[domiwei]

Fixes #25239. Stacked on #25211.

Problem

ReadParticipations resolves the parent of every Gloas block that has attestations through kv.BlockRootToSlot. When that entry is missing and the parent is not the genesis block, it returned a plain error. initializeStateAntiquaryIfNeeded treated it as fatal, so the state antiquary could not start from that resume point.

Change

• ReadParticipations returns the new ErrMissingParentBlock sentinel when the parent root is neither in the block index nor the genesis block root. A genesis root row that is present but not 32 bytes long stays a hard error (invalid genesis block root length), the same as other corrupt root rows.
• initializeStateAntiquaryIfNeeded backs off on ErrMissingParentBlock, the same as on ErrMissingGloasData and ErrMissingHistoryVectorData. The check moved into shouldBackoffHistoricalStateRead.

Backing off only moves the resume point to a state before the gap. The antiquary does not write kv.BlockRootToSlot, so this does not rebuild the block index.

Tests

• TestReadParticipationsReturnsMissingParentBlockError: missing index entry with a non-genesis parent returns the sentinel.
• TestReadParticipationsReturnsMalformedGenesisRootError: a 31-byte genesis root returns the length error, not the sentinel.
• TestReadHistoricalStateReturnsMissingParentBlockError: the sentinel survives the %w wrap in ReadHistoricalState.
• TestShouldBackoffHistoricalStateRead: classification, including a wrapped sentinel.

There is no end-to-end test through initializeStateAntiquaryIfNeeded: the Gloas fixtures are a few slots long and cannot reach the first backoff step of 10 * SlotsPerDump slots.

Not in this PR

The backoff loop in initializeStateAntiquaryIfNeeded does not check ctx between attempts. This is the same for every backoff sentinel and predates this change.

[Copilot]

🟢 Approval recommended

The narrow change uses the existing backoff mechanism, preserves corruption handling, and includes focused regression coverage.

No open issues 🎉

What changed in this PR

This PR addresses #25239 by routing missing parent-block index entries into the antiquary’s existing historical-state backoff path. It builds on #25211 without attempting to rebuild the block index.

Changes:

• Adds ErrMissingParentBlock while preserving hard errors for malformed genesis-root rows.
• Makes the antiquary recognize the sentinel as a reason to back off.
• Adds regression coverage for sentinel propagation, corruption handling, and backoff classification.

File	Description
cl/​persistence/​state/​historical_states_reader/​historical_states_reader.go	Defines and returns the missing-parent sentinel; distinguishes malformed genesis roots.
cl/​persistence/​state/​historical_states_reader/​historical_states_reader_test.go	Tests missing-parent errors, wrapping, and malformed genesis-root handling.
cl/​antiquary/​state_antiquary.go	Adds the sentinel to the existing backoff classifier.
cl/​antiquary/​state_antiquary_test.go	Extends backoff classification coverage to missing-parent errors.

_{Review effort: Balanced}

[domiwei]

Closing. A missing BlockRootToSlot entry for a parent block only happens with corrupt or inconsistent data. WriteHeaderSlot writes the entry on every block write path and in the snapshot indexer, nothing deletes from the table, and the snapshot index is rebuilt before the state antiquary starts. Backing off would also not repair the index, so reads of those states through the API would still fail. A loud error is the better outcome here.