Fix threading example for sync.

examples/threading.rs

@@ -6,6 +6,12 @@ fn main() {
     // Channel: Master -> Script
     let (tx_master, rx_script) = std::sync::mpsc::channel();
 
+    #[cfg(feature = "sync")]
+    let (tx_script, rx_script) = (
+        std::sync::Mutex::new(tx_script),
+        std::sync::Mutex::new(rx_script),
+    );
+
     // Spawn thread with Engine
     std::thread::spawn(move || {
         // Create Engine
@@ -13,10 +19,19 @@ fn main() {
 
         // Register API
         // Notice that the API functions are blocking
+
+        #[cfg(not(feature = "sync"))]
         engine
             .register_fn("get", move || rx_script.recv().unwrap())
             .register_fn("put", move |v: INT| tx_script.send(v).unwrap());
 
+        #[cfg(feature = "sync")]
+        engine
+            .register_fn("get", move || rx_script.lock().unwrap().recv().unwrap())
+            .register_fn("put", move |v: INT| {
+                tx_script.lock().unwrap().send(v).unwrap()
+            });
+
         // Run script
         engine
             .consume(