The validator beacon APIs `getAttesterDuties`, `getProposerDuties`, and
`getSyncCommitteeDuties`, have reported the `execution_optimistic`
state for the current head block. This can lead to a race if duties are
requested around the slot start, if a new head block is currently being
processed by the EL, during which the BN head may be briefly optimistic.
`execution_optimistic` is documented in beacon APIs as:
> True if the response references an unverified execution payload.
> Optimistic information may be invalidated at a later time.
> If the field is not present, assume the False value.
As the duty endpoints reference the shuffling dependent root instead of
the currently selected head block, `execution_optimistic` is now fetched
based on that shuffling dependent block root. As this dependent block is
in the past it doesn't usually become optimistic when adding new blocks.
Note that the endpoints requested 4/8 seconds into the slot that perform
the actual duties instead of just querying for duty schedule, still
report `execution_optimistic` based on the BN head block.
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