Welcome to the 8th edition of Git Rev News, a digest of all things Git. For our goals, the archives, the way we work, and how to contribute or to subscribe, see the Git Rev News page on git.github.io.
This edition covers what happened during the month of September 2015.
Lars Schneider wrote that to avoid breaking tests in the future he configured Travis-CI to run all Git tests including git-p4 and git-lfs tests on both Ubuntu and MacOS using gcc and clang.
If his config was enabled on https://github.com/git/git then the status of all official branches and all pull requests could be known by any contributor for free.
Junio, the Git maintainer, replied that the last time he looked at it, Travis wanted write access to the repository, and that for security reason he could not allow that.
Dennis Kaarsemaker replied with the following:
It does not need write access to the git data, only to auxiliary GitHub data: commit status and deployment status (where it can put “this commit failed tests”), repository hooks (to set up build triggers), team membership (ro) and email addresses (ro).
Then a lot of long time members of the community, like Jeff King, Johannes Schindelin, Shawn Pearce, Stefan Beller, Roberto Tyley, Luke Diamand and Sebastian Schuberth, started discussing if it would be better for Travis to be configured only on a fork of git/git and about the annoyance that emails sent by Travis might be.
Roberto Tyley, the author of submitGit, then talked about possible enhancement to the submitGit workflow if Travis or another CI system are set up, like enabling sending a patch email to the Git mailing list only after the test results are available.
And then Matthieu Moy explained the benefits for everyone like this:
The very nice thing with Travis-CI is that it does not only test the repository’s branches, but also all pull-requests. So, if it is activated on git/git, it will become possible to have a flow like
1) User pushes to his own repo, sends a pull-request,
2) Travis-CI notices the pull-request and builds it (no action needed from anyone),
3) Once the build is finished, the user can use e.g. SubmitGit to actually submit the code.
This has real benefits for the submitter (know if your code is broken early), for the reviewers (things like “you have a def-after-use” would be noticed by a computer before human beings start spending time on the review), and for you (some issues noticed before a topic enters pu).
There’s no extra work for the user at all compared to the standard pull-request flow (nothing to do, just submit a PR), and a one-time setup for the project.
This appear to have convinced Junio of the value of a CI tool linked to git/git, and Lars has since posted a version 2 and a version 3 of his patches, so an interesting way to test patches will perhaps be available soon to Git developers.
Shortly after git version 2.6.0 was released, Nazri Ramliy noticed that:
the format of the comment lines in a rebase instruction sheet has become stricter - it could no longer begin with spaces or tabs. The comment char (“#” for example) has to appear on the first column.
and that this change broke his script.
As Matthieu Moy found out, the cause of the regression was a patch to pre check the rebase instruction sheet before starting an interactive rebase. The goal of this pre checking patch was to avoid discovering a syntax error in the instruction sheet in the middle of a rebase.
Matthieu then posted a patch to fix the regression
based on a suggestion by Junio Hamano. Matthieu’s patch just
prefilters the instruction sheet by piping it through
sed 's/^[[:space:]]*//'.
But Junio replied that Matthieu’s patch would fix one aspect of the problems caused by the pre checking patch. There could be more problems as the pre checking code is different from the actual parsing when the instruction sheet is processed. Junio also said he was tempted to revert the pre checking patch, but he provided a patch to fix the problem anyway by using the same code in both the pre checking step and the actual parsing.
Matthieu agreed that Junio’s patch was the way to go, and after a few more patches and emails, they agreed on a solution.
Q. Who are you, and what do you do?
A. I’ve been writing software since my uncle got me hooked with a TI-56 calculator and a TRS-80 model 2. Right now, I’m working for Roku, (making cool new video streaming hardware :-) which currently involves a lot of fairly low-level digging around in SoC software.
Q. What would you name your most important contribution to Git?
A. My fix to git-p4, to make it stream commits into fast-import, rather than buffer them all up in memory first. I did it out of desperation.
At the time, I’d just been working for a tiny startup which was also making video streaming products, but sadly not making any profits. While there, I’d become hooked on git after watching “those” videos.
So when I found myself at Broadcom in about 2008, and started trying to use Perforce, almost the first thing I did was try to use git-p4. But at the time, it read everything into memory using an O(n^2) algorithm, which meant it would use up all of the swap space available (no matter how much you had) and then crash after a couple of days.
These days of course, if you’ve got a problem the answer is on stackoverflow (if you can think of it, it’s already been done) but back then, we had nothing like that. Eventually I found a patch from Andrew Oakley which hinted at the solution. Inspired by this, I taught git-p4 to stream commits at fast-import (as the writers of fast-import presumably intended) and suddenly I could use git at work. Which was completely awesome!
With a good deal of help from Pete Wyckoff and Junio I then got it submitted; that process itself was a massive eye opener as to how to really review code (and how to write shell script as well).
I spent the next few years trying to encourage everyone else in the office to use git and submitted a few more patches to git-p4 along the way. We finally switched over, and then the office was closed down!
Which is how it is I now find myself working side-by-side with the very same Andrew Oakley who wrote that original patch - he’s now at Roku as well!
Q. What are you doing on the Git project these days, and why?
A. It’s all git-p4 related. I’ve got a tiny fix that lets you work on a detached head; that’s just waiting for a bit of bandwidth.
After that, there’s a few things I’ve thought about. It would be very handy to be able to turn a P4 shelved changelist into a git commit (and vice-versa). And I think it ought to be possible to teach git-p4 to act in a “daemon” mode, whereby it gets notified of changes and automatically pushes them into Perforce, and pulls Perforce changes back into git. I’ve had something like that going in the past but it was an ugly hack.
A. If you could get a team of expert developers to work full time on something in Git for a full year, what would it be?
Performance on big repos would be nice. Quite a few people have tried in the past so maybe it’s just impossible, but for example, I did wonder if more could be done with cache prefetching (pld instruction on ARM) to slurp data into the cache faster. Since I’m pretty much completely ignorant of the internals of git, I’m able to be free of the boring constraints of reality :-)
And I’d really like a way to painlessly teach people to use git who only know about traditional revision control systems (e.g. Perforce).
Q. If you could remove something from Git without worrying about backwards compatibility, what would it be?
I think Junio already removed it. That “feature” where git push would push all of the branches rather than just HEAD. It had me completely confused when I was first starting.
Q. What is your favorite Git-related tool/library, outside of Git itself?
I really like Gerrit, although the prolog configuration makes my head hurt. And of course Github has to get a mention - it’s making git (and hence sane version control) ubiquitous. There was a question about it the other day on a mountain bike forum I follow!
Q. What is your favourite new feature in git?
git commit --fixup
I only found this recently, it’s just brilliant. Thank you, whoever it was put that in!
Updates
Various
Light reading
Git tools and sites
This edition of Git Rev News was curated by Christian Couder <christian.couder@gmail.com>, Thomas Ferris Nicolaisen <tfnico@gmail.com> and Nicola Paolucci <npaolucci@atlassian.com>, with help from Luke Diamand, Roberto Tyley, Matthieu Moy, Johannes Schindelin and David Turner.