MINOR: Some optimizations

cpp/src/arrow/array/array_base.h

@@ -160,7 +160,7 @@ class ARROW_EXPORT Array {
   /// Input-checking variant of Array::Slice
   Result<std::shared_ptr<Array>> SliceSafe(int64_t offset) const;
 
-  std::shared_ptr<ArrayData> data() const { return data_; }
+  const std::shared_ptr<ArrayData>& data() const { return data_; }
 
   int num_fields() const { return static_cast<int>(data_->child_data.size()); }
 

java/flight/flight-core/src/main/java/org/apache/arrow/flight/grpc/AddWritableBuffer.java

@@ -17,6 +17,7 @@
 
 package org.apache.arrow.flight.grpc;
 
+import java.io.IOException;
 import java.io.OutputStream;
 import java.lang.reflect.Constructor;
 import java.lang.reflect.Field;
@@ -87,8 +88,11 @@ public class AddWritableBuffer {
    * @param buf The buffer to add.
    * @param stream The Candidate OutputStream to add to.
    * @return True if added. False if not possible.
+   * @throws IOException on error
    */
-  public static boolean add(ByteBuf buf, OutputStream stream) {
+  public static boolean add(ByteBuf buf, OutputStream stream) throws IOException {
+    buf.readBytes(stream, buf.readableBytes());
+
     if (bufChainOut == null) {
       return false;
     }

java/flight/flight-core/src/test/java/org/apache/arrow/flight/perf/PerformanceTestServer.java

@@ -68,7 +68,7 @@ public void register(FlightProducer.ServerStreamListener listener) {
 
       @Override
       public WaitResult waitForListener(long timeout) {
-        while (!listener.isReady() && !listener.isCancelled()) {
+        while (!listener.isReady()) {
           // busy wait
         }
         return WaitResult.READY;