comments and add warn for error

kernel/src/engine/default/json.rs

@@ -13,6 +13,7 @@ use futures::stream::{self, BoxStream};
 use futures::{StreamExt, TryStreamExt};
 use object_store::path::Path;
 use object_store::{DynObjectStore, GetResultPayload};
+use tracing::warn;
 use url::Url;
 
 use super::executor::TaskExecutor;
@@ -91,18 +92,22 @@ impl<E: TaskExecutor> JsonHandler for DefaultJsonHandler<E> {
         let readahead = self.readahead;
 
         self.task_executor.spawn(async move {
+            // an iterator of futures that open each file
             let file_futures = files.into_iter().map(|file| file_opener.open(file, None));
 
+            // create a stream from that iterator which buffers up to `readahead` futures at a time
             let mut stream = stream::iter(file_futures)
                 .buffered(readahead)
                 .try_flatten()
                 .map_ok(|record_batch| {
                     Box::new(ArrowEngineData::new(record_batch)) as Box<dyn EngineData>
                 });
 
+            // send each record batch over the channel
             while let Some(item) = stream.next().await {
-                // check err?
-                let _ = tx.send(item);
+                if tx.send(item).is_err() {
+                    warn!("read_json receiver end of channel dropped before sending completed");
+                }
             }
         });