## What changes were proposed in this pull request?
have the`FileFormatWriter.ExecuteWriteTask.releaseResources()` implementations set `currentWriter=null` in a finally clause. This guarantees that if the first call to `currentWriter()` throws an exception, the second releaseResources() call made during the task cancel process will not trigger a second attempt to close the stream.
## How was this patch tested?
Tricky. I've been fixing the underlying cause when I saw the problem [HADOOP-14204](https://issues.apache.org/jira/browse/HADOOP-14204), but SPARK-10109 shows I'm not the first to have seen this. I can't replicate it locally any more, my code no longer being broken.
code review, however, should be straightforward
Author: Steve Loughran <stevel@hortonworks.com>
Closes#17364 from steveloughran/stevel/SPARK-20038-close.
7536e2849d