{"abstract":"An acknowledgment from a prior assignment is accepted after a rebalance reuses the message identifier.","category":"Distributed coordination","checks":7,"contract":"The active delivery is [assignment generation,message ID]. Acks have the same shape. Return whether any acknowledgment exactly matches both fields. Other deliveries do not share this state.","contract_signature":"generation, message, acknowledgments","evaluation_group":"model-df2464e237d341a5","failed_approach":"Validating only the generation accepts acknowledgments for a different message in the same assignment.","family":"dist-ack-assignment-generation","id":"FA-091","implementations":{"attempt":{"sha256":"91a96bff2c4e467511178f12ac25f2e1ccbbc6f38bcf59de422c742e676fe701","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(generation, message, acknowledgments):\n    return any(ack_generation == generation for ack_generation, ack_message in acknowledgments)\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ng, message = N+1, 'message-'+str(N)\ncheck('prior owner late ack', solve(g, message, [[g-1, message]]), False)\ncheck('wrong message same assignment', solve(g, message, [[g, 'different']]), False)\ncheck('valid acknowledgment', solve(g, message, [[g, message]]), True)\ncheck('future generation is not this delivery', solve(g, message, [[g+1, message]]), False)\ncheck('valid among delayed noise', solve(g, message, [[g-1, message], [g, 'different'], [g, message]]), True)\ncheck('no acknowledgment yet', solve(g, message, []), False)\ncheck('invalid after valid cannot undo ack', solve(g, message, [[g, message], [g-1, message]]), True)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"broken":{"sha256":"bf7b0151932184a1d99329cf8c0becc151bfa0693f531464ed449a3746277f48","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(generation, message, acknowledgments):\n    return any(ack_message == message for ack_generation, ack_message in acknowledgments)\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ng, message = N+1, 'message-'+str(N)\ncheck('prior owner late ack', solve(g, message, [[g-1, message]]), False)\ncheck('wrong message same assignment', solve(g, message, [[g, 'different']]), False)\ncheck('valid acknowledgment', solve(g, message, [[g, message]]), True)\ncheck('future generation is not this delivery', solve(g, message, [[g+1, message]]), False)\ncheck('valid among delayed noise', solve(g, message, [[g-1, message], [g, 'different'], [g, message]]), True)\ncheck('no acknowledgment yet', solve(g, message, []), False)\ncheck('invalid after valid cannot undo ack', solve(g, message, [[g, message], [g-1, message]]), True)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"}},"limitations":" 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.","method":"Deterministic executable model with adversarial boundary fixtures.","provenance":{"created_by":"Failure Map","dependencies":"Python standard library","family":"dist-ack-assignment-generation","generated_at":"2026-09-29T14:36:50.439436+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"Models fencing of consumer acknowledgments across partition ownership changes; unlike checkpoint prefix accounting, the failure concerns ownership of a particular in-flight delivery.","root_cause":"Acknowledgment validation omits either the assignment generation or the message identity.","sha256":"2398dbbfbf600627f6786f0378f6c1849882c31577c68fabeadfdff67318fd3d","title":"A revoked consumer acknowledges a new assignment's message · case 01","variant":1,"variant_policy":"Five numbered records share a model and may reuse boundary fixtures.","verified":true,"visibility":"public","verification":{"attempt":{"elapsed_ms":34.343,"exit_code":1,"observations":[{"actual":false,"check":"prior owner late ack","expected":false,"passed":true},{"actual":true,"check":"wrong message same assignment","expected":false,"passed":false},{"actual":true,"check":"valid acknowledgment","expected":true,"passed":true},{"actual":false,"check":"future generation is not this delivery","expected":false,"passed":true},{"actual":true,"check":"valid among delayed noise","expected":true,"passed":true},{"actual":false,"check":"no acknowledgment yet","expected":false,"passed":true},{"actual":true,"check":"invalid after valid cannot undo ack","expected":true,"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"prior owner late ack\", \"actual\": false, \"expected\": false, \"passed\": true}, {\"check\": \"wrong message same assignment\", \"actual\": true, \"expected\": false, \"passed\": false}, {\"check\": \"valid acknowledgment\", \"actual\": true, \"expected\": true, \"passed\": true}, {\"check\": \"future generation is not this delivery\", \"actual\": false, \"expected\": false, \"passed\": true}, {\"check\": \"valid among delayed noise\", \"actual\": true, \"expected\": true, \"passed\": true}, {\"check\": \"no acknowledgment yet\", \"actual\": false, \"expected\": false, \"passed\": true}, {\"check\": \"invalid after valid cannot undo ack\", \"actual\": true, \"expected\": true, \"passed\": true}], \"passed\": false}\n"},"broken":{"elapsed_ms":33.893,"exit_code":1,"observations":[{"actual":true,"check":"prior owner late ack","expected":false,"passed":false},{"actual":false,"check":"wrong message same assignment","expected":false,"passed":true},{"actual":true,"check":"valid acknowledgment","expected":true,"passed":true},{"actual":true,"check":"future generation is not this delivery","expected":false,"passed":false},{"actual":true,"check":"valid among delayed noise","expected":true,"passed":true},{"actual":false,"check":"no acknowledgment yet","expected":false,"passed":true},{"actual":true,"check":"invalid after valid cannot undo ack","expected":true,"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"prior owner late ack\", \"actual\": true, \"expected\": false, \"passed\": false}, {\"check\": \"wrong message same assignment\", \"actual\": false, \"expected\": false, \"passed\": true}, {\"check\": \"valid acknowledgment\", \"actual\": true, \"expected\": true, \"passed\": true}, {\"check\": \"future generation is not this delivery\", \"actual\": true, \"expected\": false, \"passed\": false}, {\"check\": \"valid among delayed noise\", \"actual\": true, \"expected\": true, \"passed\": true}, {\"check\": \"no acknowledgment yet\", \"actual\": false, \"expected\": false, \"passed\": true}, {\"check\": \"invalid after valid cannot undo ack\", \"actual\": true, \"expected\": true, \"passed\": true}], \"passed\": false}\n"}},"member_only":{"stages":["fixed"],"fields":["implementations.fixed","verification.fixed","harness","repair"],"note":"The verified repair, its recorded checks, the repair description, and the scoring harness are available to members."}}