FA-76216 / Chat ordering and read receipts / Open access
Batch outgoing read receipts per conversation: background defer · case 01
Receipts are sent while the app is in the background, exposing activity the user has not seen.
ROOT CAUSE
The background defer decision evaluates `if not foreground and not rows:` where the contract requires `if not foreground:`.
VERIFIED REPAIR
Use `if not foreground:` for the background defer decision and keep every other rule of the model unchanged.
Unsuccessful approach: Deferring only multi-conversation batches still sends a single background receipt. The attempted `if not foreground and len(rows) > 1:` still disagrees with a fixture.
Case contract
pending_reads are [conv, seq] events produced while scrolling, in any order. Per conversation only the highest seq is sent, and only if it is above the marker already sent (missing marker means nothing sent, -1). In the background nothing is sent: the eligible conversations are deferred. Result {send: [[conv, seq]...] sorted by conv, defer: [conv...]}.
Why this case matters
Receipt batching limits traffic; batching errors send stale receipts or none at all.
1 / The failure
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(pending_reads, sent_markers, foreground):
best = {}
for conv, seq in pending_reads:
if seq > best.get(conv, -1):
best[conv] = seq
rows = [[c, s] for c, s in sorted(best.items()) if s > sent_markers.get(c, -1)]
if not foreground and not rows:
return {'send': [], 'defer': [c for c, _ in rows]}
return {'send': rows, 'defer': []}
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('scroll events out of order', ([['a', 10], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 10], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 8]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 8]]}), ('lower than sent', ([['a', 2]], {'a': 7}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 2]], {'b': 2}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 2: [('scroll events out of order', ([['a', 11], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 11], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 9]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 9]]}), ('lower than sent', ([['a', 2]], {'a': 8}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 3]], {'b': 3}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 3: [('scroll events out of order', ([['a', 12], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 12], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 10]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 10]]}), ('lower than sent', ([['a', 2]], {'a': 9}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 4]], {'b': 4}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 4: [('scroll events out of order', ([['a', 13], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 13], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 11]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 11]]}), ('lower than sent', ([['a', 2]], {'a': 10}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 5]], {'b': 5}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 5: [('scroll events out of order', ([['a', 14], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 14], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 12]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 12]]}), ('lower than sent', ([['a', 2]], {'a': 11}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 6]], {'b': 6}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})]}
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 |
|---|---|---|---|
| scroll events out of order | {'defer': [], 'send': [['a', 10], ['b', 2]]} | {'defer': [], 'send': [['a', 10], ['b', 2]]} | Passed |
| first message has seq zero | {'defer': [], 'send': [['n', 0]]} | {'defer': [], 'send': [['n', 0]]} | Passed |
| not advancing | {'defer': [], 'send': [['b', 8]]} | {'defer': [], 'send': [['b', 8]]} | Passed |
| lower than sent | {'defer': [], 'send': []} | {'defer': [], 'send': []} | Passed |
| background defers | {'defer': [], 'send': [['a', 3]]} | {'defer': ['a'], 'send': []} | Failed |
| empty | {'defer': [], 'send': []} | {'defer': [], 'send': []} | Passed |
SHA-256 / 1ecc591cd5e8ee495786e1fd53447317a3f21c438ceebc9447292bbb54f2c66b
2 / The unsuccessful fix
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(pending_reads, sent_markers, foreground):
best = {}
for conv, seq in pending_reads:
if seq > best.get(conv, -1):
best[conv] = seq
rows = [[c, s] for c, s in sorted(best.items()) if s > sent_markers.get(c, -1)]
if not foreground and len(rows) > 1:
return {'send': [], 'defer': [c for c, _ in rows]}
return {'send': rows, 'defer': []}
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('scroll events out of order', ([['a', 10], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 10], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 8]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 8]]}), ('lower than sent', ([['a', 2]], {'a': 7}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 2]], {'b': 2}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 2: [('scroll events out of order', ([['a', 11], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 11], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 9]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 9]]}), ('lower than sent', ([['a', 2]], {'a': 8}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 3]], {'b': 3}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 3: [('scroll events out of order', ([['a', 12], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 12], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 10]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 10]]}), ('lower than sent', ([['a', 2]], {'a': 9}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 4]], {'b': 4}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 4: [('scroll events out of order', ([['a', 13], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 13], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 11]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 11]]}), ('lower than sent', ([['a', 2]], {'a': 10}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 5]], {'b': 5}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 5: [('scroll events out of order', ([['a', 14], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 14], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 12]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 12]]}), ('lower than sent', ([['a', 2]], {'a': 11}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 6]], {'b': 6}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})]}
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 |
|---|---|---|---|
| scroll events out of order | {'defer': [], 'send': [['a', 10], ['b', 2]]} | {'defer': [], 'send': [['a', 10], ['b', 2]]} | Passed |
| first message has seq zero | {'defer': [], 'send': [['n', 0]]} | {'defer': [], 'send': [['n', 0]]} | Passed |
| not advancing | {'defer': [], 'send': [['b', 8]]} | {'defer': [], 'send': [['b', 8]]} | Passed |
| lower than sent | {'defer': [], 'send': []} | {'defer': [], 'send': []} | Passed |
| background defers | {'defer': [], 'send': [['a', 3]]} | {'defer': ['a'], 'send': []} | Failed |
| empty | {'defer': [], 'send': []} | {'defer': [], 'send': []} | Passed |
SHA-256 / 0683f72206ba31e91cc2bbfef8e2a633cf1a320ab38518db85b33f54fcba4d70
3 / The verified repair
Exit 0"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(pending_reads, sent_markers, foreground):
best = {}
for conv, seq in pending_reads:
if seq > best.get(conv, -1):
best[conv] = seq
rows = [[c, s] for c, s in sorted(best.items()) if s > sent_markers.get(c, -1)]
if not foreground:
return {'send': [], 'defer': [c for c, _ in rows]}
return {'send': rows, 'defer': []}
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('scroll events out of order', ([['a', 10], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 10], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 8]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 8]]}), ('lower than sent', ([['a', 2]], {'a': 7}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 2]], {'b': 2}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 2: [('scroll events out of order', ([['a', 11], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 11], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 9]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 9]]}), ('lower than sent', ([['a', 2]], {'a': 8}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 3]], {'b': 3}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 3: [('scroll events out of order', ([['a', 12], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 12], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 10]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 10]]}), ('lower than sent', ([['a', 2]], {'a': 9}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 4]], {'b': 4}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 4: [('scroll events out of order', ([['a', 13], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 13], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 11]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 11]]}), ('lower than sent', ([['a', 2]], {'a': 10}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 5]], {'b': 5}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})], 5: [('scroll events out of order', ([['a', 14], ['a', 4], ['b', 2]], {}, True), {'defer': [], 'send': [['a', 14], ['b', 2]]}), ('first message has seq zero', ([['n', 0]], {}, True), {'defer': [], 'send': [['n', 0]]}), ('not advancing', ([['a', 5], ['b', 12]], {'a': 5, 'b': 3}, True), {'defer': [], 'send': [['b', 12]]}), ('lower than sent', ([['a', 2]], {'a': 11}, True), {'defer': [], 'send': []}), ('background defers', ([['a', 3], ['b', 6]], {'b': 6}, False), {'defer': ['a'], 'send': []}), ('empty', ([], {'a': 1}, True), {'defer': [], 'send': []})]}
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 |
|---|---|---|---|
| scroll events out of order | {'defer': [], 'send': [['a', 10], ['b', 2]]} | {'defer': [], 'send': [['a', 10], ['b', 2]]} | Passed |
| first message has seq zero | {'defer': [], 'send': [['n', 0]]} | {'defer': [], 'send': [['n', 0]]} | Passed |
| not advancing | {'defer': [], 'send': [['b', 8]]} | {'defer': [], 'send': [['b', 8]]} | Passed |
| lower than sent | {'defer': [], 'send': []} | {'defer': [], 'send': []} | Passed |
| background defers | {'defer': ['a'], 'send': []} | {'defer': ['a'], 'send': []} | Passed |
| empty | {'defer': [], 'send': []} | {'defer': [], 'send': []} | Passed |
SHA-256 / 0b9d0c38fb9b77cba0190af534fc4ad09f3294a1edef2cef856ee2ce6edb4015
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 2026-09-29T14:49:14.181465+00:00.
Case digest / 262b6e8e31d31962a6335d6ab0482da2b5c20df45bcd48d671197d2565a2c1db