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FA-27046 / HTTP ranges / Open access

A proxy coalesces nearby requested spans into economical origin reads: empty logical selectors must not allocate backend fetches · case 01

A proxy coalesces nearby requested spans into economical origin reads: empty logical selectors must not allocate backend fetches.

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

ROOT CAUSE

The coalesced-origin-fetch-empty-read 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: mapping.append(None); continue index=assignments[i] mapping.append([index,a-fetches[index][0],b-a]) return [fetches,mapping].

THE FAILURE

The coalesced-origin-fetch-empty-read 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: 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 or a==0] 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], 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: 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-read 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-read 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-read 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-read 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-read fixture 4', json.loads(json.dumps(solve({'requests':[[0,0],[3,3]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[],[None,None]])))
check('empty-read 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-read 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-read 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-read fixture 0[[[0, 6]], [[0, 4, 2], [0, 0, 2]]][[[0, 6]], [[0, 4, 2], [0, 0, 2]]]Passed
empty-read fixture 1[[[0, 2], [4, 6]], [[0, 0, 2], [1, 0, 2]]][[[0, 2], [4, 6]], [[0, 0, 2], [1, 0, 2]]]Passed
empty-read fixture 2[[[0, 5]], [[0, 0, 5], [0, 2, 1]]][[[0, 5]], [[0, 0, 5], [0, 2, 1]]]Passed
empty-read fixture 3[[[0, 4]], [[0, 0, 2], [0, 2, 2]]][[[0, 4]], [[0, 0, 2], [0, 2, 2]]]Passed
empty-read fixture 4[[[0, 0], [3, 3]], [None, None]][[], [None, None]]Failed
empty-read fixture 5[[[2, 4], [8, 9]], [[0, 0, 2], [1, 0, 1]]][[[2, 4], [8, 9]], [[0, 0, 2], [1, 0, 1]]]Passed
empty-read fixture 6[[[0, 2]], [[0, 0, 1], [0, 1, 1]]][[[0, 2]], [[0, 0, 1], [0, 1, 1]]]Passed
empty-read fixture 7[[], []][[], []]Passed

SHA-256 / 112cdcbb4e97967b3b5cd888c8b0c65bd333379b28a4eb274df0653a183a12c4

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 or a==0]
    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-read 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-read 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-read 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-read 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-read fixture 4', json.loads(json.dumps(solve({'requests':[[0,0],[3,3]],'gap':0,'forbidden':[]}))), json.loads(json.dumps([[],[None,None]])))
check('empty-read 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-read 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-read 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-read fixture 0[[[0, 6]], [[0, 4, 2], [0, 0, 2]]][[[0, 6]], [[0, 4, 2], [0, 0, 2]]]Passed
empty-read fixture 1[[[0, 2], [4, 6]], [[0, 0, 2], [1, 0, 2]]][[[0, 2], [4, 6]], [[0, 0, 2], [1, 0, 2]]]Passed
empty-read fixture 2[[[0, 5]], [[0, 0, 5], [0, 2, 1]]][[[0, 5]], [[0, 0, 5], [0, 2, 1]]]Passed
empty-read fixture 3[[[0, 4]], [[0, 0, 2], [0, 2, 2]]][[[0, 4]], [[0, 0, 2], [0, 2, 2]]]Passed
empty-read fixture 4[[[0, 0]], [None, None]][[], [None, None]]Failed
empty-read fixture 5[[[2, 4], [8, 9]], [[0, 0, 2], [1, 0, 1]]][[[2, 4], [8, 9]], [[0, 0, 2], [1, 0, 1]]]Passed
empty-read fixture 6[[[0, 2]], [[0, 0, 1], [0, 1, 1]]][[[0, 2]], [[0, 0, 1], [0, 1, 1]]]Passed
empty-read fixture 7[[], []][[], []]Passed

SHA-256 / 28ed9d6bc16d7a546e6ae43782587ac5f741f609c269cbd21916f616bd602bf1

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 8 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

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 2026-09-29T14:41:21.471867+00:00.

Case digest / a2f03133338ba33fab170b8e330eb55660248643273ed134efc9155546f12195