chore: add test for tolerating empty responses

crates/net/downloaders/src/bodies/bodies.rs

@@ -762,4 +762,30 @@ mod tests {
             header += resp.len();
         }
     }
+
+    // Check that the downloader can tolerate a few completely empty responses
+    #[tokio::test]
+    async fn can_tolerate_empty_responses() {
+        // Generate some random blocks
+        let db = create_test_rw_db();
+        let (headers, mut bodies) = generate_bodies(0..=99);
+
+        insert_headers(db.db(), &headers);
+
+        // respond with empty bodies for every other request.
+        let client = Arc::new(
+            TestBodiesClient::default().with_bodies(bodies.clone()).with_empty_responses(2),
+        );
+        let mut downloader = BodiesDownloaderBuilder::default()
+            .with_request_limit(3)
+            .with_stream_batch_size(100)
+            .build(client.clone(), Arc::new(TestConsensus::default()), db);
+
+        // Download the requested range
+        downloader.set_download_range(0..=99).expect("failed to set download range");
+        assert_matches!(
+            downloader.next().await,
+            Some(Ok(res)) => assert_eq!(res, zip_blocks(headers.iter().take(100), &mut bodies))
+        );
+    }
 }

crates/net/downloaders/src/test_utils/bodies_client.rs

@@ -22,6 +22,7 @@ pub struct TestBodiesClient {
     should_delay: bool,
     max_batch_size: Option<usize>,
     times_requested: AtomicU64,
+    empty_response_mod: Option<u64>,
 }
 
 impl TestBodiesClient {
@@ -35,6 +36,13 @@ impl TestBodiesClient {
         self
     }
 
+    /// Instructs the client to respond with empty responses some portion of the time. Every
+    /// `empty_mod` responses, the client will respond with an empty response.
+    pub(crate) fn with_empty_responses(mut self, empty_mod: u64) -> Self {
+        self.empty_response_mod = Some(empty_mod);
+        self
+    }
+
     pub(crate) fn with_max_batch_size(mut self, max_batch_size: usize) -> Self {
         self.max_batch_size = Some(max_batch_size);
         self
@@ -43,6 +51,18 @@ impl TestBodiesClient {
     pub(crate) fn times_requested(&self) -> u64 {
         self.times_requested.load(Ordering::Relaxed)
     }
+
+    /// Returns whether or not the client should respond with an empty response.
+    ///
+    /// This will only return true if `empty_response_mod` is `Some`, and `times_requested %
+    /// empty_response_mod == 0`.
+    pub(crate) fn should_respond_empty(&self) -> bool {
+        if let Some(empty_response_mod) = self.empty_response_mod {
+            self.times_requested.load(Ordering::Relaxed) % empty_response_mod == 0
+        } else {
+            false
+        }
+    }
 }
 
 impl DownloadClient for TestBodiesClient {
@@ -68,8 +88,13 @@ impl BodiesClient for TestBodiesClient {
         let max_batch_size = self.max_batch_size;
 
         self.times_requested.fetch_add(1, Ordering::Relaxed);
+        let should_respond_empty = self.should_respond_empty();
 
         Box::pin(async move {
+            if should_respond_empty {
+                return Ok((PeerId::default(), vec![]).into());
+            }
+
             if should_delay {
                 tokio::time::sleep(Duration::from_millis((hashes[0][0] % 100) as u64)).await;
             }