.TH dgit 7 "" "Debian Project" "dgit"
.SH NAME
dgit \- principles of operation
.SH SUMMARY
.B 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
.B dgit-repos
which lives on a dedicated git server.
.SH MODEL
You may use any suitable git workflow with dgit, provided you
satisfy dgit's requirements:

dgit maintains a pseudo-remote called
.BR dgit ,
with one branch per suite.  This remote cannot be used with
plain git.

The
.B dgit-repos
repository for each package contains one ref per suite named
\fBrefs/dgit/\fR\fIsuite\fR.  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 (by default) make signed tags called
.BI debian/ version
and push them to dgit-repos, but nothing depends on these tags
existing.

dgit push can operate on any commit which is a descendant of the
current dgit/suite tip in dgit-repos.

Uploads made by dgit contain an additional field
.B 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.

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 an origin commit with the contents, and a
psuedo-merge from last known upload - that is, from the contents of
the dgit/suite branch.

dgit expects repos that it works with to have a
.B dgit
remote.  This refers to the well-known dgit-repos location (on a
dedicated Debian VM).  dgit fetch updates the remote tracking branch
for dgit/suite.

dgit does not (currently) represent the orig tarball(s) in git.  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.

dgit repositories could be cloned with standard (git) methods. The
only exception is that for sourcefull builds / uploads the orig
tarball(s) need to be present in the parent directory.

To a user looking at the archive, changes pushed using dgit look like
changes made in an NMU: in a `3.0 (quilt)' package the delta from the
previous upload is recorded in a new patch constructed by dpkg-source.
.SH 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.
.SH 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.
.SH 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.
.SH FILES IN THE SOURCE PACKAGE BUT NOT IN GIT
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:
.TP
\(bu
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.
.IP
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.
.TP
\(bu
Have separate git branches which do contain the extra files, and after
regenerating the extra files (whenever you would have to anyway),
commit the result onto those branches.
.TP
\(bu
Provide source packages which lack the files you don't want
in git, and arrange for your package build to create them as needed.
This may mean not using upstream source tarballs and makes the Debian
source package less useful for people without Debian build
infrastructure.
.LP
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.
.LP
.SH PROBLEMS WITH PACKAGE CLEAN TARGETS ETC.
A related problem is unexpected behaviour by a package's
.B clean
target.
If a package's rules
remove or modify files which are distributed in the package,
or simply forgets to remove certain files,
dgit will complain that the tree is dirty.
.LP
The solution is to use
.BR "dgit -wg" " aka " "--clean=git" ,
which instructs dgit to use git clean instead of the package's
build target,
along with perhaps
.B git reset --hard
before each build.
.LP
This is 100% reliable, but has the downside
that if you forget to git add or to commit, and then use
.BR "dgit -wg" " or " "git reset --hard" ,
your changes may be lost.
.SH SEE ALSO
\fBdgit\fP(1).