HADOOP-14204 S3A multipart commit failing,

"UnsupportedOperationException at java.util.Collections$UnmodifiableList.sort".

Contributed by Steve Loughran

(cherry picked from commit 595501c9dd)

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/S3AFileSystem.java

@@ -2403,11 +2403,13 @@ public class S3AFileSystem extends FileSystem {
           "No partitions have been uploaded");
       LOG.debug("Completing multipart upload {} with {} parts",
           uploadId, partETags.size());
+      // a copy of the list is required, so that the AWS SDK doesn't
+      // attempt to sort an unmodifiable list.
       return s3.completeMultipartUpload(
           new CompleteMultipartUploadRequest(bucket,
               key,
               uploadId,
-              partETags));
+              new ArrayList<>(partETags)));
     }
 
     /**