Maintaining parallel releases simultaneously has implications for how daily development is done. In particular, it makes practically mandatory a discipline that would be recommended anyway: have each commit be a single logical change, and never mix unrelated changes in the same commit. If a change is too big or too disruptive to do in one commit, break it across N commits, where each commit is a well-partitioned subset of the overall change, and includes nothing unrelated to the overall change.
Here's an example of an ill-thought-out commit:
------------------------------------------------------------------------ r6228 | jrandom | 2004-06-30 22:13:07 -0500 (Wed, 30 Jun 2004) | 8 lines Fix Issue #1729: Make indexing gracefully warn the user when a file is changing as it is being indexed. * ui/repl.py (ChangingFile): New exception class. (DoIndex): Handle new exception. * indexer/index.py (FollowStream): Raise new exception if file changes during indexing. (BuildDir): Unrelatedly, remove some obsolete comments, reformat some code, and fix the error check when creating a directory. Other unrelated cleanups: * www/index.html: Fix some typos, set next release date. ------------------------------------------------------------------------
The problem with it becomes apparent as soon as someone needs to
BuildDir error check fix over to a
branch for an upcoming maintenance release. The porter doesn't want
any of the other changes—for example, perhaps the fix for ticket
#1729 wasn't approved for the maintenance branch at all, and the
index.html tweaks would simply be irrelevant
there. But she cannot easily grab just the
BuildDir change via the version control tool's
merge functionality, because the version control system was told that
that change is logically grouped with all these other unrelated
things. In fact, the problem would become apparent even before the
merge. Merely listing the change for voting would become problematic:
instead of just giving the revision number, the proposer would have to
make a special patch or change branch just to isolate the portion of
the commit being proposed. That would be a lot of work for others to
suffer through, and all because the original committer couldn't be
bothered to break things into logical groups.
In fact, that commit really should have been
four separate commits: one to fix issue
#1729, another to remove obsolete comments and reformat code in
BuildDir, another to fix the error check in
BuildDir, and finally, one to tweak
index.html. The third of those commits would be
the one proposed for the maintenance release branch.
Of course, release stabilization is not the only reason why having each commit be one logical change is desirable. Psychologically, a semantically unified commit is easier to review, and easier to revert if necessary (in some version control systems, reversion is really a special kind of merge anyway). A little up-front discipline on everyone's part can save the project a lot of headache later.
One area where open source projects have historically differed from proprietary projects is in release planning. Proprietary projects usually have firmer deadlines. Sometimes it's because customers were promised that an upgrade would be available by a certain date, because the new release needs to be coordinated with some other effort for marketing purposes, or because the venture capitalists who invested in the whole thing need to see some results before they put in any more funding. Free software projects, on the other hand, were until recently mostly motivated by amateurism in the most literal sense: they were written for the love of it. No one felt the need to ship before all the features were ready, and why should they? It wasn't as if anyone's job was on the line.
Nowadays, many open source projects are funded by corporations, and are correspondingly more and more influenced by deadline-conscious corporate culture. This is in many ways a good thing, but it can cause conflicts between the priorities of those developers who are being paid and those who are volunteering their time. These conflicts often happen around the issue of when and how to schedule releases. The salaried developers who are under pressure will naturally want to just pick a date when the releases will occur, and have everyone's activities fall into line. But the volunteers may have other agendas—perhaps features they want to complete, or some testing they want to have done—that they feel the release should wait on.
There is no general solution to this problem except discussion and compromise, of course. But you can minimize the frequency and degree of friction caused, by decoupling the proposed existence of a given release from the date when it would go out the door. That is, try to steer discussion toward the subject of which releases the project will be making in the near- to medium-term future, and what features will be in them, without at first mentioning anything about dates, except for rough guesses with wide margins of error. By nailing down feature sets early, you reduce the complexity of the discussion centered on any individual release, and therefore improve predictability. This also creates a kind of inertial bias against anyone who proposes to expand the definition of a release by adding new features or other complications. If the release's contents are fairly well defined, the onus is on the proposer to justify the expansion, even though the date of the release may not have been set yet.
In his multi-volume biography of Thomas Jefferson, Jefferson and His Time, Dumas Malone tells the story of how Jefferson handled the first meeting held to decide the organization of the future University of Virginia. The University had been Jefferson's idea in the first place, but (as is the case everywhere, not just in open source projects) many other parties had climbed on board quickly, each with their own interests and agendas. When they gathered at that first meeting to hash things out, Jefferson made sure to show up with meticulously prepared architectural drawings, detailed budgets for construction and operation, a proposed curriculum, and the names of specific faculty he wanted to import from Europe. No one else in the room was even remotely as prepared; the group essentially had to capitulate to Jefferson's vision, and the University was eventually founded more or less in accordance with his plans. The facts that construction went far over budget, and that many of his ideas did not, for various reasons, work out in the end, were all things Jefferson probably knew perfectly well would happen. His purpose was strategic: to show up at the meeting with something so substantive that everyone else would have to fall into the role of simply proposing modifications to it, so that the overall shape, and therefore schedule, of the project would be roughly as he wanted.
In the case of a free software project, there is no single "meeting", but instead a series of small proposals made mostly by means of the ticket tracker. But if you have some credibility in the project to start with, and you start assigning various features, enhancements, and bugs to target releases in the ticket tracker, according to some announced overall plan, people will mostly go along with you. Once you've got things laid out more or less as you want them, the conversations about actual release dates will go much more smoothly.
It is crucial, of course, to never present any individual decision as written in stone. In the comments associated with each assignment of a ticket to a specific future release, invite discussion, dissent, and be genuinely willing to be persuaded whenever possible. Never exercise control merely for the sake of exercising control: the more deeply others participate in the release planning process (see the section called “Share Management Tasks as Well as Technical Tasks” in Chapter 8, Managing Volunteers), the easier it will be to persuade them to share your priorities on the issues that really count for you.
The other way the project can lower tensions around release planning is to make releases fairly often. When there's a long time between releases, the importance of any individual release is magnified in everyone's minds; people are that much more crushed when their code doesn't make it in, because they know how long it might be until the next chance. Depending on the complexity of the release process and the nature of your project, somewhere between every three and six months is usually about the right gap between releases, though maintenance lines may put out micro releases a bit faster, if there is demand for them.
 For an alternative approach, you may wish to read Martin Michlmayr's Ph.D. thesis Quality Improvement in Volunteer Free and Open Source Software Projects: Exploring the Impact of Release Management (cyrius.com/publications/michlmayr-phd.html). It is about using time-based release processes, as opposed to feature-based, in large free software projects. Michlmayr also gave a talk at Google on the subject, available on Google Video at video.google.com/videoplay?docid=-5503858974016723264.