FA-76031 / Chat ordering and read receipts / Open access
Plan history backfill requests for missing sequences: horizon floor · case 01
Messages between the retention horizon and the first received message are never backfilled.
ROOT CAUSE
The horizon floor decision evaluates `lo = min(got) if got else horizon` where the contract requires `lo = horizon`.
VERIFIED REPAIR
Use `lo = horizon` for the horizon floor decision and keep every other rule of the model unchanged.
Unsuccessful approach: Clamping to the horizon still starts scanning at the first received message. The attempted `lo = max(horizon, min(got)) if got else horizon` still disagrees with a fixture.
Case contract
have lists received seqs (unsorted, duplicates possible); horizon is the oldest seq the server still retains; head is the newest known seq. Every seq in [horizon, head] not received is missing; missing seqs are merged into inclusive [start, end] ranges, returned newest first and limited to max_ranges. complete is true only when nothing is missing.
Why this case matters
Chat clients repair gaps after reconnecting; wrong ranges refetch too much or leave holes in the timeline.
1 / The failure
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(have, horizon, head, max_ranges):
got = set(have)
lo = min(got) if got else horizon
hi = head
ranges = []
start = None
for s in range(lo, hi + 1):
if s not in got:
if start is None:
start = s
elif start is not None:
ranges.append([start, s - 1])
start = None
if start is not None:
ranges.append([start, hi])
ranges.reverse()
return {'fetch': ranges[:max_ranges], 'complete': not ranges}
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('gap before first received', ([6, 7, 10], 3, 10, 5), {'complete': False, 'fetch': [[8, 9], [3, 5]]}), ('head not yet received', ([1, 2, 3], 1, 5, 3), {'complete': False, 'fetch': [[4, 5]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 11], 1, 11, 2), {'complete': False, 'fetch': [[8, 10], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 4, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 9, 5), {'complete': False, 'fetch': [[9, 9], [3, 7]]})], 2: [('gap before first received', ([7, 8, 11], 4, 11, 5), {'complete': False, 'fetch': [[9, 10], [4, 6]]}), ('head not yet received', ([1, 2, 3], 1, 6, 3), {'complete': False, 'fetch': [[4, 6]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 12], 1, 12, 2), {'complete': False, 'fetch': [[8, 11], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 5, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 10, 5), {'complete': False, 'fetch': [[9, 10], [3, 7]]})], 3: [('gap before first received', ([8, 9, 12], 5, 12, 5), {'complete': False, 'fetch': [[10, 11], [5, 7]]}), ('head not yet received', ([1, 2, 3], 1, 7, 3), {'complete': False, 'fetch': [[4, 7]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 13], 1, 13, 2), {'complete': False, 'fetch': [[8, 12], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 6, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 11, 5), {'complete': False, 'fetch': [[9, 11], [3, 7]]})], 4: [('gap before first received', ([9, 10, 13], 6, 13, 5), {'complete': False, 'fetch': [[11, 12], [6, 8]]}), ('head not yet received', ([1, 2, 3], 1, 8, 3), {'complete': False, 'fetch': [[4, 8]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 14], 1, 14, 2), {'complete': False, 'fetch': [[8, 13], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 7, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 12, 5), {'complete': False, 'fetch': [[9, 12], [3, 7]]})], 5: [('gap before first received', ([10, 11, 14], 7, 14, 5), {'complete': False, 'fetch': [[12, 13], [7, 9]]}), ('head not yet received', ([1, 2, 3], 1, 9, 3), {'complete': False, 'fetch': [[4, 9]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 15], 1, 15, 2), {'complete': False, 'fetch': [[8, 14], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 8, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 13, 5), {'complete': False, 'fetch': [[9, 13], [3, 7]]})]}
for _label, _args, _expected in _CASES[N]:
check(_label, solve(*_args), _expected)
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 |
|---|---|---|---|
| gap before first received | {'complete': False, 'fetch': [[8, 9]]} | {'complete': False, 'fetch': [[8, 9], [3, 5]]} | Failed |
| head not yet received | {'complete': False, 'fetch': [[4, 5]]} | {'complete': False, 'fetch': [[4, 5]]} | Passed |
| single missing seqs | {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]} | {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]} | Passed |
| several gaps limited | {'complete': False, 'fetch': [[8, 10], [5, 6]]} | {'complete': False, 'fetch': [[8, 10], [5, 6]]} | Passed |
| nothing missing | {'complete': True, 'fetch': []} | {'complete': True, 'fetch': []} | Passed |
| limit zero with gaps | {'complete': False, 'fetch': []} | {'complete': False, 'fetch': []} | Passed |
| uneven gaps newest first | {'complete': False, 'fetch': [[9, 9], [3, 7]]} | {'complete': False, 'fetch': [[9, 9], [3, 7]]} | Passed |
SHA-256 / b1c16d6f4208a73b96a4b09e90f2b4d2ae0d5203916b9ddac90bed5ffd7613b6
2 / The unsuccessful fix
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(have, horizon, head, max_ranges):
got = set(have)
lo = max(horizon, min(got)) if got else horizon
hi = head
ranges = []
start = None
for s in range(lo, hi + 1):
if s not in got:
if start is None:
start = s
elif start is not None:
ranges.append([start, s - 1])
start = None
if start is not None:
ranges.append([start, hi])
ranges.reverse()
return {'fetch': ranges[:max_ranges], 'complete': not ranges}
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('gap before first received', ([6, 7, 10], 3, 10, 5), {'complete': False, 'fetch': [[8, 9], [3, 5]]}), ('head not yet received', ([1, 2, 3], 1, 5, 3), {'complete': False, 'fetch': [[4, 5]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 11], 1, 11, 2), {'complete': False, 'fetch': [[8, 10], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 4, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 9, 5), {'complete': False, 'fetch': [[9, 9], [3, 7]]})], 2: [('gap before first received', ([7, 8, 11], 4, 11, 5), {'complete': False, 'fetch': [[9, 10], [4, 6]]}), ('head not yet received', ([1, 2, 3], 1, 6, 3), {'complete': False, 'fetch': [[4, 6]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 12], 1, 12, 2), {'complete': False, 'fetch': [[8, 11], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 5, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 10, 5), {'complete': False, 'fetch': [[9, 10], [3, 7]]})], 3: [('gap before first received', ([8, 9, 12], 5, 12, 5), {'complete': False, 'fetch': [[10, 11], [5, 7]]}), ('head not yet received', ([1, 2, 3], 1, 7, 3), {'complete': False, 'fetch': [[4, 7]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 13], 1, 13, 2), {'complete': False, 'fetch': [[8, 12], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 6, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 11, 5), {'complete': False, 'fetch': [[9, 11], [3, 7]]})], 4: [('gap before first received', ([9, 10, 13], 6, 13, 5), {'complete': False, 'fetch': [[11, 12], [6, 8]]}), ('head not yet received', ([1, 2, 3], 1, 8, 3), {'complete': False, 'fetch': [[4, 8]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 14], 1, 14, 2), {'complete': False, 'fetch': [[8, 13], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 7, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 12, 5), {'complete': False, 'fetch': [[9, 12], [3, 7]]})], 5: [('gap before first received', ([10, 11, 14], 7, 14, 5), {'complete': False, 'fetch': [[12, 13], [7, 9]]}), ('head not yet received', ([1, 2, 3], 1, 9, 3), {'complete': False, 'fetch': [[4, 9]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 15], 1, 15, 2), {'complete': False, 'fetch': [[8, 14], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 8, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 13, 5), {'complete': False, 'fetch': [[9, 13], [3, 7]]})]}
for _label, _args, _expected in _CASES[N]:
check(_label, solve(*_args), _expected)
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 |
|---|---|---|---|
| gap before first received | {'complete': False, 'fetch': [[8, 9]]} | {'complete': False, 'fetch': [[8, 9], [3, 5]]} | Failed |
| head not yet received | {'complete': False, 'fetch': [[4, 5]]} | {'complete': False, 'fetch': [[4, 5]]} | Passed |
| single missing seqs | {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]} | {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]} | Passed |
| several gaps limited | {'complete': False, 'fetch': [[8, 10], [5, 6]]} | {'complete': False, 'fetch': [[8, 10], [5, 6]]} | Passed |
| nothing missing | {'complete': True, 'fetch': []} | {'complete': True, 'fetch': []} | Passed |
| limit zero with gaps | {'complete': False, 'fetch': []} | {'complete': False, 'fetch': []} | Passed |
| uneven gaps newest first | {'complete': False, 'fetch': [[9, 9], [3, 7]]} | {'complete': False, 'fetch': [[9, 9], [3, 7]]} | Passed |
SHA-256 / 2a85cd149054c11cadd1b50b1236988c1d2bd748467ae44ea731681495914c82
3 / The verified repair
Exit 0"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(have, horizon, head, max_ranges):
got = set(have)
lo = horizon
hi = head
ranges = []
start = None
for s in range(lo, hi + 1):
if s not in got:
if start is None:
start = s
elif start is not None:
ranges.append([start, s - 1])
start = None
if start is not None:
ranges.append([start, hi])
ranges.reverse()
return {'fetch': ranges[:max_ranges], 'complete': not ranges}
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('gap before first received', ([6, 7, 10], 3, 10, 5), {'complete': False, 'fetch': [[8, 9], [3, 5]]}), ('head not yet received', ([1, 2, 3], 1, 5, 3), {'complete': False, 'fetch': [[4, 5]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 11], 1, 11, 2), {'complete': False, 'fetch': [[8, 10], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 4, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 9, 5), {'complete': False, 'fetch': [[9, 9], [3, 7]]})], 2: [('gap before first received', ([7, 8, 11], 4, 11, 5), {'complete': False, 'fetch': [[9, 10], [4, 6]]}), ('head not yet received', ([1, 2, 3], 1, 6, 3), {'complete': False, 'fetch': [[4, 6]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 12], 1, 12, 2), {'complete': False, 'fetch': [[8, 11], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 5, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 10, 5), {'complete': False, 'fetch': [[9, 10], [3, 7]]})], 3: [('gap before first received', ([8, 9, 12], 5, 12, 5), {'complete': False, 'fetch': [[10, 11], [5, 7]]}), ('head not yet received', ([1, 2, 3], 1, 7, 3), {'complete': False, 'fetch': [[4, 7]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 13], 1, 13, 2), {'complete': False, 'fetch': [[8, 12], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 6, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 11, 5), {'complete': False, 'fetch': [[9, 11], [3, 7]]})], 4: [('gap before first received', ([9, 10, 13], 6, 13, 5), {'complete': False, 'fetch': [[11, 12], [6, 8]]}), ('head not yet received', ([1, 2, 3], 1, 8, 3), {'complete': False, 'fetch': [[4, 8]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 14], 1, 14, 2), {'complete': False, 'fetch': [[8, 13], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 7, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 12, 5), {'complete': False, 'fetch': [[9, 12], [3, 7]]})], 5: [('gap before first received', ([10, 11, 14], 7, 14, 5), {'complete': False, 'fetch': [[12, 13], [7, 9]]}), ('head not yet received', ([1, 2, 3], 1, 9, 3), {'complete': False, 'fetch': [[4, 9]]}), ('single missing seqs', ([1, 3, 5, 7], 1, 7, 10), {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]}), ('several gaps limited', ([1, 4, 7, 15], 1, 15, 2), {'complete': False, 'fetch': [[8, 14], [5, 6]]}), ('nothing missing', ([3, 4, 5, 5], 3, 5, 0), {'complete': True, 'fetch': []}), ('limit zero with gaps', ([1, 3], 1, 8, 0), {'complete': False, 'fetch': []}), ('uneven gaps newest first', ([1, 2, 8], 1, 13, 5), {'complete': False, 'fetch': [[9, 13], [3, 7]]})]}
for _label, _args, _expected in _CASES[N]:
check(_label, solve(*_args), _expected)
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 |
|---|---|---|---|
| gap before first received | {'complete': False, 'fetch': [[8, 9], [3, 5]]} | {'complete': False, 'fetch': [[8, 9], [3, 5]]} | Passed |
| head not yet received | {'complete': False, 'fetch': [[4, 5]]} | {'complete': False, 'fetch': [[4, 5]]} | Passed |
| single missing seqs | {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]} | {'complete': False, 'fetch': [[6, 6], [4, 4], [2, 2]]} | Passed |
| several gaps limited | {'complete': False, 'fetch': [[8, 10], [5, 6]]} | {'complete': False, 'fetch': [[8, 10], [5, 6]]} | Passed |
| nothing missing | {'complete': True, 'fetch': []} | {'complete': True, 'fetch': []} | Passed |
| limit zero with gaps | {'complete': False, 'fetch': []} | {'complete': False, 'fetch': []} | Passed |
| uneven gaps newest first | {'complete': False, 'fetch': [[9, 9], [3, 7]]} | {'complete': False, 'fetch': [[9, 9], [3, 7]]} | Passed |
SHA-256 / a624374b2c66ba3fc7efc0b86c51a62e48841f499ba502b93d7b0fd06f55a8dd
Verification & scope
Stipulated offline chat model; not a complete messaging protocol, client or server implementation. 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 2026-09-29T14:49:12.578643+00:00.
Case digest / de30e03369b7a8a93cb48be9273b73d010c9502105fc3adedb95b92ab375b649