The backfillProgress column was read by four hand-rolled walkers over
untyped records and written inline in the sync core. One module now owns
the column: a Zod schema reads it, lenient per entry so a malformed cursor
or end is dropped on its own, and one function writes it. The sync core
keeps only the bookkeeping of a running backfill.
A walk that stopped for lack of progress ended its data type for good, so
a passing fault in Google's paging closed that history until a reconnect.
A walk that had written nothing yet was also noted as ending today.
The first end without progress now parks the data type with its edge: it
is not finished, not published, and is walked again a day later. Once
nothing else is left, the next slice is queued for when it is due, and
the marker is never stamped over a parked type. A second end without
progress is final; a second attempt that moves on is an ordinary walk
again. A walk that stalls before writing anything records no end and
fails the slice as stuck.
Refs #1220
A resumable collection counted the pages it had written across every
backfill attempt and stopped at its ceiling (10,000 for heart rate). A
watch that records heart rate every few seconds reaches that count two to
three years back, and since it never reset, every later slice stopped at
the same place, failed as stuck, and the backfill was never stamped
complete.
A walk is now stopped only when it stops moving: 20 pages in a row that
reach no further back than the pages before them end it. A history
backfill reads back at most GOOGLE_HEALTH_BACKFILL_HORIZON_YEARS (20 when
unset). A data type that ends either way counts as finished, and where and
why it ended is kept in backfillProgress past the completion stamp. An
oldest-first list is read whole, as before. The slice chain's ceiling is
derived from sixty days of back-to-back slices instead of the old page
count. An account left at the old ceiling continues from its edge with
the next slice.
Refs #1220
Marc-André Bombeck