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Johnathan Corgan
959134657f
The FSP XK rekey handshake has a race condition where the initiator can cut over (K-bit flip) and send data encrypted with the new session before msg3 reaches the responder. The responder has no pending session yet, so K-bit detection fails and packets are dropped. Defer FSP initiator cutover by 2 seconds after handshake completion (FSP_CUTOVER_DELAY_MS) to give msg3 time to traverse the mesh. FMP (IK, 2 messages) is unaffected since the responder completes during msg1 processing. Also fix MMP metric corruption after rekey cutover: the new session starts with counter 0 but MMP state carries highest_counter and GapTracker.expected_next from the old session, producing false reorder counts, jitter spikes, and invalid OWD trends. Add reset_for_rekey() methods that clear counter-dependent state while preserving RTT estimates. Additional fixes: - Remove stale peers_by_index entry on abandon_rekey error path - Replace redundant peers_by_index inserts with debug assertions verifying the pre-registration invariant - Tighten rekey integration test to zero tolerance (was 4 failures)