A login created after the revamp got a random account UUID that never
left the device, so its credential was keyed to an ID no other device
had and the login was unusable there. Migrated accounts were fine only
because their ID is deterministically the instance ID.
Accounts, the permanent legacy-migration markers, the permanent deletion
tombstones and the selected data context now ride the existing
CKSyncEngine as four record types, with the rules the category needs:
- Absence proves nothing. A fetch resumes from a change token and never
enumerates the zone, so deletions are records of their own and they
dominate the account wherever they are seen — including the engine's
retry-as-insert path, which is how a queued offline edit would
otherwise resurrect a deleted account.
- Fetched records are staged durably before each delegate event returns
and applied as a set, tombstones first, gated on the zone's own result
rather than didFetchChanges, which fires after failed fetches too.
- Nothing activates for an identity that has not bootstrapped: a device
left on the previous release consumed these record types as unknown
and advanced its token past them, so each identity fetches the zone
once from a nil token, outside the engine's own token.
- A disabled category defers records rather than discarding them; every
persisted artifact is namespaced by identity, and async work is
generation-checked so a fetch begun under one Apple account can never
land in another's state.
- Uploads are ordered: a marker never outruns the account it describes,
and a context naming an account is never published before that account
is server-confirmed.
