Indeed, this is the fragility that I'm not too happy with. However, the force flag only applies to optimize (not regular merges) and we protect all optimize calls with `optimizeMutex` so this works fine. Maybe the ability to "force a forceMerge" should be added to Lucene (the only way to do it right now would be to close the writer, open a new one with a custom merge policy that ignores the number of segments already present in the index, run a force merge, and then re-open with a regular merge policy).