Closes the TOCTOU window flagged in review on #93369 (merged via
#93430): the divergence guard compared EXPORT-TIME message counts, but
another backend can append donor messages between the export snapshot
and the retire loop — that growth would be stamped behind the
non-recoverable adopted_by_profile archive, the exact H2 class the
guard exists to prevent, just via a narrower race.
The retire loop now re-reads live donor vs local counts immediately
before end_session and leaves the donor unretired (donor_retired=False,
warn-logged) on any donor-ahead signal; the next resume's export-time
guard then handles the divergence normally. Equal-count CONTENT
divergence (donor rewind+rewrite) remains invisible to count comparison
— documented as accepted: bytes stay in the donor store either way.
New red-first-verified regression simulates the exact race by appending
to the donor from inside an export_session_lineage wrapper.
adoption+ownership suites: 25 passed; ruff clean.
kshitijk4poor
dc50f02090