Avoid extra byte array copying when downloading to memory with AsyncResponseTransformer (#6348)

* Avoid extra byte array copying when downloading to memory with AsyncResponseTransformer

* Add comments

Author: davidh44

.changes/next-release/feature-AWSSDKforJavav2-d4e19d2.json

@@ -0,0 +1,6 @@
+{
+    "type": "feature",
+    "category": "AWS SDK for Java v2",
+    "contributor": "",
+    "description": "Avoid extra byte array copying when downloading to memory with AsyncResponseTransformer"
+}

core/sdk-core/src/main/java/software/amazon/awssdk/core/internal/async/ByteArrayAsyncResponseTransformer.java

@@ -47,7 +47,9 @@ public final class ByteArrayAsyncResponseTransformer<ResponseT> implements
     @Override
     public CompletableFuture<ResponseBytes<ResponseT>> prepare() {
         cf = new CompletableFuture<>();
-        return cf.thenApply(arr -> ResponseBytes.fromByteArray(response, arr));
+        // Using fromByteArrayUnsafe() to avoid unnecessary extra copying of byte array. The data writing has completed and the
+        // byte array will not be further modified so this is safe
+        return cf.thenApply(arr -> ResponseBytes.fromByteArrayUnsafe(response, arr));
     }
 
     @Override