Hi, this is the first draft of my proposal. --- ABSTRACT
git is a modular source control management software, and all of its subcommands are programs on their own. A lot of them are written in C, but a couple of them are shell or Perl scripts. This is the case of =git-rebase--interactive= (or interactive rebase), which is a shell script. Rewriting it in C would improve its performance, its portability, and maybe its robustness. ABOUT `git-rebase` AND `git-rebase--interactive` git-rebase allows to re-apply changes on top of another branch. For instance, when a local branch and a remote branch have diverged, git-rebase can re-unify them, applying each change made on the local branch on top of the remote branch. git-rebase--interactive is used to reorganize commits by reordering, rewording, or squashing them. To achieve this purpose, =git= opens the list of commits to be modified in a text editor (hence the interactivity), as well as the actions to be performed for each of them. PROJECT GOALS Back in 2016, Johannes Schindelin discussed[1] about retiring git-rebase.sh (described here as a “hacky shell script”) in favor of a builtin. He explained that, as it’s only a command-line parser for git-rebase--am.sh, git-rebase--interactive.sh, and git-rebase--merge.sh, these 3 scripts should be rewritten first. The goal of this project is to rewrite git-rebase--interactive.sh in C, for multiple reasons : Performance improvements Shell scripts are inherently slow. When Johannes Schindelin began to rewrite some parts of git-rebase--interactive in C, its performance increased from 3 to 5 times, depending on the platform[2]. That’s because each command is a program by itself. So, for each command, the shell interpreter has to spawn a new process and to load a new program (with fork() and exec() syscalls), which is an expensive process. Those commands can be other git commands. Sometimes, they are wrappers to call internal C functions (eg. git-rebase--helper), something shell scripts can’t do natively. These wrappers basically parse the parameters, then start the appropriate function, which is obviously slower than just calling a function from C. Other commands can be POSIX utilities (eg. sed, cut, etc.). They have their own problems (speed aside), namely portability. Portability improvements Shell scripts often relies on many of those POSIX utilities, which are not necessarily natively available on all platforms (most notably, Windows), or may have more or less features depending on the implementation. Although C is not perfect portability-wise, it’s still better than shell scripts. For instance, the resulting binaries will not necessarily depend on third-party programs or libraries. RISKS Of course, rewriting a piece of software takes time, and can lead to regressions (ie. new bugs). To mitigate that risk, I should understand well the functions I want to rewrite, run tests on a regular basis and write new if needed, and of course discuss about my code with the community during reviews. APPROXIMATIVE TIMELINE Normally, I would be able to work 35 to 40 hours a week. When I have courses or exams at university, I could work between 20 and 25 hours a week. Community bonding --- April 23, 2018 -- May 14, 2018 /I’ll still have courses at the university during this period./ During the community bonding, I would like to dive into git’s codebase, and to understand what git-rebase--interactive does under the hood. At the same time, I’d communicate with the community and my mentor, seeking for clarifications, and asking questions about how things should or should not be done. Weeks 1 & 2 --- May 14, 2018 -- May 27, 2018 /From May 14 to 18, I have exams at the university, so I won’t be able to work full time./ I would search for edge cases not covered by current tests and write some if needed. Week 3 --- May 28, 2018 -- June 3, 2018 At the same time, I would refactor --preserve-merges in its own shell script, if it has not been deprecated or moved in the meantime. Dscho explained that this would be the first step of the conversion[1]. This operation is not really tricky by itself, as --preserve-merges is about only 50 lines of code into git_rebase__interactive(), plus some specific functions (eg. pick_one()). Weeks 4 to 7 --- June 4, 2018 -- July 1, 2018 Then, I would start to incrementally rewrite git-rebase--interactive.sh functions in C, and move them git-rebase--helper.c. Newly-rewritten C functions are then associated to command-line parameters to be able to use them from shell scripts. Examples of such conversion can be found in commits 0cce4a2756[3] (rebase -i -x: add exec commands via the rebase--helper) and b903674b35[4] (bisect--helper: `is_expected_rev` & `check_expected_revs` shell function in C). There is a lot of functions into git-rebase--interactive.sh to rewrite. Most of them are small, and some of them are even wrappers for a single command (eg. do_next()), so they shouldn’t be really problematic. A couple of them are quite long (eg. pick_one()), and will probably be even longer once rewritten in C due to the low-level nature of the language. They also tend to depend a lot on other smaller functions. The plan here would be to start rewriting the smaller functions when applicable (ie. they’re not a simple command wrapper) before working on the biggest of them. Of course, rewriting a function should not cause any breakage in the test suite. Week 8 --- July 2, 2018 -- July 8, 2018 Then, I plan to polish the new code, in order to improve its performance or to make it more readable. Benchmarking will be done using the existing script in t/perf. Weeks 9 & 10 --- July 9, 2018 -- July 22, 2018 When all majors functions from git-rebase--interactive.sh have been rewritten in C, I would retire the script in favor of a builtin. Weeks 11 & 12 --- July 23, 2018 -- August 5, 2018 In the last two weeks, I would improve the code coverage where needed. I also plan to use this time as a backup if I am late on my planning. IF TIMES PERMITS * Add an option to include the patch in the message of commits to be reworded, as proposed by Ævar Arnfjörð Bjarmason[5]. COMMUNICATING ABOUT MY WORK I will communicate every week about the state of my work on the mailing list. ABOUT ME My name is Alban Gruin. I am an undergraduate at the Paul Sabatier University in Toulouse, France, where I have been studying Computer Sciences for a year and a half. My timezone is UTC+02:00. I have been programming in C for the last 5 years. I learned using freely available resources online, and by attending class ever since last year. I am also quite familiar with shell scripts, and I have been using git for the last 3 years. My e-mail address is alban <dot> gruin <at> gmail <dot> com, my IRC nick is abngrn. My micro-project was “userdiff: add built-in pattern for golang”[6][7]. --- You can find the Google Doc version here[8]. Regards, Alban Gruin [1] https://public-inbox.org/git/alpine.DEB.2.20.1609021432070.129229@virtualbox/ [2] https://public-inbox.org/git/cover.1472457609.git.johannes.schinde...@gmx.de/ [3] https://git.kernel.org/pub/scm/git/git.git/commit/?id=0cce4a2756 [4] https://git.kernel.org/pub/scm/git/git.git/commit/?id=b903674b35 [5] https://public-inbox.org/git/87in9ucsbb....@evledraar.gmail.com/ [6] https://public-inbox.org/git/20180228172906.30582-1-alban.gr...@gmail.com/ [7] https://git.kernel.org/pub/scm/git/git.git/commit/?id=1dbf0c0a [8] https://docs.google.com/document/d/1Jx0w867tVAht7QI1_prieiXg_iQ_nTloOyaIIOnm85g/edit?usp=sharing
