Major Description
IndexWriter's exception handling is overly complicated. Every method in general reads like this:
try { try { try { ... // lock order: COMPLICATED synchronized(this or that) { } ... } finally { if (!success5) { deleter.deleteThisFileOrThat(); } ... } }
Part of the problem is it acts like its an invincible superhero, e.g. can take a disk full on merge or flush to the face and just keep on trucking, and you can somehow fix the root cause and then just go about making commits on the same instance.
But we have a hard enough time ensuring exceptions dont do the wrong thing (e.g. cause corruption), and I don't think we really test this crazy behavior anywhere: e.g. making commits AFTER hitting disk full and so on.
It would probably be simpler if when such things happen, IW just considered them "tragic" just like OOM and rolled itself back, instead of doing all kinds of really scary stuff to try to "keep itself healthy" (like the little dance it plays with IFD in mergeMiddle manually deleting CFS files).
Besides, without something like a WAL, Indexwriter isn't really fit to be a superhero anyway: it can't prevent you from losing data in such situations. It just doesn't have the right tools for the job.
edit: just to be clear I am referring to abort (low level exception during flush) and exceptions during merge. For simple non-aborting cases like analyzer errors, of course we can deal with this. We already made great progress on turning a lot of BS exceptions that would cause aborts into non-aborting ones recently.