Enhance rpath handling in fdb build scripts

Author: Choochmeque

rust/crates/fdb-sys/build.rs

@@ -67,13 +67,25 @@ fn build_system() {
         .expect("DEP_ECKIT_ROOT not set - eckit-sys must be a dependency");
     let metkit_root = env::var("DEP_METKIT_ROOT")
         .expect("DEP_METKIT_ROOT not set - metkit-sys must be a dependency");
+    let eccodes_root = env::var("DEP_ECCODES_ROOT")
+        .expect("DEP_ECCODES_ROOT not set - eccodes-sys must be a dependency");
 
     println!("cargo:rustc-link-search=native={eckit_root}/lib");
     println!("cargo:rustc-link-lib=dylib=eckit");
     println!("cargo:rustc-link-search=native={metkit_root}/lib");
     println!("cargo:rustc-link-lib=dylib=metkit");
     bindman_utils::link_cpp_stdlib();
 
+    // Re-publish each dependency's install lib dir so the downstream
+    // `fdb` crate's build script can emit matching absolute rpath
+    // entries on the final binary. `rustc-link-arg` emitted by a
+    // library crate's build.rs does not reach binaries that link the
+    // crate, so the rpath flags have to come from `fdb/build.rs`.
+    println!("cargo:system_fdb5_lib={}", lib_dir.display());
+    println!("cargo:system_eckit_lib={eckit_root}/lib");
+    println!("cargo:system_metkit_lib={metkit_root}/lib");
+    println!("cargo:system_eccodes_lib={eccodes_root}/lib");
+
     // Export for downstream crates
     println!("cargo:root={}", root.display());
     println!("cargo:include={}", fdb_include.display());

rust/crates/fdb/build.rs

@@ -2,8 +2,39 @@
 //!
 //! Emits RPATH linker flags so binaries can find dynamic libraries
 //! at runtime without setting `LD_LIBRARY_PATH`/`DYLD_LIBRARY_PATH`.
-
+//!
+//! Two layouts are supported:
+//!
+//! - **Vendored** (default): dynamic libs are copied into
+//!   `fdb_libs/` and `eccodes_libs/` subdirectories next to the
+//!   final binary. The rpath entries are binary-relative
+//!   (`@executable_path/fdb_libs` on macOS, `$ORIGIN/fdb_libs` on
+//!   Linux), so the binary is portable as long as the user ships
+//!   those two directories alongside it.
+//!
+//! - **System**: libraries live wherever `find_package` resolved
+//!   them (e.g. `/usr/lib`, `/opt/.../lib`, or a custom prefix).
+//!   `fdb-sys`'s build script re-publishes each dependency's lib dir
+//!   via `cargo:system_*_lib` metadata keys, and we emit an
+//!   absolute rpath entry for each one so the binary still loads
+//!   without `LD_LIBRARY_PATH` / `DYLD_LIBRARY_PATH`.
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     bindman_utils::emit_rpath_flags(&["fdb_libs", "eccodes_libs"]);
+
+    // When fdb-sys is in system mode, it re-publishes each
+    // dependency's install lib dir so we can stamp matching
+    // absolute rpath entries onto the final binary. The vendored
+    // build leaves these unset, so this block is a no-op there.
+    for key in [
+        "DEP_FDB_SYS_SYSTEM_FDB5_LIB",
+        "DEP_FDB_SYS_SYSTEM_ECKIT_LIB",
+        "DEP_FDB_SYS_SYSTEM_METKIT_LIB",
+        "DEP_FDB_SYS_SYSTEM_ECCODES_LIB",
+    ] {
+        println!("cargo:rerun-if-env-changed={key}");
+        if let Ok(lib_dir) = std::env::var(key) {
+            println!("cargo:rustc-link-arg=-Wl,-rpath,{lib_dir}");
+        }
+    }
 }