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FA-45946 / Bounded deques / Open access

Unlink retains links on the removed intrusive node · case 01

Unlink retains links on the removed intrusive node.

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

ROOT CAUSE

The detached node remains traversable and can falsely appear owned.

THE FAILURE

The detached node remains traversable and can falsely appear owned.

Unsuccessful approach: The partial repair still applies the incorrect transition to an admitted boundary or multi-element case.

Case contract

Unlink one selected node from a finite doubly linked deque with sentinel S. Nodes are keyed by identity; return forward IDs, backward IDs, removed node links, size, sentinel endpoint links and reciprocal-link validity. Every input ID is unique.

Why this case matters

Controlled bounded deque implementation model with explicit storage and lifecycle observations.

1 / The failure

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

N = 1
observations = []
def solve(x):
    ids, victim = x
    nodes = {'S': ['S','S']}
    for i, value in enumerate(ids):
        nodes[value] = [ids[i-1] if i else 'S', ids[i+1] if i+1 < len(ids) else 'S']
    if ids:
        nodes['S'] = [ids[-1],ids[0]]
    size = len(ids)
    left, right = nodes[victim]
    nodes[left][1] = right
    nodes[right][0] = left
    nodes[victim] = [left,right]
    size -= 1
    forward = []
    current = nodes['S'][1]
    for _ in range(len(ids)+1):
        if current == 'S' or current is None: break
        forward.append(current)
        current = nodes[current][1]
    backward = []
    current = nodes['S'][0]
    for _ in range(len(ids)+1):
        if current == 'S' or current is None: break
        backward.append(current)
        current = nodes[current][0]
    valid = all(nodes[nodes[v][0]][1] == v and nodes[nodes[v][1]][0] == v for v in forward if nodes[v][0] is not None and nodes[v][1] is not None)
    return [forward,backward,nodes[victim],size,nodes['S'],valid]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('middle unlink', solve([[N,N+1,N+2],N+1]), [[N,N+2],[N+2,N],[None,None],2,[N+2,N],True])
check('head unlink', solve([[N,N+1,N+2],N]), [[N+1,N+2],[N+2,N+1],[None,None],2,[N+2,N+1],True])
check('tail unlink', solve([[N,N+1,N+2],N+2]), [[N,N+1],[N+1,N],[None,None],2,[N+1,N],True])
check('only node', solve([[N],N]), [[],[],[None,None],0,["S","S"],True])
check('pair head', solve([[N,N+1],N]), [[N+1],[N+1],[None,None],1,[N+1,N+1],True])
check('long middle', solve([[N,N+1,N+2,N+3,N+4],N+2]), [[N,N+1,N+3,N+4],[N+4,N+3,N+1,N],[None,None],4,[N+4,N],True])
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
middle unlink[[1, 3], [3, 1], [1, 3], 2, [3, 1], True][[1, 3], [3, 1], [None, None], 2, [3, 1], True]Failed
head unlink[[2, 3], [3, 2], ['S', 2], 2, [3, 2], True][[2, 3], [3, 2], [None, None], 2, [3, 2], True]Failed
tail unlink[[1, 2], [2, 1], [2, 'S'], 2, [2, 1], True][[1, 2], [2, 1], [None, None], 2, [2, 1], True]Failed
only node[[], [], ['S', 'S'], 0, ['S', 'S'], True][[], [], [None, None], 0, ['S', 'S'], True]Failed
pair head[[2], [2], ['S', 2], 1, [2, 2], True][[2], [2], [None, None], 1, [2, 2], True]Failed
long middle[[1, 2, 4, 5], [5, 4, 2, 1], [2, 4], 4, [5, 1], True][[1, 2, 4, 5], [5, 4, 2, 1], [None, None], 4, [5, 1], True]Failed

SHA-256 / 671ed7577aa4f9335b374d7b19c0403b5038b3c5a3367561e179ad1de33c75d2

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(x):
    ids, victim = x
    nodes = {'S': ['S','S']}
    for i, value in enumerate(ids):
        nodes[value] = [ids[i-1] if i else 'S', ids[i+1] if i+1 < len(ids) else 'S']
    if ids:
        nodes['S'] = [ids[-1],ids[0]]
    size = len(ids)
    left, right = nodes[victim]
    nodes[left][1] = right
    nodes[right][0] = left
    nodes[victim] = [None,right]
    size -= 1
    forward = []
    current = nodes['S'][1]
    for _ in range(len(ids)+1):
        if current == 'S' or current is None: break
        forward.append(current)
        current = nodes[current][1]
    backward = []
    current = nodes['S'][0]
    for _ in range(len(ids)+1):
        if current == 'S' or current is None: break
        backward.append(current)
        current = nodes[current][0]
    valid = all(nodes[nodes[v][0]][1] == v and nodes[nodes[v][1]][0] == v for v in forward if nodes[v][0] is not None and nodes[v][1] is not None)
    return [forward,backward,nodes[victim],size,nodes['S'],valid]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('middle unlink', solve([[N,N+1,N+2],N+1]), [[N,N+2],[N+2,N],[None,None],2,[N+2,N],True])
check('head unlink', solve([[N,N+1,N+2],N]), [[N+1,N+2],[N+2,N+1],[None,None],2,[N+2,N+1],True])
check('tail unlink', solve([[N,N+1,N+2],N+2]), [[N,N+1],[N+1,N],[None,None],2,[N+1,N],True])
check('only node', solve([[N],N]), [[],[],[None,None],0,["S","S"],True])
check('pair head', solve([[N,N+1],N]), [[N+1],[N+1],[None,None],1,[N+1,N+1],True])
check('long middle', solve([[N,N+1,N+2,N+3,N+4],N+2]), [[N,N+1,N+3,N+4],[N+4,N+3,N+1,N],[None,None],4,[N+4,N],True])
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
middle unlink[[1, 3], [3, 1], [None, 3], 2, [3, 1], True][[1, 3], [3, 1], [None, None], 2, [3, 1], True]Failed
head unlink[[2, 3], [3, 2], [None, 2], 2, [3, 2], True][[2, 3], [3, 2], [None, None], 2, [3, 2], True]Failed
tail unlink[[1, 2], [2, 1], [None, 'S'], 2, [2, 1], True][[1, 2], [2, 1], [None, None], 2, [2, 1], True]Failed
only node[[], [], [None, 'S'], 0, ['S', 'S'], True][[], [], [None, None], 0, ['S', 'S'], True]Failed
pair head[[2], [2], [None, 2], 1, [2, 2], True][[2], [2], [None, None], 1, [2, 2], True]Failed
long middle[[1, 2, 4, 5], [5, 4, 2, 1], [None, 4], 4, [5, 1], True][[1, 2, 4, 5], [5, 4, 2, 1], [None, None], 4, [5, 1], True]Failed

SHA-256 / 7dcb2ff5637a71eb135448cacf848c386fb34ebfa77e8dbc96fc2119cc8f2281

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.

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Verification & scope

Offline finite deterministic model; no claim of production implementation or concurrent memory-model conformance. 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 2026-09-29T14:44:27.429374+00:00.

Case digest / c0fe95f9fde3533fa2942df866d62fcea2f53b96074d3dc8b8b9545d2cfc622f