Re: [PATCH 0/3] Makefile update libgit.a: Include xdiff and reftable in libgit.a

From: Ezekiel Newren <ezekielnewren@gmail.com>
Date: Oct 2, 2025, 19:17 UTC
Message-ID: <CAH=ZcbCJjuKtVGmFv0xHTHt8XaAVPwh-Cjyba0jTaQMH7Dkh2Q@mail.gmail.com>
In-Reply-To: <xmqqecrmyz9j.fsf@gitster.g>

On Wed, Oct 1, 2025 at 5:32 PM Junio C Hamano <gitster@pobox.com> wrote:

Show 22 quoted lines

>
> "Ezekiel Newren via GitGitGadget" <gitgitgadget@gmail.com> writes:
>
> > The Rust compiler only needs to know how to link against libgit.a in 2 cases
> > that I can think of:
> >
> >  * Rust unit tests
> >  * Rust defining the main function
> >
> > Otherwise Rust can be compiled without linking, and then Makefile and Meson
> > can use Cargo's produced static lib files to build Git.
>
> It is a bit unclear why two (or three) are so much more hassle than
> one, but OK.  Allowing both build systems to agree on the same set
> of artifacts is very much desirable, and if meson based build rolls
> everything into a single library archive, the the other one should
> do the same.
>
> Of course we could run "ar" ourselves and combine the three into a
> single library archive, but as an approach, what you have here is a
> perfectly fine, and more preferable, way to achieve the goal of
> ending up with a single archive file.

In order for Cargo to tell the Rust compiler how to link against the C
archives they need to be specified in the build.rs file for each
crate. That would look something like this:
tree gitcore/
gitcore/
├── build.rs
├── Cargo.toml
└── src
    ├── do_that.rs
    ├── do_this.rs
    └── lib.rs

Where gitcore/build.rs would look something like this:
fn main() {
    ...
    println!("cargo:rustc-link-search=native={}", git_build_dir);
    println!("cargo:rustc-link-lib=static=git");
    if let Some(built_with_makefile) = std::env::var("BUILT_WITH_MAKEFILE") {
        println!("cargo:rustc-link-search=native={}",
git_build_dir.join("xdiff"));
        println!("cargo:rustc-link-lib=static=xdiff");
        println!("cargo:rustc-link-search=native={}",
git_build_dir.join("reftable"));
        println!("cargo:rustc-link-lib=static=reftable");
    }
}

But `cargo build` is invoked by a shell script by both Makefile and Meson, and build.rs would need to have an environment variable set. Something like BUILT_WITH_MAKEFILE for only when build_rust.sh is invoked by makefile and then you'd have to remember to update both Makefile and every build.rs if the library files produced by Makefile are ever changed. It seems a lot cleaner to hardcode just libgit.a and leave it up to the build systems to ensure that everything inside that static library contains everything that is needed.

Show 5 quoted lines

> This topic, however, especially its first step, had caused rather
> unpleasant textual conflicts when merged to 'seen' (I didn't check
> which other topic was the most heavily conflicting, though).  I may
> attempt to get a clean merge again tomorrow, but due to time
> pressure, tonight's 'seen' was done without these patches merged.

I wonder if Patrick's approach would cause fewer merge conflicts. If I add xdiff and reftable objects to LIB_OBJS directly then I don't need to bother with the move commit.