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FA-27086 / HTTP ranges / Open access

A proxy coalesces nearby requested spans into economical origin reads: empty selectors retain a null slot in caller mapping · case 01

A proxy coalesces nearby requested spans into economical origin reads: empty selectors retain a null slot in caller mapping.

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The coalesced-origin-fetch-empty-mapping decision uses rows=[(a,b,i) for i,(a,b) in enumerate(x['requests']) if a<b] rows.sort(key=lambda r:(r[0],r[1])) fetches=[] assignments={} for a,b,i in rows: merge=bool(fetches) and a<=fetches[-1][1]+x['gap'] if merge and any(p in x['forbidden'] for p in range(fetches[-1][1],a)): merge=False if merge: index=len(fetches)-1 fetches[index][1]=max(fetches[index][1],b) else: index=len(fetches) fetches.append([a,b]) assignments[i]=index mapping=[] for i,(a,b) in enumerate(x['requests']): if a==b: continue index=assignments[i] mapping.append([index,a-fetches[index][0],b-a]) return [fetches,mapping].

VERIFIED REPAIR

Apply the bounded decision exactly: rows=[(a,b,i) for i,(a,b) in enumerate(x['requests']) if a<b] rows.sort(key=lambda r:(r[0],r[1])) fetches=[] assignments={} for a,b,i in rows: merge=bool(fetches) and a<=fetches[-1][1]+x['gap'] if merge and any(p in x['forbidden'] for p in range(fetches[-1][1],a)): merge=False if merge: index=len(fetches)-1 fetches[index][1]=max(fetches[index][1],b) else: index=len(fetches) fetches.append([a,b]) assignments[i]=index mapping=[] for i,(a,b) in enumerate(x['requests']): if a==b: mapping.append(None); continue index=assignments[i] mapping.append([index,a-fetches[index][0],b-a]) return [fetches,mapping]

Unsuccessful approach: The partial repair uses rows=[(a,b,i) for i,(a,b) in enumerate(x['requests']) if a<b] rows.sort(key=lambda r:(r[0],r[1])) fetches=[] assignments={} for a,b,i in rows: merge=bool(fetches) and a<=fetches[-1][1]+x['gap'] if merge and any(p in x['forbidden'] for p in range(fetches[-1][1],a)): merge=False if merge: index=len(fetches)-1 fetches[index][1]=max(fetches[index][1],b) else: index=len(fetches) fetches.append([a,b]) assignments[i]=index mapping=[] for i,(a,b) in enumerate(x['requests']): if a==b: mapping.append([0,0,0]); continue index=assignments[i] mapping.append([index,a-fetches[index][0],b-a]) return [fetches,mapping], which still violates the stated contract.

Case contract

x contains request rows [start,stop] with half-open nonnegative bounds, a nonnegative max-gap, and a set of forbidden byte positions. Empty requests contribute no work. Sort requests by start/stop, merge overlapping or nearby reads only if the gap has no forbidden byte. Return fetch spans and, in original request order, [fetch-index,offset-within-fetch,count] mappings. Empty requests map to null. This model plans origin reads only and does not redefine inclusive wire range semantics.

Why this case matters

Range responses combine representation identity, conditional requests, framing, and partial-object state.

1 / The failure

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

N = 1
observations = []
def solve(x):
    rows=[(a,b,i) for i,(a,b) in enumerate(x['requests']) if a<b]
    rows.sort(key=lambda r:(r[0],r[1]))
    fetches=[]
    assignments={}
    for a,b,i in rows:
        merge=bool(fetches) and a<=fetches[-1][1]+x['gap']
        if merge and any(p in x['forbidden'] for p in range(fetches[-1][1],a)): merge=False
        if merge:
            index=len(fetches)-1
            fetches[index][1]=max(fetches[index][1],b)
        else:
            index=len(fetches)
            fetches.append([a,b])
        assignments[i]=index
    mapping=[]
    for i,(a,b) in enumerate(x['requests']):
        if a==b: continue
        index=assignments[i]
        mapping.append([index,a-fetches[index][0],b-a])
    return [fetches,mapping]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('empty-mapping fixture 0', json.loads(json.dumps(solve({'requests':[[4,6],[0,2]],'gap':2,'forbidden':[]}))), json.loads(json.dumps([[[0,6]],[[0,4,2],[0,0,2]]])))
check('empty-mapping fixture 1', json.loads(json.dumps(solve({'requests':[[0,2],[4,6]],'gap':2,'forbidden':[3]}))), json.loads(json.dumps([[[0,2],[4,6]],[[0,0,2],[1,0,2]]])))
check('empty-mapping fixture 2', json.loads(json.dumps(solve({'requests':[[0,5],[2,3]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[[0,5]],[[0,0,5],[0,2,1]]])))
check('empty-mapping fixture 3', json.loads(json.dumps(solve({'requests':[[0,2],[2,4]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[[0,4]],[[0,0,2],[0,2,2]]])))
check('empty-mapping fixture 4', json.loads(json.dumps(solve({'requests':[[0,0],[3,3]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[],[None,None]])))
check('empty-mapping fixture 5', json.loads(json.dumps(solve({'requests':[[2,4],[8,9]],'gap':1,'forbidden':[]}))), json.loads(json.dumps([[[2,4],[8,9]],[[0,0,2],[1,0,1]]])))
check('empty-mapping fixture 6', json.loads(json.dumps(solve({'requests':[[0,N],[N,2*N]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[[0,2*N]],[[0,0,N],[0,N,N]]])))
check('empty-mapping fixture 7', json.loads(json.dumps(solve({'requests':[],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[],[]])))
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
empty-mapping fixture 0[[[0, 6]], [[0, 4, 2], [0, 0, 2]]][[[0, 6]], [[0, 4, 2], [0, 0, 2]]]Passed
empty-mapping fixture 1[[[0, 2], [4, 6]], [[0, 0, 2], [1, 0, 2]]][[[0, 2], [4, 6]], [[0, 0, 2], [1, 0, 2]]]Passed
empty-mapping fixture 2[[[0, 5]], [[0, 0, 5], [0, 2, 1]]][[[0, 5]], [[0, 0, 5], [0, 2, 1]]]Passed
empty-mapping fixture 3[[[0, 4]], [[0, 0, 2], [0, 2, 2]]][[[0, 4]], [[0, 0, 2], [0, 2, 2]]]Passed
empty-mapping fixture 4[[], []][[], [None, None]]Failed
empty-mapping fixture 5[[[2, 4], [8, 9]], [[0, 0, 2], [1, 0, 1]]][[[2, 4], [8, 9]], [[0, 0, 2], [1, 0, 1]]]Passed
empty-mapping fixture 6[[[0, 2]], [[0, 0, 1], [0, 1, 1]]][[[0, 2]], [[0, 0, 1], [0, 1, 1]]]Passed
empty-mapping fixture 7[[], []][[], []]Passed

SHA-256 / 3eb3008bdfe39085f56ed7d32de5814976a18f637b94f7748b85739193c6bc08

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(x):
    rows=[(a,b,i) for i,(a,b) in enumerate(x['requests']) if a<b]
    rows.sort(key=lambda r:(r[0],r[1]))
    fetches=[]
    assignments={}
    for a,b,i in rows:
        merge=bool(fetches) and a<=fetches[-1][1]+x['gap']
        if merge and any(p in x['forbidden'] for p in range(fetches[-1][1],a)): merge=False
        if merge:
            index=len(fetches)-1
            fetches[index][1]=max(fetches[index][1],b)
        else:
            index=len(fetches)
            fetches.append([a,b])
        assignments[i]=index
    mapping=[]
    for i,(a,b) in enumerate(x['requests']):
        if a==b: mapping.append([0,0,0]); continue
        index=assignments[i]
        mapping.append([index,a-fetches[index][0],b-a])
    return [fetches,mapping]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('empty-mapping fixture 0', json.loads(json.dumps(solve({'requests':[[4,6],[0,2]],'gap':2,'forbidden':[]}))), json.loads(json.dumps([[[0,6]],[[0,4,2],[0,0,2]]])))
check('empty-mapping fixture 1', json.loads(json.dumps(solve({'requests':[[0,2],[4,6]],'gap':2,'forbidden':[3]}))), json.loads(json.dumps([[[0,2],[4,6]],[[0,0,2],[1,0,2]]])))
check('empty-mapping fixture 2', json.loads(json.dumps(solve({'requests':[[0,5],[2,3]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[[0,5]],[[0,0,5],[0,2,1]]])))
check('empty-mapping fixture 3', json.loads(json.dumps(solve({'requests':[[0,2],[2,4]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[[0,4]],[[0,0,2],[0,2,2]]])))
check('empty-mapping fixture 4', json.loads(json.dumps(solve({'requests':[[0,0],[3,3]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[],[None,None]])))
check('empty-mapping fixture 5', json.loads(json.dumps(solve({'requests':[[2,4],[8,9]],'gap':1,'forbidden':[]}))), json.loads(json.dumps([[[2,4],[8,9]],[[0,0,2],[1,0,1]]])))
check('empty-mapping fixture 6', json.loads(json.dumps(solve({'requests':[[0,N],[N,2*N]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[[0,2*N]],[[0,0,N],[0,N,N]]])))
check('empty-mapping fixture 7', json.loads(json.dumps(solve({'requests':[],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[],[]])))
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
empty-mapping fixture 0[[[0, 6]], [[0, 4, 2], [0, 0, 2]]][[[0, 6]], [[0, 4, 2], [0, 0, 2]]]Passed
empty-mapping fixture 1[[[0, 2], [4, 6]], [[0, 0, 2], [1, 0, 2]]][[[0, 2], [4, 6]], [[0, 0, 2], [1, 0, 2]]]Passed
empty-mapping fixture 2[[[0, 5]], [[0, 0, 5], [0, 2, 1]]][[[0, 5]], [[0, 0, 5], [0, 2, 1]]]Passed
empty-mapping fixture 3[[[0, 4]], [[0, 0, 2], [0, 2, 2]]][[[0, 4]], [[0, 0, 2], [0, 2, 2]]]Passed
empty-mapping fixture 4[[], [[0, 0, 0], [0, 0, 0]]][[], [None, None]]Failed
empty-mapping fixture 5[[[2, 4], [8, 9]], [[0, 0, 2], [1, 0, 1]]][[[2, 4], [8, 9]], [[0, 0, 2], [1, 0, 1]]]Passed
empty-mapping fixture 6[[[0, 2]], [[0, 0, 1], [0, 1, 1]]][[[0, 2]], [[0, 0, 1], [0, 1, 1]]]Passed
empty-mapping fixture 7[[], []][[], []]Passed

SHA-256 / b0277d4ad1036300da2959d7386c1e3ec16f7c4040ea1f63b2731409ff727cc8

3 / The verified repair

Exit 0
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    rows=[(a,b,i) for i,(a,b) in enumerate(x['requests']) if a<b]
    rows.sort(key=lambda r:(r[0],r[1]))
    fetches=[]
    assignments={}
    for a,b,i in rows:
        merge=bool(fetches) and a<=fetches[-1][1]+x['gap']
        if merge and any(p in x['forbidden'] for p in range(fetches[-1][1],a)): merge=False
        if merge:
            index=len(fetches)-1
            fetches[index][1]=max(fetches[index][1],b)
        else:
            index=len(fetches)
            fetches.append([a,b])
        assignments[i]=index
    mapping=[]
    for i,(a,b) in enumerate(x['requests']):
        if a==b: mapping.append(None); continue
        index=assignments[i]
        mapping.append([index,a-fetches[index][0],b-a])
    return [fetches,mapping]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('empty-mapping fixture 0', json.loads(json.dumps(solve({'requests':[[4,6],[0,2]],'gap':2,'forbidden':[]}))), json.loads(json.dumps([[[0,6]],[[0,4,2],[0,0,2]]])))
check('empty-mapping fixture 1', json.loads(json.dumps(solve({'requests':[[0,2],[4,6]],'gap':2,'forbidden':[3]}))), json.loads(json.dumps([[[0,2],[4,6]],[[0,0,2],[1,0,2]]])))
check('empty-mapping fixture 2', json.loads(json.dumps(solve({'requests':[[0,5],[2,3]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[[0,5]],[[0,0,5],[0,2,1]]])))
check('empty-mapping fixture 3', json.loads(json.dumps(solve({'requests':[[0,2],[2,4]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[[0,4]],[[0,0,2],[0,2,2]]])))
check('empty-mapping fixture 4', json.loads(json.dumps(solve({'requests':[[0,0],[3,3]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[],[None,None]])))
check('empty-mapping fixture 5', json.loads(json.dumps(solve({'requests':[[2,4],[8,9]],'gap':1,'forbidden':[]}))), json.loads(json.dumps([[[2,4],[8,9]],[[0,0,2],[1,0,1]]])))
check('empty-mapping fixture 6', json.loads(json.dumps(solve({'requests':[[0,N],[N,2*N]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[[0,2*N]],[[0,0,N],[0,N,N]]])))
check('empty-mapping fixture 7', json.loads(json.dumps(solve({'requests':[],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[],[]])))
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
empty-mapping fixture 0[[[0, 6]], [[0, 4, 2], [0, 0, 2]]][[[0, 6]], [[0, 4, 2], [0, 0, 2]]]Passed
empty-mapping fixture 1[[[0, 2], [4, 6]], [[0, 0, 2], [1, 0, 2]]][[[0, 2], [4, 6]], [[0, 0, 2], [1, 0, 2]]]Passed
empty-mapping fixture 2[[[0, 5]], [[0, 0, 5], [0, 2, 1]]][[[0, 5]], [[0, 0, 5], [0, 2, 1]]]Passed
empty-mapping fixture 3[[[0, 4]], [[0, 0, 2], [0, 2, 2]]][[[0, 4]], [[0, 0, 2], [0, 2, 2]]]Passed
empty-mapping fixture 4[[], [None, None]][[], [None, None]]Passed
empty-mapping fixture 5[[[2, 4], [8, 9]], [[0, 0, 2], [1, 0, 1]]][[[2, 4], [8, 9]], [[0, 0, 2], [1, 0, 1]]]Passed
empty-mapping fixture 6[[[0, 2]], [[0, 0, 1], [0, 1, 1]]][[[0, 2]], [[0, 0, 1], [0, 1, 1]]]Passed
empty-mapping fixture 7[[], []][[], []]Passed

SHA-256 / 1e9ce1edeec50b06311979bee320ae531e758f3b067d58a53b34465153ab31e5

Verification & scope

Deterministic simplified range service, with stipulated local policies and already parsed trusted inputs; not a complete HTTP implementation. 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 2026-09-29T14:41:22.032073+00:00.

Case digest / 911873b6e7d1283985e4dd7bf5c487fe298cdb4ae5785892fed2347b96c9d91f