[test] add comprehensive tests for select_backend ensuring proper backend priority and error guidance

src/backend.rs

@@ -443,3 +443,98 @@ pub(crate) fn transcribe_with_whisper_rs(
         "Transcription requires the 'whisper' feature. Rebuild with --features whisper (and optional gpu-cuda/gpu-hip)."
     ))
 }
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::env as std_env;
+    use std::sync::{Mutex, OnceLock};
+
+    // Serialize environment variable modifications across tests in this module
+    static ENV_LOCK: OnceLock<Mutex<()>> = OnceLock::new();
+
+    #[test]
+    fn test_select_backend_auto_prefers_cuda_then_hip_then_vulkan_then_cpu() {
+        let _guard = ENV_LOCK.get_or_init(|| Mutex::new(())).lock().unwrap();
+        // Clear overrides
+        unsafe {
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_CUDA");
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_HIP");
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_VULKAN");
+        }
+        // No GPU -> CPU
+        let sel = select_backend(BackendKind::Auto, &crate::Config::default()).unwrap();
+        assert_eq!(sel.chosen, BackendKind::Cpu);
+
+        // Vulkan only -> Vulkan
+        unsafe { std_env::set_var("POLYSCRIBE_TEST_FORCE_VULKAN", "1"); }
+        let sel = select_backend(BackendKind::Auto, &crate::Config::default()).unwrap();
+        assert_eq!(sel.chosen, BackendKind::Vulkan);
+
+        // HIP only -> HIP (and preferred over Vulkan)
+        unsafe {
+            std_env::set_var("POLYSCRIBE_TEST_FORCE_HIP", "1");
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_VULKAN");
+        }
+        let sel = select_backend(BackendKind::Auto, &crate::Config::default()).unwrap();
+        assert_eq!(sel.chosen, BackendKind::Hip);
+
+        // CUDA only -> CUDA (and preferred over HIP)
+        unsafe { std_env::set_var("POLYSCRIBE_TEST_FORCE_CUDA", "1"); }
+        let sel = select_backend(BackendKind::Auto, &crate::Config::default()).unwrap();
+        assert_eq!(sel.chosen, BackendKind::Cuda);
+
+        // Cleanup
+        unsafe {
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_CUDA");
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_HIP");
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_VULKAN");
+        }
+    }
+
+    #[test]
+    fn test_select_backend_explicit_unavailable_errors_with_guidance() {
+        let _guard = ENV_LOCK.get_or_init(|| Mutex::new(())).lock().unwrap();
+        // Ensure all off
+        unsafe {
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_CUDA");
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_HIP");
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_VULKAN");
+        }
+        // CUDA requested but unavailable -> error with guidance
+        let err = select_backend(BackendKind::Cuda, &crate::Config::default()).err().expect("expected error");
+        let msg = format!("{}", err);
+        assert!(msg.contains("Requested CUDA backend"), "unexpected msg: {msg}");
+        assert!(msg.contains("How to fix"), "expected guidance text in: {msg}");
+
+        // HIP requested but unavailable -> error with guidance
+        let err = select_backend(BackendKind::Hip, &crate::Config::default()).err().expect("expected error");
+        let msg = format!("{}", err);
+        assert!(msg.contains("ROCm/HIP"), "unexpected msg: {msg}");
+        assert!(msg.contains("How to fix"), "expected guidance text in: {msg}");
+
+        // Vulkan requested but unavailable -> error with guidance
+        let err = select_backend(BackendKind::Vulkan, &crate::Config::default()).err().expect("expected error");
+        let msg = format!("{}", err);
+        assert!(msg.contains("Vulkan"), "unexpected msg: {msg}");
+        assert!(msg.contains("How to fix"), "expected guidance text in: {msg}");
+
+        // Now verify success when explicitly available via overrides
+        unsafe { std_env::set_var("POLYSCRIBE_TEST_FORCE_CUDA", "1"); }
+        assert!(select_backend(BackendKind::Cuda, &crate::Config::default()).is_ok());
+        unsafe {
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_CUDA");
+            std_env::set_var("POLYSCRIBE_TEST_FORCE_HIP", "1");
+        }
+        assert!(select_backend(BackendKind::Hip, &crate::Config::default()).is_ok());
+        unsafe {
+            std_env::remove_var("POLYSCRIBE_TEST_FORCE_HIP");
+            std_env::set_var("POLYSCRIBE_TEST_FORCE_VULKAN", "1");
+        }
+        assert!(select_backend(BackendKind::Vulkan, &crate::Config::default()).is_ok());
+
+        // Cleanup
+        unsafe { std_env::remove_var("POLYSCRIBE_TEST_FORCE_VULKAN"); }
+    }
+}