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FA-76176 / Chat ordering and read receipts / Open access

Choose which queued sends to dispatch now: head of line · case 01

Later messages are dispatched while an earlier one is still pending, so they arrive out of order.

Verified by executionVariant 1 · 7 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The head of line decision evaluates `out.append(m['nonce'])` where the contract requires `out.append(m['nonce']) break`.

THE FAILURE

The head of line decision evaluates `out.append(m['nonce'])` where the contract requires `out.append(m['nonce']) break`.

Unsuccessful approach: Breaking only after a dispatch still sends later messages when the head is waiting for its retry time. The attempted `out.append(m['nonce']) break` still disagrees with a fixture.

Case contract

outbox items {nonce, conv, order, next_try}. Per conversation, messages are sent strictly in numeric `order`: the head is the lowest-order message that is neither acknowledged nor dead (failed attempts >= 5). Dead messages are skipped without blocking. The head is dispatched when next_try <= now; later messages always wait for it. Result: nonces to dispatch, by conversation id then order.

Why this case matters

Outboxes must preserve per-conversation order across retries without letting a dead message block the queue.

1 / The failure

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

N = 1
observations = []
def solve(outbox, acked, failed, now):
    done = set(acked)
    out = []
    by_conv = {}
    for m in outbox:
        by_conv.setdefault(m['conv'], []).append(m)
    for conv in sorted(by_conv):
        for m in sorted(by_conv[conv], key=lambda m: m['order']):
            if m['nonce'] in done:
                continue
            if failed.get(m['nonce'], 0) >= 5:
                continue
            if m['next_try'] <= now:
                out.append(m['nonce'])
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('head not yet due', ([{'nonce': 'a1', 'conv': 'c', 'order': 1, 'next_try': 101}, {'nonce': 'a2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {}, 50), []), ('dead head skipped', ([{'nonce': 'd1', 'conv': 'c', 'order': 1, 'next_try': 999}, {'nonce': 'd2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {'d1': 6}, 10), ['d2']), ('due exactly now', ([{'nonce': 'e1', 'conv': 'k', 'order': 3, 'next_try': 41}], [], {}, 41), ['e1']), ('one tick early', ([{'nonce': 'f1', 'conv': 'k', 'order': 1, 'next_try': 41}], [], {}, 40), []), ('acked head', ([{'nonce': 'g1', 'conv': 'c', 'order': 2, 'next_try': 0}, {'nonce': 'g2', 'conv': 'c', 'order': 3, 'next_try': 0}], ['g1'], {}, 5), ['g2']), ('numeric order beyond nine', ([{'nonce': 'h10', 'conv': 'z', 'order': 10, 'next_try': 0}, {'nonce': 'h9', 'conv': 'z', 'order': 9, 'next_try': 0}], [], {}, 1), ['h9']), ('two conversations', ([{'nonce': 'q', 'conv': 'b', 'order': 1, 'next_try': 0}, {'nonce': 'p', 'conv': 'a', 'order': 1, 'next_try': 0}, {'nonce': 'r', 'conv': 'a', 'order': 2, 'next_try': 0}], [], {}, 1), ['p', 'q'])], 2: [('head not yet due', ([{'nonce': 'a1', 'conv': 'c', 'order': 1, 'next_try': 102}, {'nonce': 'a2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {}, 50), []), ('dead head skipped', ([{'nonce': 'd1', 'conv': 'c', 'order': 1, 'next_try': 999}, {'nonce': 'd2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {'d1': 5}, 10), ['d2']), ('due exactly now', ([{'nonce': 'e1', 'conv': 'k', 'order': 3, 'next_try': 42}], [], {}, 42), ['e1']), ('one tick early', ([{'nonce': 'f1', 'conv': 'k', 'order': 1, 'next_try': 41}], [], {}, 40), []), ('acked head', ([{'nonce': 'g1', 'conv': 'c', 'order': 2, 'next_try': 0}, {'nonce': 'g2', 'conv': 'c', 'order': 3, 'next_try': 0}], ['g1'], {}, 5), ['g2']), ('numeric order beyond nine', ([{'nonce': 'h10', 'conv': 'z', 'order': 10, 'next_try': 0}, {'nonce': 'h9', 'conv': 'z', 'order': 9, 'next_try': 0}], [], {}, 1), ['h9']), ('two conversations', ([{'nonce': 'q', 'conv': 'b', 'order': 1, 'next_try': 0}, {'nonce': 'p', 'conv': 'a', 'order': 1, 'next_try': 0}, {'nonce': 'r', 'conv': 'a', 'order': 2, 'next_try': 0}], [], {}, 2), ['p', 'q'])], 3: [('head not yet due', ([{'nonce': 'a1', 'conv': 'c', 'order': 1, 'next_try': 103}, {'nonce': 'a2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {}, 50), []), ('dead head skipped', ([{'nonce': 'd1', 'conv': 'c', 'order': 1, 'next_try': 999}, {'nonce': 'd2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {'d1': 6}, 10), ['d2']), ('due exactly now', ([{'nonce': 'e1', 'conv': 'k', 'order': 3, 'next_try': 43}], [], {}, 43), ['e1']), ('one tick early', ([{'nonce': 'f1', 'conv': 'k', 'order': 1, 'next_try': 41}], [], {}, 40), []), ('acked head', ([{'nonce': 'g1', 'conv': 'c', 'order': 2, 'next_try': 0}, {'nonce': 'g2', 'conv': 'c', 'order': 3, 'next_try': 0}], ['g1'], {}, 5), ['g2']), ('numeric order beyond nine', ([{'nonce': 'h10', 'conv': 'z', 'order': 10, 'next_try': 0}, {'nonce': 'h9', 'conv': 'z', 'order': 9, 'next_try': 0}], [], {}, 1), ['h9']), ('two conversations', ([{'nonce': 'q', 'conv': 'b', 'order': 1, 'next_try': 0}, {'nonce': 'p', 'conv': 'a', 'order': 1, 'next_try': 0}, {'nonce': 'r', 'conv': 'a', 'order': 2, 'next_try': 0}], [], {}, 3), ['p', 'q'])], 4: [('head not yet due', ([{'nonce': 'a1', 'conv': 'c', 'order': 1, 'next_try': 104}, {'nonce': 'a2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {}, 50), []), ('dead head skipped', ([{'nonce': 'd1', 'conv': 'c', 'order': 1, 'next_try': 999}, {'nonce': 'd2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {'d1': 5}, 10), ['d2']), ('due exactly now', ([{'nonce': 'e1', 'conv': 'k', 'order': 3, 'next_try': 44}], [], {}, 44), ['e1']), ('one tick early', ([{'nonce': 'f1', 'conv': 'k', 'order': 1, 'next_try': 41}], [], {}, 40), []), ('acked head', ([{'nonce': 'g1', 'conv': 'c', 'order': 2, 'next_try': 0}, {'nonce': 'g2', 'conv': 'c', 'order': 3, 'next_try': 0}], ['g1'], {}, 5), ['g2']), ('numeric order beyond nine', ([{'nonce': 'h10', 'conv': 'z', 'order': 10, 'next_try': 0}, {'nonce': 'h9', 'conv': 'z', 'order': 9, 'next_try': 0}], [], {}, 1), ['h9']), ('two conversations', ([{'nonce': 'q', 'conv': 'b', 'order': 1, 'next_try': 0}, {'nonce': 'p', 'conv': 'a', 'order': 1, 'next_try': 0}, {'nonce': 'r', 'conv': 'a', 'order': 2, 'next_try': 0}], [], {}, 4), ['p', 'q'])], 5: [('head not yet due', ([{'nonce': 'a1', 'conv': 'c', 'order': 1, 'next_try': 105}, {'nonce': 'a2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {}, 50), []), ('dead head skipped', ([{'nonce': 'd1', 'conv': 'c', 'order': 1, 'next_try': 999}, {'nonce': 'd2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {'d1': 6}, 10), ['d2']), ('due exactly now', ([{'nonce': 'e1', 'conv': 'k', 'order': 3, 'next_try': 45}], [], {}, 45), ['e1']), ('one tick early', ([{'nonce': 'f1', 'conv': 'k', 'order': 1, 'next_try': 41}], [], {}, 40), []), ('acked head', ([{'nonce': 'g1', 'conv': 'c', 'order': 2, 'next_try': 0}, {'nonce': 'g2', 'conv': 'c', 'order': 3, 'next_try': 0}], ['g1'], {}, 5), ['g2']), ('numeric order beyond nine', ([{'nonce': 'h10', 'conv': 'z', 'order': 10, 'next_try': 0}, {'nonce': 'h9', 'conv': 'z', 'order': 9, 'next_try': 0}], [], {}, 1), ['h9']), ('two conversations', ([{'nonce': 'q', 'conv': 'b', 'order': 1, 'next_try': 0}, {'nonce': 'p', 'conv': 'a', 'order': 1, 'next_try': 0}, {'nonce': 'r', 'conv': 'a', 'order': 2, 'next_try': 0}], [], {}, 5), ['p', 'q'])]}
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 fixtureActualExpectedOutcome
head not yet due['a2'][]Failed
dead head skipped['d2']['d2']Passed
due exactly now['e1']['e1']Passed
one tick early[][]Passed
acked head['g2']['g2']Passed
numeric order beyond nine['h9', 'h10']['h9']Failed
two conversations['p', 'r', 'q']['p', 'q']Failed

SHA-256 / a03de46709fc58d52d6a24ec1bb125e0e8df0cdfedb21cfb1c6f8e79c4d52bc5

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(outbox, acked, failed, now):
    done = set(acked)
    out = []
    by_conv = {}
    for m in outbox:
        by_conv.setdefault(m['conv'], []).append(m)
    for conv in sorted(by_conv):
        for m in sorted(by_conv[conv], key=lambda m: m['order']):
            if m['nonce'] in done:
                continue
            if failed.get(m['nonce'], 0) >= 5:
                continue
            if m['next_try'] <= now:
                out.append(m['nonce'])
                break
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('head not yet due', ([{'nonce': 'a1', 'conv': 'c', 'order': 1, 'next_try': 101}, {'nonce': 'a2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {}, 50), []), ('dead head skipped', ([{'nonce': 'd1', 'conv': 'c', 'order': 1, 'next_try': 999}, {'nonce': 'd2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {'d1': 6}, 10), ['d2']), ('due exactly now', ([{'nonce': 'e1', 'conv': 'k', 'order': 3, 'next_try': 41}], [], {}, 41), ['e1']), ('one tick early', ([{'nonce': 'f1', 'conv': 'k', 'order': 1, 'next_try': 41}], [], {}, 40), []), ('acked head', ([{'nonce': 'g1', 'conv': 'c', 'order': 2, 'next_try': 0}, {'nonce': 'g2', 'conv': 'c', 'order': 3, 'next_try': 0}], ['g1'], {}, 5), ['g2']), ('numeric order beyond nine', ([{'nonce': 'h10', 'conv': 'z', 'order': 10, 'next_try': 0}, {'nonce': 'h9', 'conv': 'z', 'order': 9, 'next_try': 0}], [], {}, 1), ['h9']), ('two conversations', ([{'nonce': 'q', 'conv': 'b', 'order': 1, 'next_try': 0}, {'nonce': 'p', 'conv': 'a', 'order': 1, 'next_try': 0}, {'nonce': 'r', 'conv': 'a', 'order': 2, 'next_try': 0}], [], {}, 1), ['p', 'q'])], 2: [('head not yet due', ([{'nonce': 'a1', 'conv': 'c', 'order': 1, 'next_try': 102}, {'nonce': 'a2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {}, 50), []), ('dead head skipped', ([{'nonce': 'd1', 'conv': 'c', 'order': 1, 'next_try': 999}, {'nonce': 'd2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {'d1': 5}, 10), ['d2']), ('due exactly now', ([{'nonce': 'e1', 'conv': 'k', 'order': 3, 'next_try': 42}], [], {}, 42), ['e1']), ('one tick early', ([{'nonce': 'f1', 'conv': 'k', 'order': 1, 'next_try': 41}], [], {}, 40), []), ('acked head', ([{'nonce': 'g1', 'conv': 'c', 'order': 2, 'next_try': 0}, {'nonce': 'g2', 'conv': 'c', 'order': 3, 'next_try': 0}], ['g1'], {}, 5), ['g2']), ('numeric order beyond nine', ([{'nonce': 'h10', 'conv': 'z', 'order': 10, 'next_try': 0}, {'nonce': 'h9', 'conv': 'z', 'order': 9, 'next_try': 0}], [], {}, 1), ['h9']), ('two conversations', ([{'nonce': 'q', 'conv': 'b', 'order': 1, 'next_try': 0}, {'nonce': 'p', 'conv': 'a', 'order': 1, 'next_try': 0}, {'nonce': 'r', 'conv': 'a', 'order': 2, 'next_try': 0}], [], {}, 2), ['p', 'q'])], 3: [('head not yet due', ([{'nonce': 'a1', 'conv': 'c', 'order': 1, 'next_try': 103}, {'nonce': 'a2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {}, 50), []), ('dead head skipped', ([{'nonce': 'd1', 'conv': 'c', 'order': 1, 'next_try': 999}, {'nonce': 'd2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {'d1': 6}, 10), ['d2']), ('due exactly now', ([{'nonce': 'e1', 'conv': 'k', 'order': 3, 'next_try': 43}], [], {}, 43), ['e1']), ('one tick early', ([{'nonce': 'f1', 'conv': 'k', 'order': 1, 'next_try': 41}], [], {}, 40), []), ('acked head', ([{'nonce': 'g1', 'conv': 'c', 'order': 2, 'next_try': 0}, {'nonce': 'g2', 'conv': 'c', 'order': 3, 'next_try': 0}], ['g1'], {}, 5), ['g2']), ('numeric order beyond nine', ([{'nonce': 'h10', 'conv': 'z', 'order': 10, 'next_try': 0}, {'nonce': 'h9', 'conv': 'z', 'order': 9, 'next_try': 0}], [], {}, 1), ['h9']), ('two conversations', ([{'nonce': 'q', 'conv': 'b', 'order': 1, 'next_try': 0}, {'nonce': 'p', 'conv': 'a', 'order': 1, 'next_try': 0}, {'nonce': 'r', 'conv': 'a', 'order': 2, 'next_try': 0}], [], {}, 3), ['p', 'q'])], 4: [('head not yet due', ([{'nonce': 'a1', 'conv': 'c', 'order': 1, 'next_try': 104}, {'nonce': 'a2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {}, 50), []), ('dead head skipped', ([{'nonce': 'd1', 'conv': 'c', 'order': 1, 'next_try': 999}, {'nonce': 'd2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {'d1': 5}, 10), ['d2']), ('due exactly now', ([{'nonce': 'e1', 'conv': 'k', 'order': 3, 'next_try': 44}], [], {}, 44), ['e1']), ('one tick early', ([{'nonce': 'f1', 'conv': 'k', 'order': 1, 'next_try': 41}], [], {}, 40), []), ('acked head', ([{'nonce': 'g1', 'conv': 'c', 'order': 2, 'next_try': 0}, {'nonce': 'g2', 'conv': 'c', 'order': 3, 'next_try': 0}], ['g1'], {}, 5), ['g2']), ('numeric order beyond nine', ([{'nonce': 'h10', 'conv': 'z', 'order': 10, 'next_try': 0}, {'nonce': 'h9', 'conv': 'z', 'order': 9, 'next_try': 0}], [], {}, 1), ['h9']), ('two conversations', ([{'nonce': 'q', 'conv': 'b', 'order': 1, 'next_try': 0}, {'nonce': 'p', 'conv': 'a', 'order': 1, 'next_try': 0}, {'nonce': 'r', 'conv': 'a', 'order': 2, 'next_try': 0}], [], {}, 4), ['p', 'q'])], 5: [('head not yet due', ([{'nonce': 'a1', 'conv': 'c', 'order': 1, 'next_try': 105}, {'nonce': 'a2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {}, 50), []), ('dead head skipped', ([{'nonce': 'd1', 'conv': 'c', 'order': 1, 'next_try': 999}, {'nonce': 'd2', 'conv': 'c', 'order': 2, 'next_try': 0}], [], {'d1': 6}, 10), ['d2']), ('due exactly now', ([{'nonce': 'e1', 'conv': 'k', 'order': 3, 'next_try': 45}], [], {}, 45), ['e1']), ('one tick early', ([{'nonce': 'f1', 'conv': 'k', 'order': 1, 'next_try': 41}], [], {}, 40), []), ('acked head', ([{'nonce': 'g1', 'conv': 'c', 'order': 2, 'next_try': 0}, {'nonce': 'g2', 'conv': 'c', 'order': 3, 'next_try': 0}], ['g1'], {}, 5), ['g2']), ('numeric order beyond nine', ([{'nonce': 'h10', 'conv': 'z', 'order': 10, 'next_try': 0}, {'nonce': 'h9', 'conv': 'z', 'order': 9, 'next_try': 0}], [], {}, 1), ['h9']), ('two conversations', ([{'nonce': 'q', 'conv': 'b', 'order': 1, 'next_try': 0}, {'nonce': 'p', 'conv': 'a', 'order': 1, 'next_try': 0}, {'nonce': 'r', 'conv': 'a', 'order': 2, 'next_try': 0}], [], {}, 5), ['p', 'q'])]}
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 fixtureActualExpectedOutcome
head not yet due['a2'][]Failed
dead head skipped['d2']['d2']Passed
due exactly now['e1']['e1']Passed
one tick early[][]Passed
acked head['g2']['g2']Passed
numeric order beyond nine['h9']['h9']Passed
two conversations['p', 'q']['p', 'q']Passed

SHA-256 / 51f6593120b2493036ffa1eb11b77284437f293e9dac03a057d45fb1eb9b5bf4

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This mechanism has 7 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.

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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 2026-09-29T14:49:14.051003+00:00.

Case digest / 1d5d95c54920c5327b2b244b8698b4db22e37f816d9a453565de6d1757e1f589