Problem. 

While foreground reindexing, disk space usage could surge to double the current size of the index directory. This could lead to indexing process to fail midway when there is insufficient disk space availability leaving the instance with broken index. 

From Lucene document
https://lucene.apache.org/core/5_0_0/core/org/apache/lucene/index/IndexWriter.html#addDocument(java.lang.Iterable)

Merges temporarily consume space in the directory. The amount of space required is up to 1X the size of all segments being merged, when no readers/searchers are open against the index, and up to 2X the size of all segments being merged when readers/searchers are open against the index (see forceMerge(int) for details). The sequence of primitive merge operations performed is governed by the merge policy.

Suggestion. 

Jira should check for disk space availability (at least double the current index size) or display warning to user.