Check format of received messages in sink example

examples/rust-dataflow/sink/src/main.rs

@@ -1,5 +1,5 @@
 use dora_node_api::{self, DoraNode};
-use eyre::bail;
+use eyre::{bail, Context};
 use futures::StreamExt;
 use std::time::Duration;
 
@@ -19,7 +19,15 @@ async fn main() -> eyre::Result<()> {
 
         match input.id.as_str() {
             "message" => {
-                println!("received message: {}", String::from_utf8_lossy(&input.data));
+                let received_string = String::from_utf8(input.data)
+                    .wrap_err("received message was not utf8-encoded")?;
+                println!("received message: {}", received_string);
+                if !received_string.starts_with("operator received random value ") {
+                    bail!("unexpected message format (should start with 'operator received random value')")
+                }
+                if !received_string.ends_with(" ticks") {
+                    bail!("unexpected message format (should end with 'ticks')")
+                }
             }
             other => eprintln!("Ignoring unexpected input `{other}`"),
         }