commit cbdf5400a175139d2ddf1b695578d5baea7281e3
author UebelAndre <github@uebelandre.com> Tue Sep 10 09:07:15 2024 -0700
committer GitHub <noreply@github.com> Tue Sep 10 16:07:15 2024 +0000
tree 923778212b32b1be3d71b02b18db85d462637e59
parent 05d54a46b1c7d58ec352c7881fb13b8935931b4e

Avoid double building `cargo_build_script.data` targets (#2826)

Before this change targets passed to `cargo_build_script.data` would get
built multiple times (once per configuration)
```
% bazel cquery 'filter(//test/cargo_build_script/location_expansion:target_data.txt, deps(//test/cargo_build_script/location_expansion:build_rs))'
//test/cargo_build_script/location_expansion:target_data.txt (be33641)
//test/cargo_build_script/location_expansion:target_data.txt (a1c326f)
```
We can see that this is the exec (`a1c326f`) and target (`be33641`)
configurations.
```
% bazel config a1c326f be33641
INFO: Displaying diff between configs a1c326f and be33641
Displaying diff between configs a1c326f and be33641
FragmentOptions com.google.devtools.build.lib.analysis.PlatformOptions {
  platforms: [@@internal_platforms_do_not_use//host:host], [@@bazel_tools//tools:host_platform]
}
FragmentOptions com.google.devtools.build.lib.analysis.config.CoreOptions {
  compilation_mode: opt, fastbuild
  is exec configuration: true, false
  platform_suffix: exec, null
}
FragmentOptions com.google.devtools.build.lib.rules.android.AndroidConfiguration$Options {
  fat_apk_cpu: [], [armeabi-v7a]
}
FragmentOptions com.google.devtools.build.lib.rules.cpp.CppOptions {
  copt: [-g0], []
  cxxopt: [-g0], []
  strip: always, sometimes
}
FragmentOptions com.google.devtools.build.lib.rules.java.JavaOptions {
  java_runtime_version: remotejdk_11, local_jdk
  jvmopt: [-XX:ErrorFile=/dev/stderr], []
}
```

This is caused by `data` being passed to the underlying rust binary in
`cargo_build_script_wrapper.bzl`.

The fix for this ends up requiring some gymnastics as many existing uses
of `cargo_build_script` rely on `data` files being available within the
runfiles of `cargo_build_script.script`. This change accounts for that
by creating a wrapper rule for the build script that symlinks the
`cfg=exec` Rust binary (which is the actual [Cargo build
script](https://doc.rust-lang.org/cargo/reference/build-scripts.html))
and updating the `cargo_build_script` rule to consume it as
`cfg=target`. Within the `cargo_build_script` rule this means the script
target and it's `data` attribute files will be in `cfg=target` but the
binary itself will have been built for `cfg=exec` and will be runnable
on the exec platform.

We see after this change that data targets only appear once now
```
% bazel cquery 'filter(//test/cargo_build_script/location_expansion:target_data.txt, deps(//test/cargo_build_script/location_expansion:build_rs))'
//test/cargo_build_script/location_expansion:target_data.txt (be33641)
```