+From: Ian Jackson <ijackson@chiark.greenend.org.uk>
+To: Russ Allbery <rra@debian.org>
+Cc: debian-devel@lists.debian.org
+Subject: Re: Feedback on 3.0 source format problems
+Date: Wed, 4 Jan 2017 12:08:57 +0000
+
+Russ Allbery writes ("Re: Feedback on 3.0 source format problems"):
+> Even if we never used tarballs, and instead our unit of operation was the
+> upstream Git repository plus Debian branches, I would maintain a rebased
+> branch of Debian changes to upstream
+
+I think this is definitely the right thing for many packages.
+
+I don't think this depends on the interchange format representing the
+changes as patches rather than commits. At a technical level, they
+represent roughly the same information and it is possible to
+interconvert. However:
+
+ * There are many changes that patches cannot represent.
+
+ * git (working with a rebasing branch) is a much more powerful tool
+ for editing a patch series than quilt or diff/patch or emacs
+ (working with a series of patches as files). Ie, patches are a
+ poor editing format. Consequently most people actually gateway to
+ git at the first opportunity, and out again for export.
+
+ * Few people outside Debian work with quilt stacks any more.
+
+All of these things mean that patches are a really poor interchange
+format.
+
+The only difficulty is this one:
+
+> This is actually, in a way, *harder* if we were using pure Git, since if I
+> have a rebased branch of Debian changes on top of upstream, and I need a
+> place to integrate that with Debian packaging, what does that
+> debian/master branch look like? I don't really want it to be a constantly
+> rebased branch; I want it to be a conventional branch. But I want to keep
+> merging the changes against upstream into it (but not maintain them on
+> that branch, only maintain the Debian packaging files on that branch).
+
+My preferred answer is that it is a constantly rebasing branch topped
+with a series of pseudomerges to make it fast-forwarding.
+
+git-dpm sort of does this. I have been experimenting with and
+blundering towards another approach, which is closer to raw git.
+
+Something like this:
+
+ ------/--A-----/---B3---/--> interchange view
+ / / /
+ / / 3
+ / / /
+ 2 2 2
+ / / /
+ 1 1 1
+ / / /
+ ---p-----p--A----p---B---> `packaging-only' branch
+ / / /
+ --#-----#-------#-----> upstream
+
+ Key: 1,2,3 commits touching upstream files
+ A,B commits touching debian/ only
+ B3 mixed commit (eg made by an NMUer)
+ # upstream releases
+
+ -p- special merge, takes contents of debian/
+ / from the previous `packaging only'
+ commit and rest from upstream
+
+ -/- pseudomerge; contents are identical to
+ / parent lower on diagram.
+
+Looking from the tip of the interchange view, it is I think always
+possible to classify these commits appropriately: pseudomerges are
+fairly obvious (if all three trees are identical, we descend to the
+parent with the most recent commit date), and the `p' special merge is
+the only non-pseudo merge and has a special form.
+
+So it would be possible to write a `git-debian-rebase' tool which
+would take (for example) B3, above, and be able to perform functions
+like:
+
+ * Strip pseudomerges: Rewrite the current branch so it contains no
+ pseudomerges, turning ...B3 into ...p-A-1-2-B3. (This should make
+ a note, in your .git/ somewhere, of the latest pseudomerge to be
+ deleted.)
+
+ * Cleanup branch: Reorganise the current branch so that the debian/
+ changes come first, turning -p-A-1-2-B3 into ...p-A-B-1-2.
+
+ * New upstream rebase: Start rebasing onto a new upstream version,
+ turning ...#...p-A-B-1-2 into ...#'...p'-1-2. This would be a
+ wrapper around git-rebase, which prepares p' and then tries to
+ rebase 1 2 onto p'. So if you ask for an interactive rebase p'
+ doesn't appear in your commit list.
+
+ Note that the rebasing of p into p' cannot fail because p' 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 we can `git log' and get the packaging history.)
+
+ * Record pseudomerge. This is like "committing" your patch queue.
+ The LH parent is taken from the previous strip pseudomerge. (We
+ should probably check that this is consistent with what we see in
+ debian/changelog, but that is not a sufficient check.)
+
+Maybe some of these operations should automatically edit
+debian/changelog.
+
+The only thing that this can't easily do is permit nonlinear (ie,
+merging) history on the `packaging-only' branch, because upstream
+might contain debian/ files too, so it is not possible to distinguish
+a merge of two `packaging-only' branches from the special merge `p'.
+(Indeed I since upstream might copy debian/ from us, I think it is not
+easy to reliably distinguish the two parents of a `p'. In the most
+exciting edge case, upstream might `git merge' a previous instance of
+our interchange view, but I think even then everything still works.)
+
+Ian.
+
+--
+Ian Jackson <ijackson@chiark.greenend.org.uk> These opinions are my own.
+
+If I emailed you from an address @fyvzl.net or @evade.org.uk, that is
+a private address which bypasses my fierce spamfilter.
+
~ianmdlvl / dgit.git / commitdiff