spark git commit: [SPARK-21342] Fix DownloadCallback to work well with RetryingBlockFetcher.

Repository: spark Updated Branches: refs/heads/branch-2.2 3bfad9d42 -> 40fd0ce7f [SPARK-21342] Fix DownloadCallback to work well with RetryingBlockFetcher. When `RetryingBlockFetcher` retries fetching blocks. There could be two `DownloadCallback`s download the same content to the same target

spark git commit: [SPARK-21342] Fix DownloadCallback to work well with RetryingBlockFetcher.

Repository: spark Updated Branches: refs/heads/master 647963a26 -> 6a06c4b03 [SPARK-21342] Fix DownloadCallback to work well with RetryingBlockFetcher. ## What changes were proposed in this pull request? When `RetryingBlockFetcher` retries fetching blocks. There could be two