The retained-session fix was necessary and did not work, and a live two-peer
reproduction is what showed it: both second-exchange messages still vanished
with delivered:1, failed:0.
Cause. `DoubleRatchet` is a final class on iOS and `E2ESession` is a class on
Android, so the failed decrypt that PROVES the peer is on another chain mutates
the instance it was handed — skipped keys consumed, chain stepped. Both fixes
then archived that same instance. Storing a part-advanced chain is
indistinguishable from storing nothing: it cannot open what the peer sends. The
vault copy was never handed to a decrypt, so re-read it before retiring it.
Second cause, Android only. Which slot the winner belongs in is not the same
question as which chain opened it. When the retired chain opens an envelope it
has to be written back to the RETIRED slot; promoting it swaps the two and
discards the chain we are sending on. Each branch now writes back deliberately
instead of sharing one `if (session !== existing)`.
Verified against relay1, two peer accounts, no simulators:
before p1/p2 poll -> {"messages":[]} / {"messages":[]}
after p1/p2 poll -> second-from-Q2 / second-from-Q1
The repro is in PARITY.md and takes about two minutes.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
