Legacy FreeBSD Release Engineering

The development of FreeBSD is a very open process. FreeBSD is comprised of contributions from thousands of people around the world. The FreeBSD Project provides Subversion ^[1] access to the general public so that others can have access to log messages, diffs (patches) between development branches, and other productivity enhancements that formal source code management provides. This has been a huge help in attracting more talented developers to FreeBSD. However, I think everyone would agree that chaos would soon manifest if write access to the main repository was opened up to everyone on the Internet. Therefore only a "select" group of nearly 300 people are given write access to the Subversion repository. These FreeBSD committers^[2] are usually the people who do the bulk of FreeBSD development. An elected Core Team^[3] of developers provide some level of direction over the project.

The rapid pace of FreeBSD development makes the main development branch unsuitable for the everyday use by the general public. In particular, stabilizing efforts are required for polishing the development system into a production quality release. To solve this conflict, development continues on several parallel tracks. The main development branch is the HEAD or trunk of our Subversion tree, known as "FreeBSD-CURRENT" or "-CURRENT" for short.

A set of more stable branches are maintained, known as "FreeBSD-STABLE" or "-STABLE" for short. All branches live in a master Subversion repository maintained by the FreeBSD Project. FreeBSD-CURRENT is the "bleeding-edge" of FreeBSD development where all new changes first enter the system. FreeBSD-STABLE is the development branch from which major releases are made. Changes go into this branch at a different pace, and with the general assumption that they have first gone into FreeBSD-CURRENT and have been thoroughly tested by our user community.

The term stable in the name of the branch refers to the presumed Application Binary Interface stability, which is promised by the project. This means that a user application compiled on an older version of the system from the same branch works on a newer system from the same branch. The ABI stability has improved greatly from the compared to previous releases. In most cases, binaries from the older STABLE systems run unmodified on newer systems, including HEAD, assuming that the system management interfaces are not used.

In the interim period between releases, weekly snapshots are built automatically by the FreeBSD Project build machines and made available for download from https:/download.FreeBSD.org/snapshots/. The widespread availability of binary release snapshots, and the tendency of our user community to keep up with -STABLE development with Subversion and “make buildworld” ^[4] helps to keep FreeBSD-STABLE in a very reliable condition even before the quality assurance activities ramp up pending a major release.

In addition to installation ISO snapshots, weekly virtual machine images are also provided for use with VirtualBox, qemu, or other popular emulation software. The virtual machine images can be downloaded from https://download.FreeBSD.org/snapshots/VM-IMAGES/.

The virtual machine images are approximately 150MB xz(1) compressed, and contain a 10GB sparse filesystem when attached to a virtual machine.

Bug reports and feature requests are continuously submitted by users throughout the release cycle. Problems reports are entered into our Bugzilla database through the web interface provided at https://www.freebsd.org/support/bugreports/.

To service our most conservative users, individual release branches were introduced with FreeBSD 4.3. These release branches are created shortly before a final release is made. After the release goes out, only the most critical security fixes and additions are merged onto the release branch. In addition to source updates via Subversion, binary patchkits are available to keep systems on the releng/X.Y branches updated.

New releases of FreeBSD are released from the -STABLE branch at approximately four month intervals. The FreeBSD release process begins to ramp up 70-80 days before the anticipated release date when the release engineer sends an email to the development mailing lists to remind developers that they only have 15 days to integrate new changes before the code freeze. During this time, many developers perform what have become known as "MFC sweeps".

MFC stands for "Merge From CURRENT" and it describes the process of merging a tested change from our -CURRENT development branch to our -STABLE branch. Project policy requires any change to be first applied to trunk, and merged to the -STABLE branches after sufficient external testing was done by -CURRENT users (developers are expected to extensively test the change before committing to -CURRENT, but it is impossible for a person to exercise all usages of the general-purpose operating system). Minimal MFC period is 3 days, which is typically used only for trivial or critical bugfixes.

FreeBSD "releases" can be built by anyone with a fast machine and access to a source repository. (That should be everyone, since we offer Subversion access! See the Subversion section in the Handbook for details.) The only special requirement is that the md(4) device must be available. If the device is not loaded into your kernel, then the kernel module should be automatically loaded when mdconfig(8) is executed during the boot media creation phase. All of the tools necessary to build a release are available from the Subversion repository in src/release. These tools aim to provide a consistent way to build FreeBSD releases. A complete release can actually be built with only a single command, including the creation of ISO images suitable for burning to CDROM or DVD, and an FTP install directory. release(7) fully documents the src/release/generate-release.sh script which is used to build a release. generate-release.sh is a wrapper around the Makefile target: make release.

Although FreeBSD forms a complete operating system, there is nothing that forces you to use the system exactly as we have packaged it up for distribution. We have tried to design the system to be as extensible as possible so that it can serve as a platform that other commercial products can be built on top of. The only "rule" we have about this is that if you are going to distribute FreeBSD with non-trivial changes, we encourage you to document your enhancements! The FreeBSD community can only help support users of the software we provide. We certainly encourage innovation in the form of advanced installation and administration tools, for example, but we cannot be expected to answer questions about it.

The release engineering process for 4.4 formally began on August 1st, 2001. After that date all commits to the RELENG_4 branch of FreeBSD had to be explicitly approved by the Release Engineering Team <re@FreeBSD.org>. The first release candidate for the x86 architecture was released on August 16, followed by 4 more release candidates leading up to the final release on September 18th. The security officer was very involved in the last week of the process as several security issues were found in the earlier release candidates. A total of over 500 emails were sent to the Release Engineering Team <re@FreeBSD.org> in little over a month.

Our user community has made it very clear that the security and stability of a FreeBSD release should not be sacrificed for any self-imposed deadlines or target release dates. The FreeBSD Project has grown tremendously over its lifetime and the need for standardized release engineering procedures has never been more apparent. This will become even more important as FreeBSD is ported to new platforms.

It is imperative for our release engineering activities to scale with our growing userbase. Along these lines we are working very hard to document the procedures involved in producing FreeBSD releases.

I would like to thank Jordan Hubbard for giving me the opportunity to take on some of the release engineering responsibilities for FreeBSD 4.4 and also for all of his work throughout the years making FreeBSD what it is today. Of course the release would not have been possible without all of the release-related work done by Satoshi Asami <asami@FreeBSD.org>, Steve Price <steve@FreeBSD.org>, Bruce A. Mah <bmah@FreeBSD.org>, Nik Clayton <nik@FreeBSD.org>, David O’Brien <obrien@FreeBSD.org>, Kris Kennaway <kris@FreeBSD.org>, John Baldwin <jhb@FreeBSD.org> and the rest of the FreeBSD development community. I would also like to thank Rodney W. Grimes <rgrimes@FreeBSD.org>, Poul-Henning Kamp <phk@FreeBSD.org>, and others who worked on the release engineering tools in the very early days of FreeBSD. This article was influenced by release engineering documents from the CSRG ^[7] , the NetBSD Project, ^[8] , and John Baldwin’s proposed release engineering process notes. ^[9]