[Bugfix][Rust Frontend] Fix macOS build: read inherited listener family from its address (#62177)

Signed-off-by: khluu <khluu000@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

rust/src/engine-core-client/src/transport.rs

@@ -405,7 +405,9 @@ fn adopt_listener(fd: RawFd) -> Result<zeromq::Listener> {
     // transferring ownership to the listener constructed below.
     let fd = unsafe { OwnedFd::from_raw_fd(fd) };
     let socket: socket2::Socket = fd.into();
-    let domain = socket.domain()?;
+    // `Socket::domain` (SO_DOMAIN) exists only on Linux, Android, FreeBSD and
+    // Fuchsia; the bound address carries the family on every platform.
+    let domain = socket.local_addr()?.domain();
 
     if domain == socket2::Domain::IPV4 || domain == socket2::Domain::IPV6 {
         let listener: std::net::TcpListener = socket.into();
@@ -672,4 +674,29 @@ mod tests {
         assert_eq!(endpoint.to_string(), format!("tcp://{expected}"));
         assert!(router.close().await.is_empty());
     }
+
+    #[tokio::test]
+    async fn inherited_unix_listener_is_adopted_without_rebinding() {
+        let path = std::env::temp_dir().join(format!(
+            "vllm-inherited-listener-{}-{}.sock",
+            std::process::id(),
+            std::time::SystemTime::now()
+                .duration_since(std::time::UNIX_EPOCH)
+                .expect("clock after epoch")
+                .as_nanos()
+        ));
+        let listener = std::os::unix::net::UnixListener::bind(&path).expect("bind listener");
+        let mut router = RouterSocket::new();
+
+        let endpoint = router
+            .bind_listener(
+                adopt_listener(listener.into_raw_fd()).expect("adopt inherited listener fd"),
+            )
+            .await
+            .expect("bind inherited listener");
+
+        assert!(endpoint.to_string().starts_with("ipc://"), "{endpoint}");
+        assert!(router.close().await.is_empty());
+        let _ = std::fs::remove_file(&path);
+    }
 }