automatically detect dependencies between git commits
Project description
git-deps
git-deps is a tool for performing automatic analysis of dependencies between commits in a git repository. Here’s a screencast demonstration:
I also spoke about the tool in episode #32 of the GitMinutes podcast.
Background theory
It is fairly clear that two git commits within a single repo can be considered “independent” from each other in a certain sense, if they do not change the same files, or if they do not change overlapping parts of the same file(s).
In contrast, when a commit changes a line, it is “dependent” on not only the commit which last changed that line, but also any commits which were responsible for providing the surrounding lines of context, because without those previous versions of the line and its context, the commit’s diff might not cleanly apply (depending on how it’s being applied, of course). So all dependencies of a commit can be programmatically inferred by running git-blame on the lines the commit changes, plus however many lines of context make sense for the use case of this particular dependency analysis.
Therefore the dependency calculation is impacted by a “fuzz” factor parameter (c.f. patch(1)), i.e. the number of lines of context which are considered necessary for the commit’s diff to cleanly apply.
As with many dependency relationships, these dependencies form edges in a DAG (directed acyclic graph) whose nodes correspond to commits. Note that a node can only depend on a subset of its ancestors.
Motivation
Sometimes it is useful to understand the nature of parts of this DAG, as its nature will impact the success or failure of operations including merge, rebase, cherry-pick etc.
Use case 1: porting between branches
For example when porting a commit “A” between git branches via git cherry-pick, it can be useful to programmatically determine in advance the minimum number of other dependent commits which would also need to be cherry-picked to provide the context for commit “A” to cleanly apply. Here’s a quick demo!
Use case 2: splitting a patch series
Large patch series or pull requests can be quite daunting for project maintainers, since they are hard to conquer in one sitting. For this reason it’s generally best to keep the number of commits in any submission reasonably small. However during normal hacking, you might accumulate a large number of patches before you start to contemplate submitting any of them upstream. In this case, git-deps can help you determine how to break them up into smaller chunks. Simply run
git deps -e $upstream_branch -s
and then create a graph starting from the head of your local development branch, recursively expanding all the dependencies. This will allow you to untangle things and expose subgraphs which can be cleanly split off into separate patch series or pull requests for submission.
Use case 3: aiding collaborative communication
Another use case might be to better understand levels of specialism / cross-functionality within an agile team. If I author a commit which modifies (say) lines 34-37 and 102-109 of a file, the authors of the dependent commits forms a list which indicates the group of people I should potentially consider asking to review my commit, since I’m effectively changing “their” code. Monitoring those relationships over time might shed some light on how agile teams should best coordinate efforts on shared code bases.
Caveat
Note the dependency graph is likely to be semantically incomplete; for example it would not auto-detect dependencies between a commit A which changes code and another commit B which changes documentation or tests to reflect the code changes in commit A. (Although of course it’s usually best practice to logically group such changes together in a single commit.) But this should not stop it from being useful.
Other uses
I’m sure there are other use cases I haven’t yet thought of. If you have any good ideas, please submit them!
Non-use cases
At first I thought that git-deps might provide a useful way to programmatically predict whether operations such as merge / rebase / cherry-pick would succeed, but actually it’s probably cheaper and more reliable simply to perform the operation and then roll back.
Installation
Please see the ``INSTALL.md` file <INSTALL.md>`__.
Usage
The tool is not yet fully documented, but usage is fairly self-explanatory if you run git deps -h.
Currently you should run it from the root (i.e. top directory) of the git repository you want to examine; this is a known limitation.
By default it will output all dependencies of the given commit-ish(s), one per line. With --recurse, it will traverse dependencies of dependencies, and so on until it cannot find any more. In recursion mode, two SHA1s are output per line, indicating that the first depends on the second.
Development / support / feedback
Please see the CONTRIBUTING file.
History
This tool was born from experiences at SUSEcon 2013, when I attempted to help a colleague backport a bugfix in OpenStack Nova from the master branch to a stable release branch. At first sight it looked like it would only require a trivial git cherry-pick, but that immediately revealed conflicts due to related code having changed in master since the release was made. I manually found the underlying commit which the bugfix required by using git blame, and tried another cherry-pick. The same thing happened again. Very soon I found myself in a quagmire of dependencies between commits, with no idea whether the end was in sight.
In coffee breaks during the ensuing openSUSE conference at the same venue, I feverishly hacked together a prototype and it seemed to work. Then normal life intervened, and no progress was made for another year.
Thanks to SUSE’s generous Hack Week policy, I had the luxury of being able to spending some of early January 2015 working to bring this tool to the next level. I submitted a Hack Week project page and announced my intentions on the ``git` mailing list <http://article.gmane.org/gmane.comp.version-control.git/262000>`__.
Again in May 2018 I took advantage of another Hack Week to package git-deps properly as a Python module in order to improve the installation process. This was in preparation for demonstrating the software at a Meetup event of the Git London User Group.
License
Released under GPL version 2 in order to be consistent with `git’s license <https://github.com/git/git/blob/master/COPYING>`__, but I’m open to the idea of dual-licensing if there’s a convincing reason.
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