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During the search, the region between `upper_bound` and `target_pos` should contain no data items. The search lowers `upper_bound` and raises `lower_bound` until they both point to the last item before `target_pos`. The `lower_bound` is increased to the position of the last item returned by a search (`high_pos`) when that item is lower than `target_pos`. This avoids some loop iterations compared to a strict binary search algorithm, which would increase `lower_bound` only as far as `probe_pos`. When the search runs over live extent items, occasionally a new extent will appear between `upper_bound` and `target_pos`. When this happens, `lower_bound` is bumped up to the position of one of the new items, but that position is in the "unoccupied" space between `upper_bound` and `target_pos`, where no items are supposed to exist, so `seek_backward` throws an exception. To cut down on the noise, only increase `lower_bound` as far as `upper_bound`. This avoids the exception without increasing the number of loop iterations for normal cases. In the exceptional cases, extra loop iterations are needed to skip over the new items. This raises the worst-case number of loop iterations by one. Signed-off-by: Zygo Blaxell <bees@furryterror.org>