FA-956 / Reliability / Open access

Allow a half-open circuit probe: Concurrent requests occupy the single half-open probe slot · case 01

The circuit probe operation is admitted even though concurrent requests occupy the single half-open probe slot.

Verified by executionVariant 1 · 9 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The admission path omits the probe exclusive invariant while validating the other operation preconditions.

THE FAILURE

The admission path omits the probe exclusive invariant while validating the other operation preconditions.

Unsuccessful approach: Adding the probe exclusive check repairs the reported defect, but replacing the adjacent circuit generation check loses that independent invariant.

Case contract

Return a Boolean admission decision for allow a half-open circuit probe. The record r must satisfy all of: r['open_delay'][0]-r['open_delay'][1] >= r['open_delay'][2]; r['probe_exclusive'] == 0; r['circuit_generation'][0] == r['circuit_generation'][1]; r['endpoint_health'][0] in r['endpoint_health'][1]; r['caller_deadline'][0] < r['caller_deadline'][1]. Extra tracing fields are ignored; validation does not mutate the record.

Why this case matters

A deterministic local contract for reliability. Each negative fixture violates exactly one invariant. No transport timing, persistence, cryptographic verification, or full protocol implementation is claimed.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(r):
    return (r['open_delay'][0]-r['open_delay'][1] >= r['open_delay'][2]) and (r['circuit_generation'][0] == r['circuit_generation'][1]) and (r['endpoint_health'][0] in r['endpoint_health'][1]) and (r['caller_deadline'][0] < r['caller_deadline'][1])
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'open_delay': [10, 5, 5], 'probe_exclusive': 0, 'circuit_generation': [4, 4], 'endpoint_health': ['a', ['a', 'b']], 'caller_deadline': [10, 11]}
check('valid operation', solve(r), True)
check('A probe runs before the open-state cooldown elapses', solve(dict(r, **{'open_delay': [9, 5, 5]})), False)
check('Concurrent requests occupy the single half-open probe slot', solve(dict(r, **{'probe_exclusive': 1})), False)
check('A stale timer probes a newly opened circuit generation', solve(dict(r, **{'circuit_generation': [3, 4]})), False)
check('A probe targets an endpoint removed from service discovery', solve(dict(r, **{'endpoint_health': ['c', ['a', 'b']]})), False)
check('A probe is started for an already expired caller', solve(dict(r, **{'caller_deadline': [11, 11]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'open_delay': [9, 5, 5], 'probe_exclusive': 1, 'circuit_generation': [3, 4], 'endpoint_health': ['c', ['a', 'b']], 'caller_deadline': [11, 11]}
keys = list(invalid)
pair = {keys[N % len(keys)]: invalid[keys[N % len(keys)]], keys[(N+1) % len(keys)]: invalid[keys[(N+1) % len(keys)]]}
check('two independent violations in variant', solve(dict(r, **pair)), False)
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
valid operation	`True`	`True`	Passed
A probe runs before the open-state cooldown elapses	`False`	`False`	Passed
Concurrent requests occupy the single half-open probe slot	`True`	`False`	Failed
A stale timer probes a newly opened circuit generation	`False`	`False`	Passed
A probe targets an endpoint removed from service discovery	`False`	`False`	Passed
A probe is started for an already expired caller	`False`	`False`	Passed
unrelated tracing metadata	`True`	`True`	Passed
repeat validation is pure	`True`	`True`	Passed
two independent violations in variant	`False`	`False`	Passed

SHA-256 / d09f4821eb5a07023c43379d36a25e71e67e096f1bc2fb38003869e355de0173

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(r):
    return (r['open_delay'][0]-r['open_delay'][1] >= r['open_delay'][2]) and (r['probe_exclusive'] == 0) and (r['endpoint_health'][0] in r['endpoint_health'][1]) and (r['caller_deadline'][0] < r['caller_deadline'][1])
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'open_delay': [10, 5, 5], 'probe_exclusive': 0, 'circuit_generation': [4, 4], 'endpoint_health': ['a', ['a', 'b']], 'caller_deadline': [10, 11]}
check('valid operation', solve(r), True)
check('A probe runs before the open-state cooldown elapses', solve(dict(r, **{'open_delay': [9, 5, 5]})), False)
check('Concurrent requests occupy the single half-open probe slot', solve(dict(r, **{'probe_exclusive': 1})), False)
check('A stale timer probes a newly opened circuit generation', solve(dict(r, **{'circuit_generation': [3, 4]})), False)
check('A probe targets an endpoint removed from service discovery', solve(dict(r, **{'endpoint_health': ['c', ['a', 'b']]})), False)
check('A probe is started for an already expired caller', solve(dict(r, **{'caller_deadline': [11, 11]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'open_delay': [9, 5, 5], 'probe_exclusive': 1, 'circuit_generation': [3, 4], 'endpoint_health': ['c', ['a', 'b']], 'caller_deadline': [11, 11]}
keys = list(invalid)
pair = {keys[N % len(keys)]: invalid[keys[N % len(keys)]], keys[(N+1) % len(keys)]: invalid[keys[(N+1) % len(keys)]]}
check('two independent violations in variant', solve(dict(r, **pair)), False)
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
valid operation	`True`	`True`	Passed
A probe runs before the open-state cooldown elapses	`False`	`False`	Passed
Concurrent requests occupy the single half-open probe slot	`False`	`False`	Passed
A stale timer probes a newly opened circuit generation	`True`	`False`	Failed
A probe targets an endpoint removed from service discovery	`False`	`False`	Passed
A probe is started for an already expired caller	`False`	`False`	Passed
unrelated tracing metadata	`True`	`True`	Passed
repeat validation is pure	`True`	`True`	Passed
two independent violations in variant	`False`	`False`	Passed

SHA-256 / e70317fb0e112576afd5b832a35a98b638d15e985bcdc4d8d69917a83f198e7a

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 9 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.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

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 2026-09-29T14:36:57.334184+00:00.

Case digest / cf2120da95805a54b05f2a6002906e03b0792d5fc77a91580437b66c58abcb74