SYSE.34:5.4 - State the return and its end

This release retains D, the trigger and both reader contracts. Subject to the application’s other compatibility conditions, the old executable can return while preserving legitimate address writes made through the new form. It is not necessary to undo those writes.

Contraction is separate. Before removing the old contract, fence and drain all old writers and compatibility adapters, select the new write authority and validate its consumers. Once a lossy transformation or incompatible new write has occurred, an old binary alone cannot restore the prior usable state.

Suppose a later contraction removes a trailing LF and retains neither the original D nor another copy of the absent-versus-empty tail distinction. Both 12 Oak St (T = null) and 12 Oak St followed by LF (T = empty text) then become 12 Oak St: whether a separator existed is lost, and returning the old executable cannot reconstruct it. An exact-return promise therefore requires retaining D or that lossless distinction before this change; accepting its removal requires the actual decision holder to authorize a narrower promise. After the information is gone, exact restoration needs an independently retained, qualified recovery source and is unavailable without one. A qualified forward repair must meet the actually accepted result; it is not an inverse obtainable from the transformed value alone.

A distinct fallback uses PostgreSQL 18 point-in-time recovery. It needs a suitable base backup and continuous required WAL archive; a logical dump is not a substitute for that mechanism. Restore the whole cluster into an appropriately isolated recovery arrangement, inspect its state and evaluate achieved loss/time. Choosing an earlier recovery point does not reconcile all later business events.

If an archive gap prevents that fallback, its recovery claim stops. A qualified forward repair may still be possible, and an independent non-data-changing artifact test can continue.

What changes in practice is that the team can explain which state remains usable after each change, why concurrent writes are preserved, and where “return to the old version” ceases to mean recovery.