test(fc): add attestation target walkback bound filler#614
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tcoratger merged 2 commits intoleanEthereum:mainfrom Apr 15, 2026
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Adds test_attestation_target_walkback_bounded_by_lookback to verify that get_attestation_target caps the walk from head at JUSTIFICATION_LOOKBACK_SLOTS, preventing target collapse to safe_target when the head runs far ahead. Head and target are derived symbolically from JUSTIFICATION_LOOKBACK_SLOTS and Slot.is_justifiable_after, so the test stays valid if either the constant or the justifiability rules change.
… targeting, not genesis collapse Without the walkback bound the target stays near head (too aggressive), it does not collapse to genesis. The docstring previously described the opposite failure mode. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
tcoratger
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Apr 15, 2026
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Summary
Adds
test_attestation_target_walkback_bounded_by_lookbacktotests/consensus/devnet/fc/test_attestation_target_selection.py, verifying thatStore.get_attestation_targetcaps its walk from head atJUSTIFICATION_LOOKBACK_SLOTSand does not collapse the target back tosafe_targetwhen the head runs farahead.
Motivation
The existing target-selection fillers exercise
get_attestation_targetend-to-end but none isolate the bounded-walk property in a "head far ahead of safe_target"configuration. Without explicit coverage, a regression that removed or weakened the lookback bound (e.g. letting the walk run all the way back to
safe_target) wouldnot be caught — the justification frontier would silently collapse.
What the test does
safe_targetprovably stays at genesis.StoreChecks(attestation_target_slot=...)that the target sits exactlyJUSTIFICATION_LOOKBACK_SLOTSslots behind head, notat
safe_target.Note on the issue's literal example (head=10, target=7)
The issue's example numbers (
head=10→attestation_target_slot=Slot(7)) do not hold against the current implementation. After the bounded walk,get_attestation_targetcontinues backward until the result is justifiable (store.py:1240-1243). At head=10 withsafe_target=0,head - LOOKBACK = 7anddelta = 7is not ≤5, not a perfect square, and not pronic — so the algorithm walks further back to slot 6 (pronic). The existingtest_attestation_target_justifiable_constraintalready verifies this at line 423.The new test bumps head to 12 — the smallest "long chain" head where
head - LOOKBACK = 9is justifiable from genesis (perfect square) — so the bounded-walkproperty can be asserted cleanly without the secondary justifiability walk masking the result.
Why it is symbolic, not hardcoded
Both
headandtarget_slotare derived fromJUSTIFICATION_LOOKBACK_SLOTSandSlot.is_justifiable_aftervia a short inline loop, mirroring the convention fromtests/consensus/devnet/fc/test_block_attestation_limits.py:15,35. If either the constant or the justifiability rules change, the loop recomputes a valid head andthe test continues to express the same property — no edits required.
closes #560