FA-4491 / Borrow checking / Open access

Write requires exclusive token · case 01

The operation returns a result or retained state that violates this contract: A single local object grants either any number of shared borrows or one exclusive borrow. Commands are [operation,token,value]. Reads return the value or denied; writes and acquisitions return booleans. Ownership can move only with no borrows. Only the active exclusive token can write a live object.

Verified by executionVariant 1 · 5 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Only the active exclusive token can write a live object. The broken transition violates that invariant.

THE FAILURE

Only the active exclusive token can write a live object. The broken transition violates that invariant.

Unsuccessful approach: The attempted transition changes the behavior but still violates the same invariant in at least one independent regression fixture.

Case contract

A single local object grants either any number of shared borrows or one exclusive borrow. Commands are [operation,token,value]. Reads return the value or denied; writes and acquisitions return booleans. Ownership can move only with no borrows. Only the active exclusive token can write a live object. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.

Why this case matters

A controlled local-runtime regression for collection APIs, language semantics, or ownership wrappers. Fixtures include boundary and interaction cases.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(x, y=None):
    shared=set(); exclusive=None; value=y; live=True; out=[]
    for op,k,arg in x:
        if op=='shared':
            ok=live and exclusive is None and k not in shared
            if ok: shared.add(k)
            out.append(ok)
        elif op=='exclusive':
            ok=live and not shared and exclusive is None
            if ok: exclusive=k
            out.append(ok)
        elif op=='release':
            shared.discard(k)
            if exclusive==k: exclusive=None
            out.append(True)
        elif op=='read':
            out.append(value if live and (k in shared or (exclusive is not None and k==exclusive)) else 'denied')
        elif op=='write':
            ok=live and (k in shared or k==exclusive)
            if ok: value=arg
            out.append(ok)
        elif op=='ownerwrite':
            ok=live and not shared and exclusive is None
            if ok: value=arg
            out.append(ok)
        elif op=='move':
            ok=live and not shared and exclusive is None
            if ok: live=False
            out.append(ok)
        elif op=='inspect': out.append([value,live,sorted(shared),exclusive])
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('shared readers prohibit mutation', solve([['shared', 'a', None], ['shared', 'a', None], ['shared', 'b', None], ['exclusive', 'w', None], ['write', 'a', 'bad'], ['ownerwrite', None, 'bad'], ['move', None, None], ['read', 'a', None], ['read', 'ghost', None], ['inspect', None, None]], 'initial'), [True, False, True, False, False, False, False, 'initial', 'denied', ['initial', True, ['a', 'b'], None]])
check('exclusive token protects identity', solve([['exclusive', 'w', None], ['exclusive', 'other', None], ['shared', 'a', None], ['write', 'ghost', 'bad'], ['release', 'ghost', None], ['ownerwrite', None, 'bad'], ['move', None, None], ['write', 'w', 'new'], ['read', 'w', None], ['inspect', None, None]], 'initial'), [True, False, False, False, True, False, False, True, 'new', ['new', True, [], 'w']])
check('released token loses rights', solve([['shared', 'a', None], ['release', 'a', None], ['read', 'a', None], ['exclusive', 'w', None], ['release', 'w', None], ['write', 'w', 'bad'], ['ownerwrite', None, 'new'], ['inspect', None, None]], 'initial'), [True, True, 'denied', True, True, False, True, ['new', True, [], None]])
check('moved object stays unavailable', solve([['move', None, None], ['shared', 'a', None], ['exclusive', 'w', None], ['ownerwrite', None, 'bad'], ['read', None, None], ['move', None, None], ['inspect', None, None]], 'initial'), [True, False, False, False, 'denied', False, ['initial', False, [], None]])
check('None is not a borrowed token', solve([['read',None,None]], 'initial'), ['denied'])
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
shared readers prohibit mutation	`[True, False, True, False, True, False, False, 'bad', 'denied', ['bad', True, ['a', 'b'], None]]`	`[True, False, True, False, False, False, False, 'initial', 'denied', ['initial', True, ['a', 'b'], None]]`	Failed
exclusive token protects identity	`[True, False, False, False, True, False, False, True, 'new', ['new', True, [], 'w']]`	`[True, False, False, False, True, False, False, True, 'new', ['new', True, [], 'w']]`	Passed
released token loses rights	`[True, True, 'denied', True, True, False, True, ['new', True, [], None]]`	`[True, True, 'denied', True, True, False, True, ['new', True, [], None]]`	Passed
moved object stays unavailable	`[True, False, False, False, 'denied', False, ['initial', False, [], None]]`	`[True, False, False, False, 'denied', False, ['initial', False, [], None]]`	Passed
None is not a borrowed token	`['denied']`	`['denied']`	Passed

SHA-256 / 785e2107f88691c52f92d16726d8975f476a12e4ce1716a81443f086924df23f

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(x, y=None):
    shared=set(); exclusive=None; value=y; live=True; out=[]
    for op,k,arg in x:
        if op=='shared':
            ok=live and exclusive is None and k not in shared
            if ok: shared.add(k)
            out.append(ok)
        elif op=='exclusive':
            ok=live and not shared and exclusive is None
            if ok: exclusive=k
            out.append(ok)
        elif op=='release':
            shared.discard(k)
            if exclusive==k: exclusive=None
            out.append(True)
        elif op=='read':
            out.append(value if live and (k in shared or (exclusive is not None and k==exclusive)) else 'denied')
        elif op=='write':
            ok=live and exclusive is not None
            if ok: value=arg
            out.append(ok)
        elif op=='ownerwrite':
            ok=live and not shared and exclusive is None
            if ok: value=arg
            out.append(ok)
        elif op=='move':
            ok=live and not shared and exclusive is None
            if ok: live=False
            out.append(ok)
        elif op=='inspect': out.append([value,live,sorted(shared),exclusive])
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('shared readers prohibit mutation', solve([['shared', 'a', None], ['shared', 'a', None], ['shared', 'b', None], ['exclusive', 'w', None], ['write', 'a', 'bad'], ['ownerwrite', None, 'bad'], ['move', None, None], ['read', 'a', None], ['read', 'ghost', None], ['inspect', None, None]], 'initial'), [True, False, True, False, False, False, False, 'initial', 'denied', ['initial', True, ['a', 'b'], None]])
check('exclusive token protects identity', solve([['exclusive', 'w', None], ['exclusive', 'other', None], ['shared', 'a', None], ['write', 'ghost', 'bad'], ['release', 'ghost', None], ['ownerwrite', None, 'bad'], ['move', None, None], ['write', 'w', 'new'], ['read', 'w', None], ['inspect', None, None]], 'initial'), [True, False, False, False, True, False, False, True, 'new', ['new', True, [], 'w']])
check('released token loses rights', solve([['shared', 'a', None], ['release', 'a', None], ['read', 'a', None], ['exclusive', 'w', None], ['release', 'w', None], ['write', 'w', 'bad'], ['ownerwrite', None, 'new'], ['inspect', None, None]], 'initial'), [True, True, 'denied', True, True, False, True, ['new', True, [], None]])
check('moved object stays unavailable', solve([['move', None, None], ['shared', 'a', None], ['exclusive', 'w', None], ['ownerwrite', None, 'bad'], ['read', None, None], ['move', None, None], ['inspect', None, None]], 'initial'), [True, False, False, False, 'denied', False, ['initial', False, [], None]])
check('None is not a borrowed token', solve([['read',None,None]], 'initial'), ['denied'])
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
shared readers prohibit mutation	`[True, False, True, False, False, False, False, 'initial', 'denied', ['initial', True, ['a', 'b'], None]]`	`[True, False, True, False, False, False, False, 'initial', 'denied', ['initial', True, ['a', 'b'], None]]`	Passed
exclusive token protects identity	`[True, False, False, True, True, False, False, True, 'new', ['new', True, [], 'w']]`	`[True, False, False, False, True, False, False, True, 'new', ['new', True, [], 'w']]`	Failed
released token loses rights	`[True, True, 'denied', True, True, False, True, ['new', True, [], None]]`	`[True, True, 'denied', True, True, False, True, ['new', True, [], None]]`	Passed
moved object stays unavailable	`[True, False, False, False, 'denied', False, ['initial', False, [], None]]`	`[True, False, False, False, 'denied', False, ['initial', False, [], None]]`	Passed
None is not a borrowed token	`['denied']`	`['denied']`	Passed

SHA-256 / 8ea49be164cbed92e4638ba0227a1ef627ba976a9fe425bb509b6b0e1ff935d2

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 5 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

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 2026-09-29T14:37:39.897982+00:00.

Case digest / b30582b9e19d2a3d3ca3cdc994e9a345afb8eb0417c993e3685abcc55d064325