From: Ezekiel Newren <ezekielnewren@gmail.com>
Date: Tue, 28 Oct 2025 17:42:21 GMT
Subject: Re: [PATCH 10/14] rust: add a build.rs script for tests
Message-ID: <CAH=ZcbCAzxAcxdcriLz7cafeuy0ZOMp+=05b++gdX-Vx8kbuGw@mail.gmail.com>
In-Reply-To: <aQCKWtgcYQVPzIPz@pks.im>

On Tue, Oct 28, 2025 at 3:18 AM Patrick Steinhardt <ps@pks.im> wrote:
>
> On Mon, Oct 27, 2025 at 12:44:00AM +0000, brian m. carlson wrote:
> > diff --git a/Makefile b/Makefile
> > index 7e5a735ca6..7c36302717 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -2948,7 +2948,7 @@ scalar$X: scalar.o GIT-LDFLAGS $(GITLIBS)
> >  $(LIB_FILE): $(LIB_OBJS)
> >       $(QUIET_AR)$(RM) $@ && $(AR) $(ARFLAGS) $@ $^
> >
> > -$(RUST_LIB): Cargo.toml $(RUST_SOURCES)
> > +$(RUST_LIB): Cargo.toml $(RUST_SOURCES) $(XDIFF_LIB) $(LIB_FILE) $(REFTABLE_LIB)
> >       $(QUIET_CARGO)cargo build $(CARGO_ARGS)
>
> We have recently removed the separare xdiff and reftable libraries, so
> it shouldn't be necessary to have these anymore.

Patrick is referring to my Makefile update libgit.a patch series that
has been merged into master [1].

> But one thing I'm curious about: don't we have a circular dependency
> between the Rust and C library now? I guess that's somewhat expected, as
> we'll want to call Rust from C and vice versa. But on the Meson side I
> think we need to adjust our logic so that we don't pull the Rust library
> into libgit.a to break this cycle.
>
> > diff --git a/build.rs b/build.rs
> > new file mode 100644
> > index 0000000000..136d58c35a
> > --- /dev/null
> > +++ b/build.rs
> > @@ -0,0 +1,21 @@
> > +// This program is free software; you can redistribute it and/or modify
> > +// it under the terms of the GNU General Public License as published by
> > +// the Free Software Foundation: version 2 of the License, dated June 1991.
> > +//
> > +// This program is distributed in the hope that it will be useful,
> > +// but WITHOUT ANY WARRANTY; without even the implied warranty of
> > +// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > +// GNU General Public License for more details.
> > +//
> > +// You should have received a copy of the GNU General Public License along
> > +// with this program; if not, see <https://www.gnu.org/licenses/>.
> > +
> > +fn main() {
> > +    println!("cargo::rustc-link-search=.");
> > +    println!("cargo::rustc-link-search=reftable");
> > +    println!("cargo::rustc-link-search=xdiff");
> > +    println!("cargo::rustc-link-lib=git");
> > +    println!("cargo::rustc-link-lib=reftable");
> > +    println!("cargo::rustc-link-lib=z");
> > +    println!("cargo::rustc-link-lib=xdiff");
> > +}
>
> How do we ensure that the correct libraries are linked here? E.g. for
> libz, if there are multiple such libraries, which one gets precedence?

I solved this problem in my own Introduce Rust series [2,3]. When
Makefile or Meson is invoking Cargo it sets the environment variable
`USE_LINKING=false` and build.rs doesn't link against libgit.a or any
other library. When `cargo test` is called it will link against
libgit.a because if USE_LINKING is not set then it assumes true.

[1] Makefile update libgit.a
https://lore.kernel.org/git/pull.2065.v2.git.git.1759447647.gitgitgadget@gmail.com/
[2] Ezekiel's Introduce Rust
https://lore.kernel.org/git/6032a8740c0ba72420f42c3d8d801e1bdeec12d0.1758071798.git.gitgitgadget@gmail.com/
[3] Ezekiel's Introduce Rust
https://lore.kernel.org/git/6a27e07e6310b6cad0e3feae817269b9b8eaed69.1758071798.git.gitgitgadget@gmail.com/