FA-476 / Consensus / Open access

Promise a proposal ballot: An acceptor promises a ballot below its durable promise · case 01

The acceptor prepare operation is admitted even though an acceptor promises a ballot below its durable promise.

Verified by executionVariant 1 · 9 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The admission path omits the ballot floor invariant while validating the other operation preconditions.

THE FAILURE

The admission path omits the ballot floor invariant while validating the other operation preconditions.

Unsuccessful approach: Adding the ballot floor check repairs the reported defect, but replacing the adjacent slot scope check loses that independent invariant.

Case contract

Return a Boolean admission decision for promise a proposal ballot. The record r must satisfy all of: tuple(r['ballot_floor'][0]) >= tuple(r['ballot_floor'][1]); r['slot_scope'][0] == r['slot_scope'][1]; r['promise_durable'] is True; r['accepted_attached'][0] is None or r['accepted_attached'][1] == r['accepted_attached'][0]; r['configuration_epoch'][0] == r['configuration_epoch'][1]. Extra tracing fields are ignored; validation does not mutate the record.

Why this case matters

A deterministic local contract for consensus. Each negative fixture violates exactly one invariant. No transport timing, persistence, cryptographic verification, or full protocol implementation is claimed.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(r):
    return (r['slot_scope'][0] == r['slot_scope'][1]) and (r['promise_durable'] is True) and (r['accepted_attached'][0] is None or r['accepted_attached'][1] == r['accepted_attached'][0]) and (r['configuration_epoch'][0] == r['configuration_epoch'][1])
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'ballot_floor': [[5, 2], [5, 1]], 'slot_scope': ['s1', 's1'], 'promise_durable': True, 'accepted_attached': ['chosen', 'chosen'], 'configuration_epoch': [3, 3]}
check('valid operation', solve(r), True)
check('An acceptor promises a ballot below its durable promise', solve(dict(r, **{'ballot_floor': [[4, 9], [5, 1]]})), False)
check('A prepare message targets a different consensus instance', solve(dict(r, **{'slot_scope': ['s2', 's1']})), False)
check('A promise response escapes before its durable write', solve(dict(r, **{'promise_durable': False})), False)
check('The response omits an already accepted value', solve(dict(r, **{'accepted_attached': ['chosen', None]})), False)
check('A prepare is counted under the wrong voting epoch', solve(dict(r, **{'configuration_epoch': [2, 3]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'ballot_floor': [[4, 9], [5, 1]], 'slot_scope': ['s2', 's1'], 'promise_durable': False, 'accepted_attached': ['chosen', None], 'configuration_epoch': [2, 3]}
keys = list(invalid)
pair = {keys[N % len(keys)]: invalid[keys[N % len(keys)]], keys[(N+1) % len(keys)]: invalid[keys[(N+1) % len(keys)]]}
check('two independent violations in variant', solve(dict(r, **pair)), False)
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
valid operation	`True`	`True`	Passed
An acceptor promises a ballot below its durable promise	`True`	`False`	Failed
A prepare message targets a different consensus instance	`False`	`False`	Passed
A promise response escapes before its durable write	`False`	`False`	Passed
The response omits an already accepted value	`False`	`False`	Passed
A prepare is counted under the wrong voting epoch	`False`	`False`	Passed
unrelated tracing metadata	`True`	`True`	Passed
repeat validation is pure	`True`	`True`	Passed
two independent violations in variant	`False`	`False`	Passed

SHA-256 / 3e40b7e2f9f8d1d469882d7ed4ed6510f68f08fbc61e92bfa0a1cf3bbad193f5

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(r):
    return (tuple(r['ballot_floor'][0]) >= tuple(r['ballot_floor'][1])) and (r['promise_durable'] is True) and (r['accepted_attached'][0] is None or r['accepted_attached'][1] == r['accepted_attached'][0]) and (r['configuration_epoch'][0] == r['configuration_epoch'][1])
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'ballot_floor': [[5, 2], [5, 1]], 'slot_scope': ['s1', 's1'], 'promise_durable': True, 'accepted_attached': ['chosen', 'chosen'], 'configuration_epoch': [3, 3]}
check('valid operation', solve(r), True)
check('An acceptor promises a ballot below its durable promise', solve(dict(r, **{'ballot_floor': [[4, 9], [5, 1]]})), False)
check('A prepare message targets a different consensus instance', solve(dict(r, **{'slot_scope': ['s2', 's1']})), False)
check('A promise response escapes before its durable write', solve(dict(r, **{'promise_durable': False})), False)
check('The response omits an already accepted value', solve(dict(r, **{'accepted_attached': ['chosen', None]})), False)
check('A prepare is counted under the wrong voting epoch', solve(dict(r, **{'configuration_epoch': [2, 3]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'ballot_floor': [[4, 9], [5, 1]], 'slot_scope': ['s2', 's1'], 'promise_durable': False, 'accepted_attached': ['chosen', None], 'configuration_epoch': [2, 3]}
keys = list(invalid)
pair = {keys[N % len(keys)]: invalid[keys[N % len(keys)]], keys[(N+1) % len(keys)]: invalid[keys[(N+1) % len(keys)]]}
check('two independent violations in variant', solve(dict(r, **pair)), False)
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
valid operation	`True`	`True`	Passed
An acceptor promises a ballot below its durable promise	`False`	`False`	Passed
A prepare message targets a different consensus instance	`True`	`False`	Failed
A promise response escapes before its durable write	`False`	`False`	Passed
The response omits an already accepted value	`False`	`False`	Passed
A prepare is counted under the wrong voting epoch	`False`	`False`	Passed
unrelated tracing metadata	`True`	`True`	Passed
repeat validation is pure	`True`	`True`	Passed
two independent violations in variant	`False`	`False`	Passed

SHA-256 / 2008265e391042785fe49a3bfc15fbd4ade9d0113ca3676bca63c8f12b8b84a3

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 9 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

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 2026-09-29T14:36:53.725346+00:00.

Case digest / 933912c831f7f85161734ffd9ade1d5d23a6fccddfbd00d7a2d38462502cf1bf