I just ran e4defrag on an image cache updated to buster, which had had some files cleared and so had space to add new ones. This resulted in following output, demonstrating an invalid failure count close to ULONGLONG_MAX as predicted in #15:
# e4defrag -v /cache/ ... [291614/288339]/cache/3e/64d513fb1b5568145d22a2d665d58a3e: 100% extents: 1 -> 1 [ OK ] Success: [ 291356/288339 ] Failure: [ 18446744073709548599/288339 ] Total extents: 1817550->1326154 Fragmented percentage: 18%->16% # e4defrag --version e4defrag 1.44.5 (15-Dec-2018) -- Laurence "GreenReaper" Parry https://www.greenreaper.co.uk/