FA-10526 / Caching / Open access

An L2 hit fails to populate the faster cache tier · case 01

An L2 hit fails to populate the faster cache tier.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Reading the slower tier returns its value without performing promotion.

VERIFIED REPAIR

Preserve the cache-state invariant: On an L1 miss and L2 hit, copy the L2 value into L1. Existing L1 data wins. Return [new-L1,selected value or None]; this model output does not distinguish a terminal miss from a cached None.

Unsuccessful approach: Truthiness skips zero-valued L2 entries and unconditional L2 assignment overwrites an existing L1 value.

Case contract

On an L1 miss and L2 hit, copy the L2 value into L1. Existing L1 data wins. Return [new-L1,selected value or None]; this model output does not distinguish a terminal miss from a cached None.

Why this case matters

A deterministic cache state transformation. Inputs are copied or treated as immutable; no remote storage, real clock, or concurrent interleaving is simulated.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(l1, l2, key):
    return [dict(l1),l1.get(key,l2.get(key))]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1', solve(*({}, {'a': 4}, 'a')), [{'a': 4}, 4])
check('fixture 2', solve(*({}, {'a': 0}, 'a')), [{'a': 0}, 0])
check('fixture 3', solve(*({'a': 'fresh'}, {'a': 'old'}, 'a')), [{'a': 'fresh'}, 'fresh'])
check('fixture 4', solve(*({'b': 2}, {}, 'a')), [{'b': 2}, None])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 1	`[{}, 4]`	`[{'a': 4}, 4]`	Failed
fixture 2	`[{}, 0]`	`[{'a': 0}, 0]`	Failed
fixture 3	`[{'a': 'fresh'}, 'fresh']`	`[{'a': 'fresh'}, 'fresh']`	Passed
fixture 4	`[{'b': 2}, None]`	`[{'b': 2}, None]`	Passed

SHA-256 / 19060fc126f66c0c2e4a734adf7e4dbadc5ed17a30fda357405e4bf18d9800a3

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(l1, l2, key):
    out=dict(l1)
    if l2.get(key): out[key]=l2[key]
    return [out,out.get(key)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1', solve(*({}, {'a': 4}, 'a')), [{'a': 4}, 4])
check('fixture 2', solve(*({}, {'a': 0}, 'a')), [{'a': 0}, 0])
check('fixture 3', solve(*({'a': 'fresh'}, {'a': 'old'}, 'a')), [{'a': 'fresh'}, 'fresh'])
check('fixture 4', solve(*({'b': 2}, {}, 'a')), [{'b': 2}, None])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 1	`[{'a': 4}, 4]`	`[{'a': 4}, 4]`	Passed
fixture 2	`[{}, None]`	`[{'a': 0}, 0]`	Failed
fixture 3	`[{'a': 'old'}, 'old']`	`[{'a': 'fresh'}, 'fresh']`	Failed
fixture 4	`[{'b': 2}, None]`	`[{'b': 2}, None]`	Passed

SHA-256 / 30061591a338f53a3ad9b470c40fa13a6726e872d780468199e930bbf8c78538

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(l1, l2, key):
    out=dict(l1)
    if key not in out and key in l2:
        out[key]=l2[key]
    return [out,out.get(key)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1', solve(*({}, {'a': 4}, 'a')), [{'a': 4}, 4])
check('fixture 2', solve(*({}, {'a': 0}, 'a')), [{'a': 0}, 0])
check('fixture 3', solve(*({'a': 'fresh'}, {'a': 'old'}, 'a')), [{'a': 'fresh'}, 'fresh'])
check('fixture 4', solve(*({'b': 2}, {}, 'a')), [{'b': 2}, None])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 1	`[{'a': 4}, 4]`	`[{'a': 4}, 4]`	Passed
fixture 2	`[{'a': 0}, 0]`	`[{'a': 0}, 0]`	Passed
fixture 3	`[{'a': 'fresh'}, 'fresh']`	`[{'a': 'fresh'}, 'fresh']`	Passed
fixture 4	`[{'b': 2}, None]`	`[{'b': 2}, None]`	Passed

SHA-256 / 5689aa64ef9d09a9b3d3d96cfb3f26a89276b2ab96e98a0064f55578161ecc99

Verification & scope

This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:38:39.863007+00:00.

Case digest / a33414506439f059b4bf700e019bc2b4c55f148b1c68ca78cef1316f292b2f2a