Add Publisher::get_subscription_count

Author: romainreignier

rclrs/src/publisher.rs

@@ -146,6 +146,20 @@ where
         }
     }
 
+    /// Returns the number of subscriptions of the publisher.
+    pub fn get_subscription_count(&self) -> usize {
+        let mut subscription_count = 0;
+        // SAFETY: No preconditions for the function called.
+        let ret = unsafe {
+            rcl_publisher_get_subscription_count(
+                &*self.handle.rcl_publisher.lock().unwrap(),
+                &mut subscription_count,
+            )
+        };
+        debug_assert_eq!(ret, 0);
+        subscription_count
+    }
+
     /// Publishes a message.
     ///
     /// The [`MessageCow`] trait is implemented by any