uplink-sys(build): Fix cross-compilation

This fixes cross-compilation by setting the Go `GOOS` and `GOARCH` flags
for `uplink-c`.

.github/workflows/uplink-sys-cross-aarch64-linux.yml

@@ -0,0 +1,40 @@
+name: Cross-compile uplink-sys to aarch64 Linux
+
+on:
+  push:
+    branches: ["main"]
+    paths:
+      - 'uplink-sys/**'
+      - 'Cargo.lock'
+      - 'Cargo.toml'
+      - 'docker-compose.yaml'
+      - 'Makefile'
+  pull_request:
+    branches: ["main"]
+    paths:
+      - 'uplink-sys/**'
+      - 'Cargo.lock'
+      - 'Cargo.toml'
+      - 'docker-compose.yaml'
+      - 'Makefile'
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      CARGO_BUILD_TARGET: aarch64-unknown-linux-gnu
+      BINDGEN_EXTRA_CLANG_ARGS: "--sysroot=/usr/aarch64-linux-gnu"
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up `aarch64-unknown-linux-gnu` Rust target
+        run: rustup target add aarch64-unknown-linux-gnu
+
+      - name: Install dependencies
+        run: sudo apt-get update && sudo apt-get install -y --no-install-recommends clang libclang-dev libc6-dev-arm64-cross && sudo snap install zig --classic --beta
+
+      - name: Build
+        run: make -C uplink-sys build

.github/workflows/uplink-sys.yml

@@ -2,7 +2,7 @@ name: uplink-sys
 
 on:
   push:
-    branches: main
+    branches: ["main"]
     paths:
       - 'uplink-sys/**'
       - 'Cargo.lock'

uplink-sys/build.rs

@@ -4,7 +4,54 @@ use std::env;
 use std::path::PathBuf;
 use std::process::Command;
 
+/// Converts Rust's architecture name to Go's architecture name.
+fn rust_arch_to_go_arch(rust_arch: &str) -> &str {
+    match rust_arch {
+        "x86" => "386",
+        "x86_64" => "amd64",
+        "arm" => "arm",
+        "aarch64" => "arm64",
+        "riscv32" => "riscv",
+        "riscv64" => "riscv64",
+        "loongarch64" => "loong64",
+        "mips" => "mips",
+        "mipsel" => "mipsle",
+        "mips64" => "mips64",
+        "mips64el" => "mips64le",
+        "powerpc" => "ppc",
+        "powerpc64" => "ppc64",
+        "wasm64" => "wasm",
+        _ => panic!("Unsupported architecture: {rust_arch}"),
+    }
+}
+
+fn rust_os_to_go_os<'a>(rust_arch: &str, rust_os: &'a str) -> &'a str {
+    if rust_arch.starts_with("wasm") {
+        return "js";
+    }
+
+    match rust_os {
+        "macos" => "darwin",
+        _ => rust_os,
+    }
+}
+
 fn main() {
+    // Set the environment variables for `go` compilation, so that cross-compilation works.
+    let host_triple = env::var("HOST").expect("HOST not defined");
+    let target_triple = env::var("TARGET").expect("TARGET not defined");
+    let target_os = env::var("CARGO_CFG_TARGET_OS").expect("CARGO_CFG_TARGET_OS not defined");
+    let target_arch = env::var("CARGO_CFG_TARGET_ARCH").expect("CARGO_CFG_TARGET_ARCH not defined");
+    let go_arch = rust_arch_to_go_arch(&target_arch);
+    let go_os = rust_os_to_go_os(&target_arch, &target_os);
+
+    // If cross-compiling, suggest to the user that they will need to set their sysroot properly.
+    if target_triple != host_triple && !env::var("BINDGEN_EXTRA_CLANG_ARGS")
+            .unwrap_or_default()
+            .contains("sysroot") {
+        panic!("Environment variable `BINDGEN_EXTRA_CLANG_ARGS` should contain `--sysroot=/path/to/target/sysroot` when cross-compiling");
+    }
+
     let out_dir = PathBuf::from(env::var("OUT_DIR").expect("OUT_DIR not defined"));
 
     // Directory containing uplink-c project source
@@ -22,6 +69,8 @@ fn main() {
         // parent tree directory and with the same depth.
         Command::new("make")
             .arg("build")
+            .env("GOOS", go_os)
+            .env("GOARCH", go_arch)
             .current_dir(&uplink_c_src)
             .status()
             .expect("Failed to run make command from build.rs.");
@@ -83,7 +132,7 @@ fn main() {
     //
     // N.B.: `CARGO_CFG_TARGET_OS` should be read instead of `cfg(target_os = "macos")`. The latter
     // detects the host OS that is building the `build.rs` script, not the target OS.
-    if env::var("CARGO_CFG_TARGET_OS").expect("CARGO_CFG_TARGET_OS is not defined") == "macos" {
+    if target_os == "macos" {
         println!("cargo:rustc-flags=-l framework=CoreFoundation -l framework=Security");
     }