update test should not succeed by accident

nexus/tests/integration_tests/updates.rs

@@ -40,10 +40,19 @@ use tough::key_source::KeySource;
 use tough::schema::{KeyHolder, RoleKeys, RoleType, Root};
 use tough::sign::Sign;
 
+const UPDATE_IMAGE_PATH: &'static str = "/var/tmp/zones/cockroachdb";
+
 #[tokio::test]
 async fn test_update_end_to_end() {
     let mut config = load_test_config();
 
+    // If the output file already exists, record the mtime.
+    let mtime: Option<_> = match tokio::fs::metadata(UPDATE_IMAGE_PATH).await {
+        Ok(metadata) => Some(metadata.modified().unwrap()),
+        Err(e) if matches!(e.kind(), std::io::ErrorKind::NotFound) => None,
+        Err(e) => panic!("failed to stat {:?}: {:?}", UPDATE_IMAGE_PATH, e),
+    };
+
     // build the TUF repo
     let rng = SystemRandom::new();
     let tuf_repo = new_tuf_repo(&rng);
@@ -83,8 +92,14 @@ async fn test_update_end_to_end() {
     .unwrap();
 
     // check sled agent did the thing
+    if let Some(old_mtime) = mtime {
+        let new_metadata = tokio::fs::metadata(UPDATE_IMAGE_PATH)
+            .await
+            .expect("failed to stat file after it was previously found");
+        assert!(old_mtime < new_metadata.modified().unwrap());
+    }
     assert_eq!(
-        std::fs::read("/var/tmp/zones/cockroachdb").unwrap(),
+        tokio::fs::read(UPDATE_IMAGE_PATH).await.unwrap(),
         TARGET_CONTENTS
     );