Ceph

This is kind of a fun one.

1) osd 4 backfills up to 31bccdb2/mira01213209-286/head (henceforth: foo)
2) Interval change happens
3) osd 0 now finds itself backfilling to 4 (lb=foo) and osd.5 (lb=b6670ba2/mira01213209-160/snapdir//1, henceforth: bar)
4) recover_backfill causes both 4 and 5 to scan forward, so 4 has an interval starting at foo, 5 has an interval starting at bar.
5) Once those have come back, recover_backfill attempts to trim off the last_backfill_started, but 4's interval starts after that, so foo remains in osd 4's interval (this is the bug)
6) We serve a copyfrom on foo (sent to 4 as well).
6) We eventually get to foo in the backfilling. Normally, they would have the same version, but of course we don't update osd.4's interval from the log since it should not have received writes in that interval. Thus, we end up trying to recover foo on osd.4 anyway.
7) But, an interval change happens between removing foo from osd.4 and completing the recovery, leaving osd.4 without foo, but with lb >= foo --> this crash.

• firefly backport https://github.com/ceph/ceph/pull/4385

3fb97e2 ReplicatedPG: trim backfill intervals based on peer's last_backfill_started (in firefly),