Provided by: dgit_4.3_all 
NAME
dgit - principles of operation
SUMMARY
dgit treats the Debian archive as a version control system, and bidirectionally gateways between the
archive and git. The git view of the package can contain the usual upstream git history, and will be
augmented by commits representing uploads done by other developers not using dgit. This git history is
stored in a canonical location known as dgit-repos which lives on a dedicated git server.
git branches suitable for use with dgit can be edited directly in git, and used directly for building
binary packages. They can be shared using all conventional means for sharing git branches. It is not
necessary to use dgit to work with dgitish git branches. However, dgit is (usually) needed in order to
convert to or from Debian-format source packages.
SEE ALSO
dgit(1) Reference manual and documentation catalogue. dgit-*(7) Tutorials and workflow guides. See dgit(1) for a list.
MODEL
You may use any suitable git workflow with dgit, provided you satisfy dgit's requirements:
dgit maintains a pseudo-remote called dgit, with one branch per suite. This remote cannot be used with
plain git.
The dgit-repos repository for each package contains one ref per suite named refs/dgit/suite. These
should be pushed to only by dgit. They are fast forwarding. Each push on this branch corresponds to an
upload (or attempted upload).
However, it is perfectly fine to have other branches in dgit-repos; normally the dgit-repos repo for the
package will be accessible via the remote name `origin'.
dgit push will also make signed tags called archive/debian/version (with version encoded a la DEP-14) and
push them to dgit-repos. These are used at the server to authenticate pushes.
Uploads made by dgit contain an additional field Dgit in the source package .dsc. (This is added by dgit
push.) This specifies: a commit (an ancestor of the dgit/suite branch) whose tree is identical to the
unpacked source upload; the distro to which the upload was made; a tag name which can be used to fetch
the git commits; and a url to use as a hint for the dgit git server for that distro.
Uploads not made by dgit are represented in git by commits which are synthesised by dgit. The tree of
each such commit corresponds to the unpacked source; there is a commit with the contents, and a pseudo-
merge from last known upload - that is, from the contents of the dgit/suite branch. Depending on the
source package format, the contents commit may have a more complex structure, but ultimately it will be a
convergence of stubby branches from origin commits representing the components of the source package.
dgit expects trees that it works with to have a dgit (pseudo) remote. This refers to the dgit-created
git view of the corresponding archive.
The dgit archive tracking view is synthesised locally, on demand, by each copy of dgit. The tracking
view is always a descendant of the dgit-repos suite branch (if one exists), but may be ahead of it if
uploads have been done without dgit. The archive tracking view is always fast forwarding within each
suite.
dgit push can operate on any commit which is a descendant of the suite tracking branch.
dgit does not make a systematic record of its imports of orig tarball(s). So it does not work by finding
git tags or branches referring to orig tarball(s). The orig tarballs are downloaded (by dgit clone) into
the parent directory, as with a traditional (non-gitish) dpkg-source workflow. You need to retain these
tarballs in the parent directory for dgit build and dgit push. (They are not needed for purely-git-based
workflows.)
dgit repositories could be cloned with standard (git) methods. However, the dgit repositories do not
contain uploads not made with dgit. And for sourceful builds / uploads the orig tarball(s) will need to
be present in the parent directory.
To a user looking at the archive, changes pushed in a simple NMU using dgit look like reasonable changes
made in an NMU: in a `3.0 (quilt)' package the delta from the previous upload is recorded in new
patch(es) constructed by dpkg-source.
COMBINED SUITES
dgit can synthesize a combined view of several underlying suites. This is requested by specifying, for
suite, a comma-separated list:
mainsuite,subsuite...
This facility is available with dgit clone, fetch and pull, only.
dgit will fetch the same package from each specified underlying suite, separately (as if with dgit
fetch). dgit will then generate a pseudomerge commit on the tracking branch remotes/dgit/dgit/suite
which has the tip of each of the underlying suites as an ancestor, and which contains the same as the
suite which has the highest version of the package.
The package must exist in mainsuite, but need not exist in the subsuites.
If a specified subsuite starts with - then mainsuite is prepended.
So, for example, stable,-security means to look for the package in stable, and stable-security, taking
whichever is newer. If stable is currently jessie, dgit clone would leave you on the branch
dgit/jessie,-security.
Combined suites are not supported by the dgit build operations. This is because those options are
intended for building for uploading source packages, and look in the changelog to find the relevant
suite. It does not make sense to name a dgit-synthesised combined suite in a changelog, or to try to
upload to it.
When using this facility, it is important to always specify the same suites in the same order: dgit will
not be make a coherent fast-forwarding history view otherwise.
The history generated by this feature is not normally suitable for merging back into upstreams, as it
necessarily contains unattractive pseudomerges.
LIMITATIONS
Because the synthesis of the suite tracking branches is done locally based only on the current archive
state, it will not necessarily see every upload not done with dgit. Also, different versions of dgit (or
the software it calls) might import the same .dscs differently (although we try to minimise this). As a
consequence, the dgit tracking views of the same suite, made by different instances of dgit, may vary.
They will have the same contents, but may have different history.
There is no uniform linkage between the tracking branches for different suites. The Debian
infrastructure does not do any automatic import of uploads made without dgit. It would be possible for a
distro's infrastructure to do this; in that case, different dgit client instances would see exactly the
same history.
There has been no bulk import of historical uploads into Debian's dgit infrastructure. To do this it
would be necessary to decide whether to import existing vcs history (which might not be faithful to
dgit's invariants) or previous non-Dgit uploads (which would not provide a very rich history).
git represents only file executability. git does not represent empty directories, or any leaf objects
other than plain files and symlinks. The behaviour of Debian source package formats on objects with
unusual permissions is complicated. Some pathological Debian source packages will no longer build if
empty directories are pruned (or if other things not reproduced by git are changed). Such sources cannot
be worked with properly in git, and therefore not with dgit either.
READ-ONLY DISTROS
Distros which do not maintain a set of dgit history git repositories can still be used in a read-only
mode with dgit. Currently Ubuntu is configured this way.
GITATTRIBUTES
git has features which can automatically transform files as they are being copied between the working
tree and the git history. The attributes can be specified in the source tree itself, in .gitattributes.
See gitattributes(5).
These transformations are context-sensitive and not, in general, reversible, so dgit operates on the
principle that the dgit git history contains the actual contents of the package. (When dgit is
manipulating a .dsc, it does so in a private area, where the transforming gitattributes are defused, to
achieve this.)
If transforming gitattributes are used, they can cause trouble, because the working tree files can differ
from the git revision history (and therefore from the source packages). dgit warns if it finds a
.gitattributes file (in a package being fetched or imported), unless the transforming gitattributes have
been defused.
dgit clone and dgit setup-new-tree disable transforming gitattributes by default, by creating a suitable
.git/info/attributes. See dgit setup-new-tree and dgit setup-gitattributes in dgit(1).
PACKAGE SOURCE FORMATS
If you are not the maintainer, you do not need to worry about the source format of the package. You can
just make changes as you like in git. If the package is a `3.0 (quilt)' package, the patch stack will
usually not be represented in the git history.
FILE EXECUTABILITY
Debian source package formats do not always faithfully reproduce changes to executability. But dgit
insists that the result of dgit clone is identical (as far as git can represent - see Limitations, above)
to the result of dpkg-source -x.
So files that are executable in your git tree must be executable in the result of dpkg-source -x (but
often aren't). If a package has such troublesome files, they have to be non-executable in dgit-
compatible git branches.
FORMAT 3.0 (QUILT)
For a format `3.0 (quilt)' source package, dgit may have to make a commit on your current branch to
contain metadata used by quilt and dpkg-source.
This is because `3.0 (quilt)' source format represents the patch stack as files in debian/patches/
actually inside the source tree. This means that, taking the whole tree (as seen by git or ls) (i) dpkg-
source cannot represent certain trees, and (ii) packing up a tree in `3.0 (quilt)' and then unpacking it
does not always yield the same tree.
dgit will automatically work around this for you when building and pushing. The only thing you need to
know is that dgit build, sbuild, etc., may make new commits on your HEAD. If you're not a quilt user
this commit won't contain any changes to files you care about.
You can explicitly request that dgit do just this fixup, by running dgit quilt-fixup.
If you are a quilt user you need to know that dgit's git trees are `patches applied packaging branches'
and do not contain the .pc directory (which is used by quilt to record which patches are applied). If
you want to manipulate the patch stack you probably want to be looking at tools like git-dpm.
SPLIT VIEW QUILT MODE
When working with git branches intended for use with the `3.0 (quilt)' source format dgit can
automatically convert a suitable maintainer-provided git branch (in one of a variety of formats) into a
dgit branch.
When a split view mode is engaged dgit build commands and dgit push will, on each invocation, convert the
user's HEAD into the dgit view, so that it can be built and/or uploaded.
dgit push in split view mode will push the dgit view to the dgit git server. The dgit view is always a
descendant of the maintainer view. dgit push will also make a maintainer view tag according to DEP-14
and push that to the dgit git server.
Split view mode must be enabled explicitly (by the use of the applicable command line options,
subcommands, or configuration). This is because it is not possible to reliably tell (for example)
whether a git tree for a dpkg-source `3.0 (quilt)' package is a patches-applied or patches-unapplied
tree.
Split view conversions are cached in the ref dgit-intern/quilt-cache. This should not be manipulated
directly.
FILES IN THE ORIG TARBALL BUT NOT IN GIT - AUTOTOOLS ETC.
This section is mainly of interest to maintainers who want to use dgit with their existing git history
for the Debian package.
Some developers like to have an extra-clean git tree which lacks files which are normally found in source
tarballs and therefore in Debian source packages. For example, it is conventional to ship ./configure in
the source tarball, but some people prefer not to have it present in the git view of their project.
dgit requires that the source package unpacks to exactly the same files as are in the git commit on which
dgit push operates. So if you just try to dgit push directly from one of these extra-clean git branches,
it will fail.
As the maintainer you therefore have the following options:
• Delete the files from your git branches, and your Debian source packages, and carry the deletion
as a delta from upstream. (With `3.0 (quilt)' this means represeting the deletions as patches.
You may need to pass --include-removal to dpkg-source --commit, or pass corresponding options to
other tools.) This can make the Debian source package less useful for people without Debian build
infrastructure.
• Persuade upstream that the source code in their git history and the source they ship as tarballs
should be identical. Of course simply removing the files from the tarball may make the tarball
hard for people to use.
One answer is to commit the (maybe autogenerated) files, perhaps with some simple automation to
deal with conflicts and spurious changes. This has the advantage that someone who clones the git
repository finds the program just as easy to build as someone who uses the tarball.
Of course it may also be that the differences are due to build system bugs, which cause unintended files
to end up in the source package. dgit will notice this and complain. You may have to fix these bugs
before you can unify your existing git history with dgit's.
FILES IN THE SOURCE PACKAGE BUT NOT IN GIT - DOCS, BINARIES ETC.
Some upstream tarballs contain build artifacts which upstream expects some users not to want to rebuild
(or indeed to find hard to rebuild), but which in Debian we always rebuild.
Examples sometimes include crossbuild firmware binaries and documentation. To avoid problems when
building updated source packages (in particular, to avoid trying to represent as changes in the source
package uninteresting or perhaps unrepresentable changes to such files) many maintainers arrange for the
package clean target to delete these files.
dpkg-source does not (with any of the commonly used source formats) represent deletion of binaries
(outside debian/) present in upstream. Thus deleting such files in a dpkg-source working tree does not
actually result in them being deleted from the source package. Thus deleting the files in rules clean
sweeps this problem under the rug.
However, git does always properly record file deletion. Since dgit's principle is that the dgit git tree
is the same of dpkg-source -x, that means that a dgit-compatible git tree always contains these files.
For the non-maintainer, this can be observed in the following suboptimal occurrences:
• The package clean target often deletes these files, making the git tree dirty trying to build the
source package, etc. This can be fixed by using dgit -wg aka --clean=git, so that the package
clean target is never run.
• The package build modifies these files, so that builds make the git tree dirty. This can be
worked around by using `git reset --hard' after each build (or at least before each commit or
push).
From the maintainer's point of view, the main consequence is that to make a dgit-compatible git branch it
is necessary to commit these files to git. The maintainer has a few additional options for mitigation:
for example, it may be possible for the rules file to arrange to do the build in a temporary area, which
avoids updating the troublesome files; they can then be left in the git tree without seeing trouble.
PROBLEMS WITH PACKAGE CLEAN TARGETS ETC.
A related problem is other unexpected behaviour by a package's clean target. If a package's rules modify
files which are distributed in the package, or simply forget to remove certain files, dgit will complain
that the tree is dirty.
Again, the solution is to use dgit -wg aka --clean=git, which instructs dgit to use git clean instead of
the package's build target, along with perhaps git reset --hard before each build.
This is 100% reliable, but has the downside that if you forget to git add or to commit, and then use dgit
-wg or git reset --hard, your changes may be lost.