FA-76076 / Chat ordering and read receipts / Open access

Page older history with a composite (ts, id) cursor: more flag · case 01

An extra empty page is requested whenever history length is an exact multiple of the page size.

Verified by executionVariant 1 · 6 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The more flag decision evaluates `more = len(page) == limit` where the contract requires `more = len(rows) > len(page)`.

THE FAILURE

The more flag decision evaluates `more = len(page) == limit` where the contract requires `more = len(rows) > len(page)`.

Unsuccessful approach: Requiring a full page still predicts more history when exactly one page remains. The attempted `more = len(page) >= limit and len(rows) > 0` still disagrees with a fixture.

Case contract

messages are [ts, id] pairs in any order; ordering is by (ts, id). cursor is None or the exclusive [ts, id] of the oldest message already shown. A page holds the newest `limit` messages strictly older than the cursor, in ascending order; next is the [ts, id] of the page's oldest item when older messages remain, else None.

Why this case matters

Infinite-scroll history must neither skip nor duplicate messages that share a timestamp.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(messages, cursor, limit):
    rows = sorted(messages, key=lambda m: (m[0], m[1]))
    if cursor is not None:
        rows = [m for m in rows if (m[0], m[1]) < (cursor[0], cursor[1])]
    page = rows[-limit:] if limit > 0 else []
    more = len(page) == limit
    return {'page': [m[1] for m in page], 'next': [page[0][0], page[0][1]] if more and page else None}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('siblings share the cursor timestamp', ([[100, 'a'], [100, 'd'], [100, 'b'], [100, 'c'], [91, 'z']], [100, 'c'], 5), {'next': None, 'page': ['z', 'a', 'b']}), ('equal timestamps across a page edge', ([[50, 'b'], [50, 'Q'], [50, 'a'], [40, 'x1']], None, 2), {'next': [50, 'a'], 'page': ['a', 'b']}), ('first page', ([[1, 'm01'], [2, 'm02'], [3, 'm03'], [4, 'm04'], [5, 'm05'], [6, 'm06'], [7, 'm07'], [8, 'm08']], None, 3), {'next': [6, 'm06'], 'page': ['m06', 'm07', 'm08']}), ('exact multiple', ([[1, 'a'], [2, 'b'], [3, 'c'], [4, 'd']], [5, 'e'], 4), {'next': None, 'page': ['a', 'b', 'c', 'd']}), ('zero limit', ([[1, 'a'], [2, 'b']], None, 0), {'next': None, 'page': []}), ('middle page', ([[10, 'id10'], [11, 'id11'], [12, 'id12'], [13, 'id13'], [14, 'id14'], [15, 'id15'], [16, 'id16'], [17, 'id17'], [18, 'id18'], [19, 'id19']], [15, 'id15'], 3), {'next': [12, 'id12'], 'page': ['id12', 'id13', 'id14']})], 2: [('siblings share the cursor timestamp', ([[100, 'a'], [100, 'd'], [100, 'b'], [100, 'c'], [92, 'z']], [100, 'c'], 5), {'next': None, 'page': ['z', 'a', 'b']}), ('equal timestamps across a page edge', ([[50, 'b'], [50, 'Q'], [50, 'a'], [40, 'x2']], None, 2), {'next': [50, 'a'], 'page': ['a', 'b']}), ('first page', ([[1, 'm01'], [2, 'm02'], [3, 'm03'], [4, 'm04'], [5, 'm05'], [6, 'm06'], [7, 'm07'], [8, 'm08'], [9, 'm09']], None, 3), {'next': [7, 'm07'], 'page': ['m07', 'm08', 'm09']}), ('exact multiple', ([[1, 'a'], [2, 'b'], [3, 'c'], [4, 'd']], [5, 'e'], 4), {'next': None, 'page': ['a', 'b', 'c', 'd']}), ('zero limit', ([[1, 'a'], [2, 'b']], None, 0), {'next': None, 'page': []}), ('middle page', ([[10, 'id10'], [11, 'id11'], [12, 'id12'], [13, 'id13'], [14, 'id14'], [15, 'id15'], [16, 'id16'], [17, 'id17'], [18, 'id18'], [19, 'id19']], [15, 'id15'], 2), {'next': [13, 'id13'], 'page': ['id13', 'id14']})], 3: [('siblings share the cursor timestamp', ([[100, 'a'], [100, 'd'], [100, 'b'], [100, 'c'], [93, 'z']], [100, 'c'], 5), {'next': None, 'page': ['z', 'a', 'b']}), ('equal timestamps across a page edge', ([[50, 'b'], [50, 'Q'], [50, 'a'], [40, 'x3']], None, 2), {'next': [50, 'a'], 'page': ['a', 'b']}), ('first page', ([[1, 'm01'], [2, 'm02'], [3, 'm03'], [4, 'm04'], [5, 'm05'], [6, 'm06'], [7, 'm07'], [8, 'm08'], [9, 'm09'], [10, 'm10']], None, 3), {'next': [8, 'm08'], 'page': ['m08', 'm09', 'm10']}), ('exact multiple', ([[1, 'a'], [2, 'b'], [3, 'c'], [4, 'd']], [5, 'e'], 4), {'next': None, 'page': ['a', 'b', 'c', 'd']}), ('zero limit', ([[1, 'a'], [2, 'b']], None, 0), {'next': None, 'page': []}), ('middle page', ([[10, 'id10'], [11, 'id11'], [12, 'id12'], [13, 'id13'], [14, 'id14'], [15, 'id15'], [16, 'id16'], [17, 'id17'], [18, 'id18'], [19, 'id19']], [15, 'id15'], 3), {'next': [12, 'id12'], 'page': ['id12', 'id13', 'id14']})], 4: [('siblings share the cursor timestamp', ([[100, 'a'], [100, 'd'], [100, 'b'], [100, 'c'], [94, 'z']], [100, 'c'], 5), {'next': None, 'page': ['z', 'a', 'b']}), ('equal timestamps across a page edge', ([[50, 'b'], [50, 'Q'], [50, 'a'], [40, 'x4']], None, 2), {'next': [50, 'a'], 'page': ['a', 'b']}), ('first page', ([[1, 'm01'], [2, 'm02'], [3, 'm03'], [4, 'm04'], [5, 'm05'], [6, 'm06'], [7, 'm07'], [8, 'm08'], [9, 'm09'], [10, 'm10'], [11, 'm11']], None, 3), {'next': [9, 'm09'], 'page': ['m09', 'm10', 'm11']}), ('exact multiple', ([[1, 'a'], [2, 'b'], [3, 'c'], [4, 'd']], [5, 'e'], 4), {'next': None, 'page': ['a', 'b', 'c', 'd']}), ('zero limit', ([[1, 'a'], [2, 'b']], None, 0), {'next': None, 'page': []}), ('middle page', ([[10, 'id10'], [11, 'id11'], [12, 'id12'], [13, 'id13'], [14, 'id14'], [15, 'id15'], [16, 'id16'], [17, 'id17'], [18, 'id18'], [19, 'id19']], [15, 'id15'], 2), {'next': [13, 'id13'], 'page': ['id13', 'id14']})], 5: [('siblings share the cursor timestamp', ([[100, 'a'], [100, 'd'], [100, 'b'], [100, 'c'], [95, 'z']], [100, 'c'], 5), {'next': None, 'page': ['z', 'a', 'b']}), ('equal timestamps across a page edge', ([[50, 'b'], [50, 'Q'], [50, 'a'], [40, 'x5']], None, 2), {'next': [50, 'a'], 'page': ['a', 'b']}), ('first page', ([[1, 'm01'], [2, 'm02'], [3, 'm03'], [4, 'm04'], [5, 'm05'], [6, 'm06'], [7, 'm07'], [8, 'm08'], [9, 'm09'], [10, 'm10'], [11, 'm11'], [12, 'm12']], None, 3), {'next': [10, 'm10'], 'page': ['m10', 'm11', 'm12']}), ('exact multiple', ([[1, 'a'], [2, 'b'], [3, 'c'], [4, 'd']], [5, 'e'], 4), {'next': None, 'page': ['a', 'b', 'c', 'd']}), ('zero limit', ([[1, 'a'], [2, 'b']], None, 0), {'next': None, 'page': []}), ('middle page', ([[10, 'id10'], [11, 'id11'], [12, 'id12'], [13, 'id13'], [14, 'id14'], [15, 'id15'], [16, 'id16'], [17, 'id17'], [18, 'id18'], [19, 'id19']], [15, 'id15'], 3), {'next': [12, 'id12'], 'page': ['id12', 'id13', 'id14']})]}
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
siblings share the cursor timestamp	`{'next': None, 'page': ['z', 'a', 'b']}`	`{'next': None, 'page': ['z', 'a', 'b']}`	Passed
equal timestamps across a page edge	`{'next': [50, 'a'], 'page': ['a', 'b']}`	`{'next': [50, 'a'], 'page': ['a', 'b']}`	Passed
first page	`{'next': [6, 'm06'], 'page': ['m06', 'm07', 'm08']}`	`{'next': [6, 'm06'], 'page': ['m06', 'm07', 'm08']}`	Passed
exact multiple	`{'next': [1, 'a'], 'page': ['a', 'b', 'c', 'd']}`	`{'next': None, 'page': ['a', 'b', 'c', 'd']}`	Failed
zero limit	`{'next': None, 'page': []}`	`{'next': None, 'page': []}`	Passed
middle page	`{'next': [12, 'id12'], 'page': ['id12', 'id13', 'id14']}`	`{'next': [12, 'id12'], 'page': ['id12', 'id13', 'id14']}`	Passed

SHA-256 / d83b091fdba91c5e215c864999f801edf5daac15bf9ad5edbce50744c6af6fa7

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(messages, cursor, limit):
    rows = sorted(messages, key=lambda m: (m[0], m[1]))
    if cursor is not None:
        rows = [m for m in rows if (m[0], m[1]) < (cursor[0], cursor[1])]
    page = rows[-limit:] if limit > 0 else []
    more = len(page) >= limit and len(rows) > 0
    return {'page': [m[1] for m in page], 'next': [page[0][0], page[0][1]] if more and page else None}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('siblings share the cursor timestamp', ([[100, 'a'], [100, 'd'], [100, 'b'], [100, 'c'], [91, 'z']], [100, 'c'], 5), {'next': None, 'page': ['z', 'a', 'b']}), ('equal timestamps across a page edge', ([[50, 'b'], [50, 'Q'], [50, 'a'], [40, 'x1']], None, 2), {'next': [50, 'a'], 'page': ['a', 'b']}), ('first page', ([[1, 'm01'], [2, 'm02'], [3, 'm03'], [4, 'm04'], [5, 'm05'], [6, 'm06'], [7, 'm07'], [8, 'm08']], None, 3), {'next': [6, 'm06'], 'page': ['m06', 'm07', 'm08']}), ('exact multiple', ([[1, 'a'], [2, 'b'], [3, 'c'], [4, 'd']], [5, 'e'], 4), {'next': None, 'page': ['a', 'b', 'c', 'd']}), ('zero limit', ([[1, 'a'], [2, 'b']], None, 0), {'next': None, 'page': []}), ('middle page', ([[10, 'id10'], [11, 'id11'], [12, 'id12'], [13, 'id13'], [14, 'id14'], [15, 'id15'], [16, 'id16'], [17, 'id17'], [18, 'id18'], [19, 'id19']], [15, 'id15'], 3), {'next': [12, 'id12'], 'page': ['id12', 'id13', 'id14']})], 2: [('siblings share the cursor timestamp', ([[100, 'a'], [100, 'd'], [100, 'b'], [100, 'c'], [92, 'z']], [100, 'c'], 5), {'next': None, 'page': ['z', 'a', 'b']}), ('equal timestamps across a page edge', ([[50, 'b'], [50, 'Q'], [50, 'a'], [40, 'x2']], None, 2), {'next': [50, 'a'], 'page': ['a', 'b']}), ('first page', ([[1, 'm01'], [2, 'm02'], [3, 'm03'], [4, 'm04'], [5, 'm05'], [6, 'm06'], [7, 'm07'], [8, 'm08'], [9, 'm09']], None, 3), {'next': [7, 'm07'], 'page': ['m07', 'm08', 'm09']}), ('exact multiple', ([[1, 'a'], [2, 'b'], [3, 'c'], [4, 'd']], [5, 'e'], 4), {'next': None, 'page': ['a', 'b', 'c', 'd']}), ('zero limit', ([[1, 'a'], [2, 'b']], None, 0), {'next': None, 'page': []}), ('middle page', ([[10, 'id10'], [11, 'id11'], [12, 'id12'], [13, 'id13'], [14, 'id14'], [15, 'id15'], [16, 'id16'], [17, 'id17'], [18, 'id18'], [19, 'id19']], [15, 'id15'], 2), {'next': [13, 'id13'], 'page': ['id13', 'id14']})], 3: [('siblings share the cursor timestamp', ([[100, 'a'], [100, 'd'], [100, 'b'], [100, 'c'], [93, 'z']], [100, 'c'], 5), {'next': None, 'page': ['z', 'a', 'b']}), ('equal timestamps across a page edge', ([[50, 'b'], [50, 'Q'], [50, 'a'], [40, 'x3']], None, 2), {'next': [50, 'a'], 'page': ['a', 'b']}), ('first page', ([[1, 'm01'], [2, 'm02'], [3, 'm03'], [4, 'm04'], [5, 'm05'], [6, 'm06'], [7, 'm07'], [8, 'm08'], [9, 'm09'], [10, 'm10']], None, 3), {'next': [8, 'm08'], 'page': ['m08', 'm09', 'm10']}), ('exact multiple', ([[1, 'a'], [2, 'b'], [3, 'c'], [4, 'd']], [5, 'e'], 4), {'next': None, 'page': ['a', 'b', 'c', 'd']}), ('zero limit', ([[1, 'a'], [2, 'b']], None, 0), {'next': None, 'page': []}), ('middle page', ([[10, 'id10'], [11, 'id11'], [12, 'id12'], [13, 'id13'], [14, 'id14'], [15, 'id15'], [16, 'id16'], [17, 'id17'], [18, 'id18'], [19, 'id19']], [15, 'id15'], 3), {'next': [12, 'id12'], 'page': ['id12', 'id13', 'id14']})], 4: [('siblings share the cursor timestamp', ([[100, 'a'], [100, 'd'], [100, 'b'], [100, 'c'], [94, 'z']], [100, 'c'], 5), {'next': None, 'page': ['z', 'a', 'b']}), ('equal timestamps across a page edge', ([[50, 'b'], [50, 'Q'], [50, 'a'], [40, 'x4']], None, 2), {'next': [50, 'a'], 'page': ['a', 'b']}), ('first page', ([[1, 'm01'], [2, 'm02'], [3, 'm03'], [4, 'm04'], [5, 'm05'], [6, 'm06'], [7, 'm07'], [8, 'm08'], [9, 'm09'], [10, 'm10'], [11, 'm11']], None, 3), {'next': [9, 'm09'], 'page': ['m09', 'm10', 'm11']}), ('exact multiple', ([[1, 'a'], [2, 'b'], [3, 'c'], [4, 'd']], [5, 'e'], 4), {'next': None, 'page': ['a', 'b', 'c', 'd']}), ('zero limit', ([[1, 'a'], [2, 'b']], None, 0), {'next': None, 'page': []}), ('middle page', ([[10, 'id10'], [11, 'id11'], [12, 'id12'], [13, 'id13'], [14, 'id14'], [15, 'id15'], [16, 'id16'], [17, 'id17'], [18, 'id18'], [19, 'id19']], [15, 'id15'], 2), {'next': [13, 'id13'], 'page': ['id13', 'id14']})], 5: [('siblings share the cursor timestamp', ([[100, 'a'], [100, 'd'], [100, 'b'], [100, 'c'], [95, 'z']], [100, 'c'], 5), {'next': None, 'page': ['z', 'a', 'b']}), ('equal timestamps across a page edge', ([[50, 'b'], [50, 'Q'], [50, 'a'], [40, 'x5']], None, 2), {'next': [50, 'a'], 'page': ['a', 'b']}), ('first page', ([[1, 'm01'], [2, 'm02'], [3, 'm03'], [4, 'm04'], [5, 'm05'], [6, 'm06'], [7, 'm07'], [8, 'm08'], [9, 'm09'], [10, 'm10'], [11, 'm11'], [12, 'm12']], None, 3), {'next': [10, 'm10'], 'page': ['m10', 'm11', 'm12']}), ('exact multiple', ([[1, 'a'], [2, 'b'], [3, 'c'], [4, 'd']], [5, 'e'], 4), {'next': None, 'page': ['a', 'b', 'c', 'd']}), ('zero limit', ([[1, 'a'], [2, 'b']], None, 0), {'next': None, 'page': []}), ('middle page', ([[10, 'id10'], [11, 'id11'], [12, 'id12'], [13, 'id13'], [14, 'id14'], [15, 'id15'], [16, 'id16'], [17, 'id17'], [18, 'id18'], [19, 'id19']], [15, 'id15'], 3), {'next': [12, 'id12'], 'page': ['id12', 'id13', 'id14']})]}
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
siblings share the cursor timestamp	`{'next': None, 'page': ['z', 'a', 'b']}`	`{'next': None, 'page': ['z', 'a', 'b']}`	Passed
equal timestamps across a page edge	`{'next': [50, 'a'], 'page': ['a', 'b']}`	`{'next': [50, 'a'], 'page': ['a', 'b']}`	Passed
first page	`{'next': [6, 'm06'], 'page': ['m06', 'm07', 'm08']}`	`{'next': [6, 'm06'], 'page': ['m06', 'm07', 'm08']}`	Passed
exact multiple	`{'next': [1, 'a'], 'page': ['a', 'b', 'c', 'd']}`	`{'next': None, 'page': ['a', 'b', 'c', 'd']}`	Failed
zero limit	`{'next': None, 'page': []}`	`{'next': None, 'page': []}`	Passed
middle page	`{'next': [12, 'id12'], 'page': ['id12', 'id13', 'id14']}`	`{'next': [12, 'id12'], 'page': ['id12', 'id13', 'id14']}`	Passed

SHA-256 / 117982227f15d4eab7b52ae0873906fb48cf3ee6dfb315ae593c9b609186d1c6

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 6 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

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 2026-09-29T14:49:12.867271+00:00.

Case digest / d688026693394b9b3d9ea83f8566f206dd0e54391322bae39fc2717a7874d748