FA-73876 / Feature flag rollout bucketing / Open access
Weighted variant allocation: Buckets past the total receive no variant · case 01
When weights sum below 100 percent, users in the leftover buckets get no variant at all.
ROOT CAUSE
The function returns None after the loop instead of the last positively weighted variant.
VERIFIED REPAIR
Return the last positively weighted variant for buckets beyond the running total.
Unsuccessful approach: Returning the final declared variant can hand remainder traffic to a zero-weight arm.
Case contract
variants is an ordered list of [name, weight percent with at most two decimals]; bucket is an integer in [0, 10000). Walk variants in declared order, skipping non-positive weights, adding round(weight * 100) to a running total, and return the first variant whose total exceeds the bucket. Buckets beyond the total go to the last positively weighted variant; with none, None.
Why this case matters
Variant splits decide which arm of a flag a user receives and must honour declared weights exactly.
1 / The failure
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(variants, bucket):
acc = 0
last = None
for name, weight in variants:
if weight <= 0:
continue
last = name
acc += round(weight * 100)
if bucket < acc:
return name
return None
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('bucket at first cumulative edge goes to second arm',
[[['control', 50], ['treatment', 50]], 5000],
'treatment'),
('bucket just below first edge stays in first arm',
[[['control', 50], ['treatment', 50]], 4999],
'control'),
('zero weight arm is never selected', [[['control', 0], ['treatment', 100]], 0], 'treatment'),
('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('weight table sample 1', [[['green', 1.15], ['holdout', 0], ['alpha', 1.15], ['blue', 50]], 1135], 'blue'),
('weight table sample 2', [[['blue', 50], ['treatment', 10]], 2986], 'blue')],
[('bucket just below first edge stays in first arm',
[[['control', 50], ['treatment', 50]], 4999],
'control'),
('zero weight arm is never selected', [[['control', 0], ['treatment', 100]], 0], 'treatment'),
('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('weight table sample 6',
[[['alpha', 0.29], ['holdout', 12.5], ['blue', 0.57], ['treatment', 0]], 204],
'holdout'),
('weight table sample 27',
[[['green', 33.33], ['holdout', 0], ['control', 0], ['alpha', -5]], 3333],
'green')],
[('zero weight arm is never selected', [[['control', 0], ['treatment', 100]], 0], 'treatment'),
('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('weight table sample 9',
[[['blue', 0], ['alpha', 0.57], ['green', 4.35], ['treatment', -5]], 4555],
'green'),
('weight table sample 11', [[['holdout', 12.5], ['blue', 12.5], ['alpha', 0.57]], 2500], 'alpha'),
('weight table sample 51', [[['alpha', 10], ['holdout', 10], ['blue', 0.29]], 5446], 'blue')],
[('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('three way split middle arm', [[['a', 33.33], ['b', 33.33], ['c', 33.34]], 5000], 'b'),
('weight table sample 4', [[['green', 12.5], ['treatment', 0.29]], 7991], 'treatment'),
('weight table sample 16', [[['green', 0.57], ['alpha', 12.5], ['control', 25]], 1307], 'control'),
('weight table sample 27',
[[['green', 33.33], ['holdout', 0], ['control', 0], ['alpha', -5]], 3333],
'green')],
[('bucket at first cumulative edge goes to second arm',
[[['control', 50], ['treatment', 50]], 5000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('three way split middle arm', [[['a', 33.33], ['b', 33.33], ['c', 33.34]], 5000], 'b'),
('weight table sample 21', [[['green', 33.33], ['treatment', 0]], 0], 'green'),
('weight table sample 29', [[['holdout', -5], ['treatment', -5], ['blue', 4.35]], 435], 'blue'),
('weight table sample 48', [[['treatment', 0], ['green', -5], ['blue', 0]], 0], None)]]
for label, args, expected in fixtures[N - 1]:
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 fixture | Actual | Expected | Outcome |
|---|---|---|---|
| bucket at first cumulative edge goes to second arm | treatment | treatment | Passed |
| bucket just below first edge stays in first arm | control | control | Passed |
| zero weight arm is never selected | treatment | treatment | Passed |
| trailing zero weight arm does not absorb remainder | None | treatment | Failed |
| declared order beats alphabetical order | zeta | zeta | Passed |
| no positive weights yields None | None | None | Passed |
| weight table sample 1 | blue | blue | Passed |
| weight table sample 2 | blue | blue | Passed |
SHA-256 / ef82597405f4545c2517f6be1c14b5c829e27ca5bcd0d836e1770dcbcfb45d37
2 / The unsuccessful fix
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(variants, bucket):
acc = 0
last = None
for name, weight in variants:
if weight <= 0:
continue
last = name
acc += round(weight * 100)
if bucket < acc:
return name
return variants[-1][0] if variants else None
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('bucket at first cumulative edge goes to second arm',
[[['control', 50], ['treatment', 50]], 5000],
'treatment'),
('bucket just below first edge stays in first arm',
[[['control', 50], ['treatment', 50]], 4999],
'control'),
('zero weight arm is never selected', [[['control', 0], ['treatment', 100]], 0], 'treatment'),
('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('weight table sample 1', [[['green', 1.15], ['holdout', 0], ['alpha', 1.15], ['blue', 50]], 1135], 'blue'),
('weight table sample 2', [[['blue', 50], ['treatment', 10]], 2986], 'blue')],
[('bucket just below first edge stays in first arm',
[[['control', 50], ['treatment', 50]], 4999],
'control'),
('zero weight arm is never selected', [[['control', 0], ['treatment', 100]], 0], 'treatment'),
('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('weight table sample 6',
[[['alpha', 0.29], ['holdout', 12.5], ['blue', 0.57], ['treatment', 0]], 204],
'holdout'),
('weight table sample 27',
[[['green', 33.33], ['holdout', 0], ['control', 0], ['alpha', -5]], 3333],
'green')],
[('zero weight arm is never selected', [[['control', 0], ['treatment', 100]], 0], 'treatment'),
('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('weight table sample 9',
[[['blue', 0], ['alpha', 0.57], ['green', 4.35], ['treatment', -5]], 4555],
'green'),
('weight table sample 11', [[['holdout', 12.5], ['blue', 12.5], ['alpha', 0.57]], 2500], 'alpha'),
('weight table sample 51', [[['alpha', 10], ['holdout', 10], ['blue', 0.29]], 5446], 'blue')],
[('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('three way split middle arm', [[['a', 33.33], ['b', 33.33], ['c', 33.34]], 5000], 'b'),
('weight table sample 4', [[['green', 12.5], ['treatment', 0.29]], 7991], 'treatment'),
('weight table sample 16', [[['green', 0.57], ['alpha', 12.5], ['control', 25]], 1307], 'control'),
('weight table sample 27',
[[['green', 33.33], ['holdout', 0], ['control', 0], ['alpha', -5]], 3333],
'green')],
[('bucket at first cumulative edge goes to second arm',
[[['control', 50], ['treatment', 50]], 5000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('three way split middle arm', [[['a', 33.33], ['b', 33.33], ['c', 33.34]], 5000], 'b'),
('weight table sample 21', [[['green', 33.33], ['treatment', 0]], 0], 'green'),
('weight table sample 29', [[['holdout', -5], ['treatment', -5], ['blue', 4.35]], 435], 'blue'),
('weight table sample 48', [[['treatment', 0], ['green', -5], ['blue', 0]], 0], None)]]
for label, args, expected in fixtures[N - 1]:
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 fixture | Actual | Expected | Outcome |
|---|---|---|---|
| bucket at first cumulative edge goes to second arm | treatment | treatment | Passed |
| bucket just below first edge stays in first arm | control | control | Passed |
| zero weight arm is never selected | treatment | treatment | Passed |
| trailing zero weight arm does not absorb remainder | off | treatment | Failed |
| declared order beats alphabetical order | zeta | zeta | Passed |
| no positive weights yields None | b | None | Failed |
| weight table sample 1 | blue | blue | Passed |
| weight table sample 2 | blue | blue | Passed |
SHA-256 / fbc51d161cf6ad49bd659c56d3257e80fd988dc6dc24aa19017fbb36316decc1
3 / The verified repair
Exit 0"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(variants, bucket):
acc = 0
last = None
for name, weight in variants:
if weight <= 0:
continue
last = name
acc += round(weight * 100)
if bucket < acc:
return name
return last
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('bucket at first cumulative edge goes to second arm',
[[['control', 50], ['treatment', 50]], 5000],
'treatment'),
('bucket just below first edge stays in first arm',
[[['control', 50], ['treatment', 50]], 4999],
'control'),
('zero weight arm is never selected', [[['control', 0], ['treatment', 100]], 0], 'treatment'),
('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('weight table sample 1', [[['green', 1.15], ['holdout', 0], ['alpha', 1.15], ['blue', 50]], 1135], 'blue'),
('weight table sample 2', [[['blue', 50], ['treatment', 10]], 2986], 'blue')],
[('bucket just below first edge stays in first arm',
[[['control', 50], ['treatment', 50]], 4999],
'control'),
('zero weight arm is never selected', [[['control', 0], ['treatment', 100]], 0], 'treatment'),
('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('weight table sample 6',
[[['alpha', 0.29], ['holdout', 12.5], ['blue', 0.57], ['treatment', 0]], 204],
'holdout'),
('weight table sample 27',
[[['green', 33.33], ['holdout', 0], ['control', 0], ['alpha', -5]], 3333],
'green')],
[('zero weight arm is never selected', [[['control', 0], ['treatment', 100]], 0], 'treatment'),
('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('weight table sample 9',
[[['blue', 0], ['alpha', 0.57], ['green', 4.35], ['treatment', -5]], 4555],
'green'),
('weight table sample 11', [[['holdout', 12.5], ['blue', 12.5], ['alpha', 0.57]], 2500], 'alpha'),
('weight table sample 51', [[['alpha', 10], ['holdout', 10], ['blue', 0.29]], 5446], 'blue')],
[('trailing zero weight arm does not absorb remainder',
[[['control', 40], ['treatment', 40], ['off', 0]], 9000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('three way split middle arm', [[['a', 33.33], ['b', 33.33], ['c', 33.34]], 5000], 'b'),
('weight table sample 4', [[['green', 12.5], ['treatment', 0.29]], 7991], 'treatment'),
('weight table sample 16', [[['green', 0.57], ['alpha', 12.5], ['control', 25]], 1307], 'control'),
('weight table sample 27',
[[['green', 33.33], ['holdout', 0], ['control', 0], ['alpha', -5]], 3333],
'green')],
[('bucket at first cumulative edge goes to second arm',
[[['control', 50], ['treatment', 50]], 5000],
'treatment'),
('declared order beats alphabetical order', [[['zeta', 10], ['alpha', 90]], 500], 'zeta'),
('fractional weight 0.29 covers 29 buckets', [[['tiny', 0.29], ['rest', 99.71]], 28], 'tiny'),
('no positive weights yields None', [[['a', 0], ['b', 0]], 10], None),
('three way split middle arm', [[['a', 33.33], ['b', 33.33], ['c', 33.34]], 5000], 'b'),
('weight table sample 21', [[['green', 33.33], ['treatment', 0]], 0], 'green'),
('weight table sample 29', [[['holdout', -5], ['treatment', -5], ['blue', 4.35]], 435], 'blue'),
('weight table sample 48', [[['treatment', 0], ['green', -5], ['blue', 0]], 0], None)]]
for label, args, expected in fixtures[N - 1]:
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 fixture | Actual | Expected | Outcome |
|---|---|---|---|
| bucket at first cumulative edge goes to second arm | treatment | treatment | Passed |
| bucket just below first edge stays in first arm | control | control | Passed |
| zero weight arm is never selected | treatment | treatment | Passed |
| trailing zero weight arm does not absorb remainder | treatment | treatment | Passed |
| declared order beats alphabetical order | zeta | zeta | Passed |
| no positive weights yields None | None | None | Passed |
| weight table sample 1 | blue | blue | Passed |
| weight table sample 2 | blue | blue | Passed |
SHA-256 / 04b37cd1063a7de9224c80afd071b8d9e27e4893ece147db6a83ca53f3b82b83
Verification & scope
A deterministic toy flag-evaluation model with a stipulated contract; it does not reproduce any vendor SDK byte for byte. 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 2026-09-29T14:48:51.480843+00:00.
Case digest / a892e2386ecc8566e6f8a78e79286d6875e41c1a5e5539534c183cbd9ebc6f59