Fix flight compile error

ballista/rust/core/src/client.rs

@@ -17,7 +17,7 @@
 
 //! Client API for sending requests to executors.
 
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 use std::{collections::HashMap, pin::Pin};
 use std::{
     convert::{TryFrom, TryInto},
@@ -135,24 +135,28 @@ impl BallistaClient {
 }
 
 struct FlightDataStream {
-    stream: Streaming<FlightData>,
+    stream: Mutex<Streaming<FlightData>>,
     schema: SchemaRef,
 }
 
 impl FlightDataStream {
     pub fn new(stream: Streaming<FlightData>, schema: SchemaRef) -> Self {
-        Self { stream, schema }
+        Self {
+            stream: Mutex::new(stream),
+            schema,
+        }
     }
 }
 
 impl Stream for FlightDataStream {
     type Item = ArrowResult<RecordBatch>;
 
     fn poll_next(
-        mut self: std::pin::Pin<&mut Self>,
+        self: std::pin::Pin<&mut Self>,
         cx: &mut Context<'_>,
     ) -> Poll<Option<Self::Item>> {
-        self.stream.poll_next_unpin(cx).map(|x| match x {
+        let mut stream = self.stream.lock().expect("mutex is bad");
+        stream.poll_next_unpin(cx).map(|x| match x {
             Some(flight_data_chunk_result) => {
                 let converted_chunk = flight_data_chunk_result
                     .map_err(|e| ArrowError::from_external_error(Box::new(e)))