FAILURE MAP
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FA-016 / Concurrency / Open access

Stale snapshots erase a counter increment · case 01

Two updates calculated from the same snapshot leave only one increment.

Verified by executionVariant 1 · 3 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Absolute values calculated from a stale base replace each other.

VERIFIED REPAIR

Apply each delta to the current accumulator rather than replacing it from a stale base.

Unsuccessful approach: Deduplicating equal deltas mistakes independent operations for retries.

Case contract

Apply each delta to the current accumulator rather than replacing it from a stale base.

Why this case matters

A compact reasoning and repair task with executable evidence.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(base, deltas):
    return base + deltas[-1] if deltas else base
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('two independent updates', solve(N, [2, 2]), N+4)
check('mixed deltas', solve(N, [1, -1, 3]), N+3)
check('no updates', solve(N, []), N)
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 fixtureActualExpectedOutcome
two independent updates35Failed
mixed deltas44Passed
no updates11Passed

SHA-256 / 3cb19adfe6d019c217104ee3e2bc1a5304db9d3c3f8c6b0c680c0689a123ba2d

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(base, deltas):
    return base + sum(set(deltas))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('two independent updates', solve(N, [2, 2]), N+4)
check('mixed deltas', solve(N, [1, -1, 3]), N+3)
check('no updates', solve(N, []), N)
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 fixtureActualExpectedOutcome
two independent updates35Failed
mixed deltas44Passed
no updates11Passed

SHA-256 / 7a31b49393f5f97d163df4a28aadf3b70940d341eafe59da23b4f75aad144ed9

3 / The verified repair

Exit 0
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(base, deltas):
    return base + sum(deltas)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('two independent updates', solve(N, [2, 2]), N+4)
check('mixed deltas', solve(N, [1, -1, 3]), N+3)
check('no updates', solve(N, []), N)
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 fixtureActualExpectedOutcome
two independent updates55Passed
mixed deltas44Passed
no updates11Passed

SHA-256 / 6b6762836483a1fcc77f02d4b453abb78347a02cb7470f9026fab1911895b4e2

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 2026-09-29T14:36:49.861722+00:00.

Case digest / a9068f0b8bfe1d0684ddc19163ba50fd472eda5f439d5567b15c587a8e2866af