Minor Description
Spinoff from http://www.nabble.com/IllegalStateEx-thrown-when-calling-close-to20201825.html
When IndexWriter hits an OOME, it records this and then if close() is
called it calls rollback() instead. This is a defensive measure, in
case the OOME corrupted the internal buffered state (added/deleted
docs).
But there's a bug: if you opened IndexWriter with autoCommit true,
close() then incorrectly throws an IllegalStatException.
This fix is simple: allow rollback to be called even if autoCommit is
true, internally during close. (External calls to rollback with
autoCommmit true is still not allowed).