fix message offset of opendal source

Signed-off-by: Richard Chien <stdrc@outlook.com>

src/connector/src/source/filesystem/opendal_source/opendal_reader.rs

@@ -182,10 +182,16 @@ impl<Src: OpendalSource> OpendalReader<Src> {
             // note that the buffer contains the newline character
             debug_assert_eq!(n_read, line_buf.len());
 
+            // FIXME(rc): Here we have to use `offset + n_read`, i.e. the offset of the next line,
+            // as the *message offset*, because we check whether a file is finished by comparing the
+            // message offset with the file size in `FsFetchExecutor::into_stream`. However, we must
+            // understand that this message offset is not semantically consistent with the offset of
+            // other source connectors.
+            let msg_offset = (offset + n_read).to_string();
             batch.push(SourceMessage {
                 key: None,
                 payload: Some(std::mem::take(&mut line_buf).into_bytes()),
-                offset: offset.to_string(),
+                offset: msg_offset,
                 split_id: split.id(),
                 meta: SourceMeta::Empty,
             });

src/stream/src/executor/source/fetch_executor.rs

@@ -386,6 +386,11 @@ impl<S: StateStore, Src: OpendalSource> FsFetchExecutor<S, Src> {
                                     }
                                     _ => unreachable!(),
                                 };
+                                // FIXME(rc): Here we compare `offset` with `fs_split.size` to determine
+                                // whether the file is finished, where the `offset` is the starting position
+                                // of the NEXT message line in the file. However, In other source connectors,
+                                // we use the word `offset` to represent the offset of the current message.
+                                // We have to be careful about this semantical inconsistency.
                                 if offset.parse::<usize>().unwrap() >= fs_split.size {
                                     splits_on_fetch -= 1;
                                     state_store_handler.delete(split_id).await?;