Ensure RoutingFuunction can't route to itself

Author: olegz

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/config/RoutingFunction.java

@@ -193,6 +193,12 @@ else if (StringUtils.hasText(functionProperties.getRoutingExpression())) {
 			}
 		}
 
+		if (function.getTarget().equals(this)) {
+			throw new IllegalStateException("Failed to establish route, and routing to itself is not allowed as it creates a loop. Please provide: "
+					+ "'spring.cloud.function.definition' as Message header or as application property or "
+					+ "'spring.cloud.function.routing-expression' as application property.");
+		}
+
 		return function.apply(input);
 	}