Migration 0420 looked for an RMSSD twin by externalId only. A Fitbit or
Google Health SDNN row with no externalId, or another one, on the same
night as an RMSSD row would have broken the measured-at unique key and
aborted the block at boot. The twin check now matches either identity,
a row left behind by a live twin is tombstoned, and an RMSSD tombstone
that would make a live row look twinned is removed so the row moves.
The backup restore applies the same rule and no longer counts a
tombstone as a twin.
A backup written before migration 0420 files Fitbit and Google Health
HRV as HEART_RATE_VARIABILITY, and the restore wrote it back verbatim,
undoing the migration for the account (and able to duplicate an
HRV_RMSSD row of the same night, since type is part of the unique key).
The restore now applies the same move: those rows come back as
HRV_RMSSD, a legacy row whose RMSSD twin the file also holds is skipped,
and SDNN personal records set by them are left out as the migration
deleted them.
Marc-André Bombeck