MDL-60943 a test was added that catches bad preg_match output due to unexpected characters in the input.
While this is a good thing overall, that exception is never caught, and so kills all of indexing. We just noticed that none of our search areas have been in 2.5 months, because the first thing it tries to index is a block_html that is bad, and then dies. This causes the entire task to die. And now it does that once a day - tries to index and then dies immediately.
My personal opinion is that the moodle_execption (with it 'error_indexing') should be caught, maybe in the engine add_documents, logged, and then that document skipped.
At the very least, a failing search area shouldn't cause all search areas to not be indexed...