State reconstruction API doesn't work if genesis backfill hasn't been enabled

[michaelsproul]

Description

The /lighthouse/database/reconstruct API is currently faulty in the case where the node was started without --genesis-backfill or --reconstruct-historic-states. The API will only attempt to trigger reconstruction, it won't trigger block backfill.

The error logged is:

ERRO State reconstruction failed error: MissingHistoricBlocks { oldest_block_slot: Slot(6225249) }, service: beacon

Version

v4.5.0

Present Behaviour

The reconstruct API only works if block backfill is enabled & complete.

Expected Behaviour

We should either:

A) Make it so that the reconstruct API triggers block backfill by flipping that var (I think it's immutable currently), or
B) Clarify in the docs and the API response that this isn't supported.

Workaround

Restart the BN with --reconstruct-historic-states.

[michaelsproul]

I think we may as well fix the docs first (option B), and then think about option A (harder).