Miroslav Suchý: DistGit 1.0

This is host post. Written by Michal “clime” Novotný.

Prolog

If you are just looking for an immediate hands-on experience, skip to “Okay, I want to try the DistGit package out. How?”. Everything
else is an interesting background.

Introduction

Almost everyone in the programming world knows Git. It’s an alpha and omega of Source Content Management in the Linux environment and
most of the programmers there cannot live without it. Everything is being tracked and code is being distributed all around the world
to be built upon by more and more well-defined units of human work called Git commits.

As opposed to that, nobody knows DistGit. Well…almost nobody. There are a few humanoids in Delta quadrant called “packagers” that collect
all the work of the primarily application-focused programmers and make neat packages out of them that serve as a basis of any Linux operating
system. These people create an operating system so they should not be taken lightly even though their work is probably less creative than of
the people creating these cool Vims, Gimps, and Sims.

For those few humanoids and their work, Git is actually not the most suitable tool. Well…it is…but not without a few tweaks. And Git with
those tweaks included is nothing else than DistGit.

Why not Git?

We have said that Git is not the weapon of choice for packagers but why exactly? Well, the reason is that a packager does not actually care
all that much about the actual application sources (even though he needs to know them well in the end). What he cares most about are meta-data
about the package such as build or run-time dependencies on other packages or installation steps that make it possible for the package to be
installed and actually used by the end user.

Therefore, in a package, sources are usually packed into a nice little (or huge) tarball with a file next to it called .spec file containing
all those additional package information that are needed for a package to be a useful piece of the target Linux distribution. Note that here
we are talking about so-called “source packages” and not about “binary packages” that are created from the source packages by a building process.

So why not Git on its own? It’s just because the tarballs (containing the source files) are not suitable to be tracked for content changes. They
are binary and any small change in the original source might cause huge (and binary) changes in the resulting tarball. Packagers are interested
in the .spec files and other optional files called “downstream patches”, which are all in human-readable text and, hence, their diffs against
their older version are comprehensible and useful.

So what to do with that (big) dirty binary blob that cannot be put into our tiny, neat package Git repo…that’s where DistGit comes in!

Why DistGit?

DistGit alias “Distributed Git” solves the problem by storing the source tarball outside of the Git repository in the so-called lookaside cache,
while keeping a “link” in the Git repository next to the .spec and patches (if any). This “link” literally called “sources” is nothing else than
another text file with an information about location of the tarball. This location is represented in a very specific way as you can see in the
following example “sources” file:

$ cd somerepo
$ cat sources
SHA512 (prunerepo-1.10.tar.gz) = e8335bada83cc8c77050ece3b0f23871941c39a4bc8d2d7b320fac3cda32fda22d64292bcd51b0e994fcc0ae0c7ddc2ed1e249b91fe995b71e6e78699e26e66b

The format might seem a bit cryptic but the hash is really nothing more than a part of an otherwise known file-system path and the “lookaside cache” is then
nothing more than a directory at a predetermined path in that filesystem.

There is not much else to be seen around here and really, in its gist, DistGit is just Git with few additional helper scripts for setting up new (Dist)Git repos
and maintaining tarballs in the lookaside cache.

Why so much fuss around it then?

DistGit was not always this simple. The previous version 0.13 available in Fedora distribution contained a lot more including an ssh-based ACL system called
Gitolite and also an integration with a Fedora Package database. It was
quite tightly bound to a specific use-case deployed in Fedora while also being outdated with respect to the wildly volatile scripts it was originally packaged from.

DistGit 1.0, now released for Fedora and EPEL7, attempts to take much smaller bite by employing only the most fundamental parts that makes DistGit what it is while
trimming off all the rest. Both Gitolite and integration with a package db can be added back easily if needed and that’s because DistGit 1.0 is a really very simple
package consisting of just a few scripts and configuration files. What matters the most are the data being maintained (Git repos + tarballs) and here you are completely
free to:

  • use a fancier tool for Git authorization than just default ssh (e.g. gitolite)
  • init and maintain Git repos according to remote db records
  • use Kerberos for uploading tarballs into the lookaside cache
  • do anything else upon the data and the access to them

I am talking about these possibilities because they are essential for the DistGit package to be a preferred option for maintaining large package collections.
Currently, it’s being integrated into Fedora package collection system and we hope, the labor will be successful in the end (looks like it will).

Fedora basically uses DistGit already (at least scripts and the configs are all very similar to the ones in the actual DistGit package) but in a non-packaged and
highly one-purposed manner. Extraction of the essential and general DistGit’s parts into a standalone package can be useful because other distributions may then
start using it as well.

Is DistGit 1.0 already used somewhere?

Yes, it is used on copr-dist-git machine in COPR build system stack.

Are there alternatives?

There are. One of them is Git-Annex that enables storage of the binaries in multiple locations as oppossed to
DistGit where they are stored in a single place only. Another difference is in client-side tooling. With Git-Annex, a well-known git command line tool is used
as a client to manage the local repositories (with git annex subcommand to handle the binaries) and there is really not a difference between a client and server.
All nodes working with the data are essentially the same. In DistGit world, there is a difference between a client pulling/pushing by using git and
uplading/downloading using rpkg (or a derived tool like fedpkg) and a server on which the initial repository setup and authorization takes place.

Then, there is also Git-LFS working in a similar spirit to Git-Annex.

The main advantage of the DistGit package that we develop is in its focus on package collections. The rpkg client tool provides lots of useful subcommands such as
rpkg container-build, rpkg copr-build that enables packagers to immediately send the new package content to a build system to obtain a final installable
binary package and there is more (see man rpkg for other useful commands like rpkg lint or rpkg compile).

Okay, I want to try the DistGit package out. How?

Normally, you would need a server machine and a client machine but for playing around, you can just use localhost for both.

First, install the dist-git package on the DistGit server (=localhost):

$ sudo dnf install dist-git

or on CentOS, RHEL with EPEL7 enabled:

$ sudo yum install dist-git

Then, you need to setup Apache on the server (=localhost) for uploading source tarballs.
There is /etc/httpd/conf.d/dist-git/lookaside-upload.conf.example provided by the package
for ssl uploading with client certificates for authentication but we will use something
more simple for the demonstration. Put the following into /etc/httpd/conf.d/dist-git/lookaside-upload.conf:

<VirtualHost _default_:80>
    # This alias must come before the /repo/ one to avoid being overridden.
    ScriptAlias /repo/pkgs/upload.cgi /var/lib/dist-git/web/upload.cgi

    Alias /repo/ /srv/cache/lookaside/

    ServerName pkgs02.stg.phx2.fedoraproject.org
    ServerAdmin webmaster@fedoraproject.org

    LogLevel trace8

    # provide this manually for upload.cgi
    SetEnv SSL_CLIENT_S_DN_CN distgit_user

    <Location /repo/pkgs/upload.cgi>
        Options +ExecCGI
        Require all granted
    </Location>

</VirtualHost>

and reload the httpd server. Note that we fake ssl authentication purely for demonstration purposes here
because the upload.cgi script (central core of DistGit) needs some authentication (SSL or Kerberos).

Then you also need to create the distgit_user and add it to packager group on the server.

$ sudo useradd distgit_user -G packager

Then on the client install the rpkg package and put the following configuration into /etc/rpkg/rpkg.conf:

[rpkg]
lookaside = http://localhost/repo/pkgs
lookasidehash = sha512
lookaside_cgi = http://localhost/repo/pkgs/upload.cgi
gitbaseurl = ssh://%(user)s@localhost/var/lib/dist-git/git/%(module)s
anongiturl = git://localhost/%(module)s
branchre = fd$|fdd$|eld$|olpcd$|master$
kojiconfig = /etc/koji.conf
build_client = koji
clone_config = bz.default-component %(module)s

If the DistGit server is different from ‘localhost’, change it appropriately.

Finally, you need to create distgit_user on the client for the test purpose
(note that you don’t need to do this if you use localhost for both DistGit server and a client):

$ sudo useradd distgit_user

An example DistGit workflow might then look like this:

root@server $ /usr/share/dist-git/setup_git_package foo # creates Git repo on the server
root@server $ ls /var/lib/dist-git/git
foo.git

distgit_user@client $ rpkg clone foo # clones remote foo.git repo
distgit_user@client $ cd foo
distgit_user@client $ rpkg import foo.src.rpm # uploads src.rpm into dist-git and modifies local repo accordingly
distgit_user@client $ git commit -m "DistGit test update" -a # commit changes to the local Git repo
distgit_user@client $ git push # push changes to the Git repos on the DistGit server

You don’t need to use root user for the repository creation at the server. Any user in the packager group is suitable.

Anything else?

The DistGit upstream is hosted at https://github.com/release-engineering/dist-git. Please, send us patches and requests there.


Source From: fedoraplanet.org.
Original article title: Miroslav Suchý: DistGit 1.0.
This full article can be read at: Miroslav Suchý: DistGit 1.0.

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