Welcome to the 29th 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 June 2017.
Last month Brandon Williams sent a long patch series about a big internal refactoring of the Git code called “repository object”.
This work was based on an RFC patch series he had sent in May that started with:
When I first started working on the git project I found it very difficult to understand parts of the code base because of the inherently global nature of our code. It also made working on submodules very difficult. Since we can only open up a single repository per process, you need to launch a child process in order to process a submodule. But you also need to be able to communicate other stateful information to the children processes so that the submodules know how best to format their output or match against a pathspec … it ends up feeling like layering on hack after hack. What I would really like to do, is to have the ability to have a repository object so that I can open a submodule in-process.
Brandon had worked previously on opening a submodule in-process in
git ls-files which is “a pretty simple command and doesn’t have
too many dependencies”, so he based his RFC patch series on this
He finished the cover letter of his RFC patch series with the following:
This is still very much in a WIP state, though it does pass all tests. What I’m hoping for here is to get a discussion started about the feasibility of a change like this and hopefully to get the ball rolling. Is this a direction we want to move in? Is it worth the pain?
Thanks for taking the time to look at this and entertain my insane ideas :)
Many Git developers, like Jeff Hostetler, Ben Peart, Stefan Beller, Jacob Keller, Brian Carlson, Johannes Schindelin (alias Dscho), Jeff King (alias Peff), Duy Nguyen, Ævar Arnfjörð Bjarmason and Junio Hamano (the Git maintainer) commented and agreed that the patch series would improve things at least from a code readability point of view.
There was even hope expressed that it could in the end yield the following benefits:
Following the above reaction from his RFC patch series, Brandon started the improved non RFC patch series with:
Given the vast interest expressed when I sent out my RFC series I decided it would be worth it to invest more time to making a repository object a reality.
The big difference from the RFC series is that I went through and did the work to migrate key repository state from global variables in ‘environment.c’ to being stored in a repository object itself.
though there was also:
I do think that we need to be slightly cautious about moving global state into the repository object though, I don’t want ‘struct repo’ to simply become a kitchen sink where everything gets dumped. But this is just a warning for the future.
To the above Stefan Beller replied:
Or in other words: You want to have another struct e.g. ‘the_command_line_arguments’, which would carry the verbosity/color options for example as they are not related to a repo object, but to the current command being run?
Yes exactly. Library code that needs to operate on a repository would then be able to take arguments like:
some_library_function(struct repo *repo, struct lib_opts *ops)
Much like how the grep machinery takes a grep_opts struct.
Dscho and Peff also participated in reviewing the series.
Brandon then followed up with a version 2 of the series which generated many discussions.
Among them one was started by Jonathan Tan replying to the cover letter of the series:
I am concerned that “struct repository” will end up growing without bounds as we store more and more repo-specific concerns in it. Could it be restricted to just the fields populated by repo_init()? repo_read_index() will then return the index itself, instead of using “struct repository” as a cache. This means that code using repo_read_index() will need to maintain its own variable holding the returned index, but that is likely a positive - it’s better for code to just pass around the specific thing needed between functions anyway, as opposed to passing a giant “struct repository” (which partially defeats the purpose of eliminating the usage of globals).
This was followed up by Peff who used the following example:
git config core.quotepath true git -C submodule config core.quotepath false git ls-files --recurse-submodules
to say that “the repository object has to become a kitchen sink of
sorts”, because in a single process we’d need one copy of the
quote_path_fully variable (associated with the
option) for each repo object.
Brandon later replied to Peff saying that the config should be removed from the repository object:
what should happen is you pass a pair of objects to the ls-files machinery (or any other command’s machinery) (1) the repository object being operated on and (2) an options struct which can be configured based on the repository. So when recursing you can do something like the following:
repo_init(submodule, path_to_submodule); configure_opts(sub_opts, submodule, super_opts); ls_files(submodule, sub_opts);
To Stefan who suggested the same thing in another subthread of the discussion, Peff replied:
Right, I think you could have a separate kitchen sink struct that isn’t the “repo” one. But now you have to pass both of those around, which is going to get cumbersome. Almost every function is going to end up passing around the context struct.
The v2 series also prompted Ævar to suggest a new macro to both
a pointer variable and set it to
NULL, which generated a subthread.
These versions were reviewed by Jonathan Nieder, Jonathan Tan, Junio, Stefan and Jeff Hostetler, but the comments were small and technical, so version 4 got merged into master.
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This edition of Git Rev News was curated by Christian Couder <email@example.com>, Thomas Ferris Nicolaisen <firstname.lastname@example.org>, Jakub Narębski <email@example.com> and Markus Jansen <firstname.lastname@example.org> with help from Johannes Schindelin.