[Aura -> BABE] Committee rotation proceeds after session key registration failure

[Klapeyron]

Committee session key registration silently drops validators

Field	Value
Risk	High
Confidence	High
Likelihood	Medium
Grounding	Runtime review (hybrid)

Summary

register_committee_keys ignores errors from SessionInterface::set_keys. When a duplicate or stale KeyOwner entry causes upstream pallet_session to reject registration, new_session still returns the full committee. Downstream, pallet_session omits validators without registered keys from the active authority set.

What happens

1. Committee rotation calls register_committee_keys for each member.
2. SessionInterface::set_keys(account, keys) fails (e.g. DuplicatedKey when a session key is already owned by another validator).
3. The failure is logged and ignored.
4. new_session / new_session_genesis return all committee account IDs regardless.
5. When the session starts, pallet_session queue construction skips validators for which NextKeys is absent.
6. Aura/GRANDPA run with a partial or unexpectedly empty authority set.

Root cause

The registration loop treats set_keys as best-effort. It does not:

• abort rotation on failure,
• reject the committee, or
• verify that every returned validator has matching keys in pallet_session.

Why it matters

Aura/GRANDPA can omit an intended committee member or produce an unexpectedly empty/partial authority set, risking liveness and finality.