Git worktree - clone but not quite

Have you ever wanted another workarea for the current repository you're working in?  Maybe you're running some tests and need the normal workarea to stay unchanged, so you can't rebase or tweak another branch in the meantime.

Options up to now:

• $ git clone [current-repo] [temp-repo]; cd temp-repo  # too heavy-weight, have to push changes back
• $ sleep 120; check email  # interrupts flow

In Git 2.5.0, you can do this easily:

• $ git worktree add ../temp master

This creates a new workarea in ../temp with all the current branches.  It's the same repository!  Any git command you do in the new workarea is applied to (and uses the database of) the original repository: commit, rebase, push, etc.

NOTE: If you leave off the branch name, 'git worktree add' creates a branch named after the new worktree directory.

NOTE: If you want to operate on the same branch as the original repository, it is disallowed by default.  In order to operate on the same branch, you have to say $ git worktree add ../temp master -f'.  Also if you ever move off of the branch and want to switch back when another worktree has the same branch checked out, you have to use an annoyingly-long option: $ git checkout master --ignore-other-worktrees.  You could put that in an alias like so: $ git config --global alias.co "checkout --ignore-other-worktrees".

Rewriting Git History for Fun

For starters, if you're looking for a good JDBC connection pool, look no farther than c3p0 [github, doc].

Back a few years commons-dbcp wasn't very stable under load, so I went looking for an alternative and landed on c3p0.  Ever since then, Steve Waldman has been making it better and better.  Except for a 2-year hiatus when he wasn't working on it, he's been very responsive and willing to accept feedback and make improvements.  I'm seriously impressed by the project.

Anyway, since I've been a long-time user and fan, when I saw Steve put his software on GitHub, I went for a look.

Here is what I saw:

But what about all those prior releases in source snapshot form, that I was used to seeing from SourceForge?

I realized that perhaps I could contribute a little to the project, so I went and grabbed all the source release zips from SourceForge, created a local git repo, created a commit for each release, tagged it, and spliced Steve's recent work on the top.

Then I submitted a GitHub issue to ask Steve what he thought -- wasn't really a pull request because it was a completely disconnected history.

Here is the top of the new history:

And here is the earliest part of that history:

The tools used were:

- git config author.name - to give credit where credit was due

- git config author.email

- curl - to download all the releases

- unzip | sed - to figure out what the commit date should be

- git commit --date "[release date]" - to create the commits

- vi .git/info/grafts - to temporarily splice the new history on the old

- git filter-branch --tag-name-filter - to rewrite the new history permanently on top

- git tag -f - to replace the existing release tags to point to the new rewritten history

- git push --tags - to push it up to github

If I could consult for projects / companies to do this kind of VCS conversion work and actually get paid for it -- wouldn't that be awesome!

Semantic Versioning

After having skimmed the semantic versioning proposal/spec, I really like it, and I'm going back for a deep read.

The most notable violator of this that has bit me in the past has been the jersey framework, and maybe earlier versions of commons-collections.

Git as Menograph

Background

In Mar. 2010, Matz came and talked at MWRC.

The awesome talk focused around Ruby as a great invention that makes it possible for programmers to bring a new world into being that could only be imagined before.

He made reference to the inventor in Hugo Gernsback's book: Ralph 124c 41 +.  The name was a play on words "One to foresee for one", with "+" being reserved for the 10 or so greatest inventors currently living.

Although Matz's reference could be interpreted as prideful, anyone who has ever had anything to do with him would correct you -- he is a very humble programmer who does awesome stuff.

Because I wanted to understand Matz's reference very thoroughly, I went and read the book.  I was able to appreciate the scope of Matz's talk much more clearly after having read about Ralph, the great inventor.

Menograph, the invention

One thing that Ralph invented was a Menograph.  It was a mind-recording device, operated by pressing a button that started a mind wave recorder and operated a scroll of fabric on which the waves were traced in ink, similar to an old-style seismograph.

The Menograph was one of Ralph 124C 41 +'s earliest inventions, and entirely superseded the pen and pencil.  It was only necessary to press the button when an idea was to be recorded and to release the button when one merely reflected and did not wish the thought-words recorded.

Those thoughts could then be replayed again verbatim by any person by use of a device called a Hypnobioscope.

After reading an account of Ralph using this invention, which is more detailed than I am able to reproduce here, I had a vague sense that I had experienced a similar feeling before while programming.

Git as Menograph

After thinking about it for a while, I realized that how Ralph used his Menograph is how I use git.

While coding, I work on stuff, then I record commits into git.  Then I work on stuff some more, then record commits.

Then I take passes back over what I've recorded and get rid of the bad stuff and keep the good stuff.  Then I push it out to the team.  Sometimes, a pair or triple on the team do this as a group exercise, working history over a few time until it won't break the world.

The parallel to git was eye-opening.  In fact, after having noticed this behavior in myself, and seeing the behavior tendency grow over time, I'd say that git encourages multi-pass thinking styles.

P.S.  If you care at all about Ruby, listen to Matz's talk.  Some of the future stuff he was talking about is already in Ruby 1.9.2.  NOTE: I'm on linux, and I had to use the flash player to get the video to show.

Core Goals of Version Control

After having a couple of years of experience with Git, I was finally able to clearly articulate the core value that version control brings to software development. This article will be written with a bias toward Git, but I've at least tried to express the core value in abstract terms.

The value is expressible in 4 goals.

Core Goals

1. Capture source artifacts
2. Isolate stable codelines from WIP
3. Enable concurrent WIP
4. Make it easy to leave sensible history behind

WIP = Work In Progress

The continuous delivery crowd says that WIP should be basically zero. I think that there is space for a fairly short development pipeline (3-5d) that can free developers up to capture ideas and then wrangle them into shape in a second or third pass.

Capture Source Artifacts

As a developer, I feel happy when I can push a save button when I've taken a small step -- and then never have think about it again. I also feel happy when I can save, even if I'm based on a slightly out-of-date base. I also feel happy when I don't have to worry about binary vs. text -- as long as its a true source file of a reasonable size.

I feel happiest when I don't have to be distracted by superfluous concerns, and can focus on the task at hand and save the results easily.

Main Point: A good version control system makes it easy to capture source artifacts.

Isolate Stable Codelines From WIP

Software is hard to get right, and once things get stable, the only way to keep them there is to avoid making risky changes. But stability is always balanced against the common desire for enhancement and restructuring.

A typical bug fix is one or two isolated commits that fix a specific problem. Most version control systems more or less support cherry-picking a small change over into another codeline.

I feel happy when I'm able to easily take an isolated series of commits from the development codeline into a stable one, even if the fix turns out to be a little larger than a couple commits.

Main Point: A good version control system makes it easy to cherry-pick changes, and even longer patch sequences, from one codeline to another with easy reconciliation of overlapping edits.

Enable Concurrent WIP

For software developed by a team larger than 2-3 people, it is useful to be able to work in parallel with each other, even on the same feature. Sometimes you can arrange your work so as not to overlap at all, but there are real situations where you want to track someone else's development within the fairly short development pipeline (3-5d) before things have gotten stable.

I feel happy when I don't have to catch people up on what I've been doing, and when they don't have to catch me up on what they've been doing, and we can get to the point easily and quickly. I also feel happy when we can leap-frog each other with ideas and real implementation.

Main Point: A good version control system makes it easy to track other team members' work, and easy to tentatively integrate with that work.

Make It Easy To Leave Sensible History Behind

After stepping away from a piece of software, it is easy to lose context and forget the concerns that shaped the development of that software. It doesn't take long. For me, about 2 weeks is enough for me to start forgetting details and motivations.

Of course, it is important to leave a properly structured artifact behind. Proper naming means a lot; proper factoring is important. Once you comprehend the structure, it is possible to change things without introducing big problems.

But often, full comprehension isn't practical, and a flat 2D view of the artifact is insufficient for reasoning about the thing itself. Often, it is often very useful to be able to get a vector on where the software came from, and interpolate from that vector where it was headed.

The creative process is messy, filled with double steps and backtracking. Don't confuse me with all the noise. Give me a history that, given all the knowledge you had when it got stable, at least looks well-reasoned.

I feel happy when I'm able to take an easy editing pass following capture/review that lets me intentionally craft history as an email to the future about the vector of change that is inherent in a certain patch sequence. I also feel very happy to not have to think about this during the creative, messy, focus-draining capture/review pass.

Main Point: A good version control system includes tools to both: 1) enable the developers to easily leave behind meaningful history, and 2) extract focused history for all or part of a piece of software.