feat(sys): Support explicit dynamic linking of onnxruntime (#295)

Previously, if static library files were found, the build script in ort-sys
would always prefer static linking.

This PR introduces a new env variable, ORT_PREFER_DYNAMIC_LINK, that can be
set (to 1 or "true") to use dynamic linking even if the static library files are
present in the onnxruntime path.

Additionally, since cuda and tensorrt require dynamic linking, static linking is
disabled when these features are present.

ort-sys/build.rs

@@ -9,6 +9,7 @@ const ONNXRUNTIME_VERSION: &str = "1.19.2";
 
 const ORT_ENV_SYSTEM_LIB_LOCATION: &str = "ORT_LIB_LOCATION";
 const ORT_ENV_SYSTEM_LIB_PROFILE: &str = "ORT_LIB_PROFILE";
+const ORT_ENV_PREFER_DYNAMIC_LINK: &str = "ORT_PREFER_DYNAMIC_LINK";
 #[cfg(feature = "download-binaries")]
 const ORT_EXTRACT_DIR: &str = "onnxruntime";
 
@@ -148,6 +149,18 @@ fn static_link_prerequisites(using_pyke_libs: bool) {
 	}
 }
 
+fn prefer_dynamic_linking() -> bool {
+	// If the cuda or tensorrt features are enabled, we need to use dynamic linking.
+	if cfg!(feature = "cuda") || cfg!(feature = "tensorrt") {
+		return true;
+	}
+
+	match env::var(ORT_ENV_PREFER_DYNAMIC_LINK) {
+		Ok(val) => val == "1" || val.to_lowercase() == "true",
+		Err(_) => false,
+	}
+}
+
 fn prepare_libort_dir() -> (PathBuf, bool) {
 	if let Ok(lib_dir) = env::var(ORT_ENV_SYSTEM_LIB_LOCATION) {
 		let lib_dir = PathBuf::from(lib_dir);
@@ -174,7 +187,7 @@ fn prepare_libort_dir() -> (PathBuf, bool) {
 		if lib_dir.join(platform_format_lib("onnxruntime")).exists() {
 			println!("cargo:rustc-link-lib=static=onnxruntime");
 			needs_link = false;
-		} else {
+		} else if !prefer_dynamic_linking() {
 			#[allow(clippy::type_complexity)]
 			let static_configs: Vec<(PathBuf, PathBuf, PathBuf, Box<dyn Fn(PathBuf, &String) -> PathBuf>)> = vec![
 				(lib_dir.join(&profile), lib_dir.join("lib"), lib_dir.join("_deps"), Box::new(|p: PathBuf, profile| p.join(profile))),
@@ -415,6 +428,7 @@ fn try_setup_with_pkg_config() -> bool {
 fn real_main(link: bool) {
 	println!("cargo:rerun-if-env-changed={}", ORT_ENV_SYSTEM_LIB_LOCATION);
 	println!("cargo:rerun-if-env-changed={}", ORT_ENV_SYSTEM_LIB_PROFILE);
+	println!("cargo:rerun-if-env-changed={}", ORT_ENV_PREFER_DYNAMIC_LINK);
 
 	let (install_dir, needs_link) = prepare_libort_dir();
 
@@ -426,7 +440,7 @@ fn real_main(link: bool) {
 			let static_lib_file_name = if target_os.contains("windows") { "onnxruntime.lib" } else { "libonnxruntime.a" };
 
 			let static_lib_path = lib_dir.join(static_lib_file_name);
-			if static_lib_path.exists() {
+			if !prefer_dynamic_linking() && static_lib_path.exists() {
 				println!("cargo:rustc-link-lib=static=onnxruntime");
 			} else {
 				println!("cargo:rustc-link-lib=onnxruntime");