Chasing a tricking Elasticsearch test failure, it came down to the delegate.close throwing an exception (ClosedByInterruptException, disturbingly, in this case), causing MockIndexOutputWrapper.close to fail to remove that IO from MDW's map.
The question is, what should we do here, when delegate.close throws an exception? Is the delegate in fact closed, even when it throws an exception?
Java8's docs on java.io.Closeable say this:
As noted in AutoCloseable.close(), cases where the close may fail require careful attention. It is strongly advised to relinquish the underlying resources and to internally mark the Closeable as closed, prior to throwing the IOException.
And our OutputStreamIndexOutput is careful about this (flushes, then closes in a try-with-resources).
So, I think MDW should be fixed to mark the IO as closed even if delegate.close throws an exception...