an unstitched branch in similar circumstances and with similar warnings to sharing any other rebasing git branch. [1] Strictly, for a package which has never had a Debian delta queue, the interchange and breakwater branches may be identical, in which case the unstitched branch is fast forward from the interchange branch and no pseudomerge is needed. When ffq-prev is not present, C records some ancestor of refs/B, (usually, the result of last stitch). This can be used to quickly determine whether refs/B is being maintained in git-debrebase form. =head1 OTHER MERGES Note that the representation described here does not permit general merges on any of the relevant branches. For this reason the tools will try to help the user avoid divergence of the interchange branch. See dgit-maint-rebase(7) for a discussion of what kinds of behaviours should be be avoided because they might generate such merges. Automatic resolution of divergent interchange branches (or laundering of merges on the interchange branch) is thought to be possible, but there is no tooling for this yet: Nonlinear (merging) history in the interchange branch is awkward because it (obviously) does not preserve the linearity of the delta queue. Easy merging of divergent delta queues is a research problem. Nonlinear (merging) history in the breakwater branch is in principle tolerable, but each of the parents would have to be, in turn, a breakwater, and difficult questions arise if they don't have the same anchor. We use the commit message annotation to distinguish the special anchor merges from other general merges, so we can at least detect unsupported merges. =head1 LEGAL OPERATIONS The following basic operations follows from this model (refer to the diagram above): =over =item Append linear commits No matter the branch state, it is always fine to simply git commit (or cherry-pick etc.) commits containing upstream file changes, packaging changes, or both. (This may make the branch unlaundered.) =item Launder branch Record the previous head in ffq-prev, if we were stitched before (and delete debrebase-last). Reorganise the current branch so that the packaging changes come first, followed by the delta queue, turning C<-@-A-1-2-B3> into C<...@-A-B-1-2-3>. Drop pseudomerges and any quilt patch additions. =item Interactive rebase With a laundered branch, one can do an interactive git rebase of the delta queue. =item New upstream rebase Start rebasing onto a new upstream version, turning C<...#..@-A-B-1-2-3> into C<(...#..@-A-B-, ...#'-)@'-1-2>. This has to be a wrapper around git-rebase, which prepares @' and then tries to rebase 1 2 onto @'. If the user asks for an interactive rebase, @' doesn't appear in the commit list, since @' is the newbase of the rebase (see git-rebase(1)). Note that the construction of @' cannot fail because @' simply copies debian/ from B and and everything else from #'. (Rebasing A and B is undesirable. We want the debian/ files to be non-rebasing so that git log shows the packaging history.) =item Stitch Make a pseudomerge, whose contributing parent to is the unstitched branch and whose overwritten parent is ffq-prev, consuming ffq-prev in the process (and writing debrebase-last instead). Ideally the contributing parent would be a laundered branch, or perhaps a laundered branch with a quilt patch addition commit. =item Commit quilt patches To generate a tree which can be represented as a 3.0 (quilt) .dsc source packages, the delta queue must be reified inside the git tree in B. These patch files can be stripped out and/or regenerated as needed. =back =head1 COMMIT MESSAGE ANNOTATIONS git-debrebase makes annotations in the messages of commits it generates. The general form is [git-debrebase[ COMMIT-TYPE [ ARGS...]]: PROSE, MORE PROSE] git-debrebase treats anything after the colon as a comment, paying no attention to PROSE. The full set of annotations is: [git-debrebase: split mixed commit, debian part] [git-debrebase: split mixed commit, upstream-part] [git-debrebase: convert dgit import, debian changes] [git-debrebase anchor: convert dgit import, upstream changes] [git-debrebase upstream-combine . PIECE[ PIECE...]: new upstream] [git-debrebase anchor: new upstream NEW-UPSTREAM-VERSION, merge] [git-debrebase: new upstream NEW-UPSTREAM-VERSION, changelog] [git-debrebase: export and commit patches] [git-debrebase convert-from-gbp: drop patches] [git-debrebase anchor: declare upstream] [git-debrebase pseudomerge: stitch] [git-debrebase convert-to-gbp: commit patches] Only anchor merges have the C<[git-debrebase anchor: ...]> tag. Single-parent anchors are not generated by git-debrebase, and when made manually should not contain any C<[git-debrebase ...]> annotation. The C and C tags are added to the pre-existing commit message, when git-debrebase rewrites the commit. =head1 APPENDIX - DGIT IMPORT HANDLING The dgit .dsc import format is not documented or specified (so some of the following terms are not defined anywhere). The dgit import format it is defined by the implementation in dgit, of which git-debrebase has special knowledge. Consider a non-dgit NMU followed by a dgit NMU: interchange --/--B3!--%--//----D*--> / / % 4 / 3 / 2 / 1 2 &_ / /| \ 1 0 00 =XBC% / / --@--A breakwater / --#--------> upstream Supplementary key: =XBC% dgit tarball import of .debian.tar.gz containing Debian packaging including changes B C and quilt patches 0 dgit tarball import of upstream tarball 00 dgit tarball import of supplementary upstream piece &_ dgit import nearly-breakwater-anchor // dgit fetch / import-dsc pseudomerge to make fast forward &' git-debrebase converted import (upstream files only) C' git-debrebase converted packaging change import * ** before and after HEAD We want to transform this into: =over =item I. No new upstream version (0 + 00 eq #) --/--B3!--%--//-----D*-------------/--> / / / % 4 4** / 3 3 / 2 2 / 1 1 2 &_ / / /| \ / 1 0 00 =XBC% / / / / / --@--A-----B---------------------C'---D / --#-----------------------------------------> =item II. New upstream (0 + 00 neq #) --/--B3!--%--//-----D*-------------/--> / / / % 4 4** / 3 3 / 2 2 / 1 1 2 &_ / / /| \ / 1 0 00 =XBC% / / / / / --@--A-----B-----------------@---C'---D / / --#--------------------- - - / - - ---------> / &' /| 0 00 =back =head1 SEE ALSO git-debrebase(1), dgit-maint-rebase(7), dgit(1)