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FA-27166 / HTTP ranges / Open access

A multipart sender resumes framing and payload segments across transport credit windows: accepted bytes advance the cursor rather than replacing it · case 01

A multipart sender resumes framing and payload segments across transport credit windows: accepted bytes advance the cursor rather than replacing it.

Verified by executionVariant 1 · 7 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The multipart-write-cursor-offset-progress decision uses segments=x['segments'] index=0 offset=0 out=[] for budget in x['budgets']: sent='' while index<len(segments): segment=segments[index] if offset>=len(segment): index+=1 offset=0 continue if budget==0: break amount=min(budget,len(segment)-offset) sent+=segment[offset:offset+amount] offset=amount if amount==0: break budget-=amount out.append(sent) return [out,index,offset].

VERIFIED REPAIR

Apply the bounded decision exactly: segments=x['segments'] index=0 offset=0 out=[] for budget in x['budgets']: sent='' while index<len(segments): segment=segments[index] if offset>=len(segment): index+=1 offset=0 continue if budget==0: break amount=min(budget,len(segment)-offset) sent+=segment[offset:offset+amount] offset+=amount budget-=amount out.append(sent) return [out,index,offset]

Unsuccessful approach: The partial repair uses segments=x['segments'] index=0 offset=0 out=[] for budget in x['budgets']: sent='' while index<len(segments): segment=segments[index] if offset>=len(segment): index+=1 offset=0 continue if budget==0: break amount=min(budget,len(segment)-offset) sent+=segment[offset:offset+amount] offset+=amount if offset==0 else amount+1 offset=min(offset,len(segment)) budget-=amount out.append(sent) return [out,index,offset], which still violates the stated contract.

Case contract

x has nonempty ASCII segments representing alternating boundary/header/body/framing bytes, and nonnegative write budgets. Drain in order, consuming each budget across segment boundaries. Return [one emitted string per budget,segment-index,intra-segment-offset]. Empty segments are skipped even with zero budget. A exhausted plan is stable. These preframed segments isolate cursor management from wire-format construction.

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):
    segments=x['segments']
    index=0
    offset=0
    out=[]
    for budget in x['budgets']:
        sent=''
        while index<len(segments):
            segment=segments[index]
            if offset>=len(segment):
                index+=1
                offset=0
                continue
            if budget==0: break
            amount=min(budget,len(segment)-offset)
            sent+=segment[offset:offset+amount]
            offset=amount
            if amount==0: break
            budget-=amount
        out.append(sent)
    return [out,index,offset]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('offset-progress fixture 0', json.loads(json.dumps(solve({'segments':['--b','H','BODY','\r\n'],'budgets':[2,2,3,3]}))), json.loads(json.dumps([['--','bH','BOD','Y\r\n'],4,0])))
check('offset-progress fixture 1', json.loads(json.dumps(solve({'segments':['A','BC'],'budgets':[1,0,1,1,1]}))), json.loads(json.dumps([['A','','B','C',''],2,0])))
check('offset-progress fixture 2', json.loads(json.dumps(solve({'segments':['','A','','BC',''],'budgets':[0,2,1]}))), json.loads(json.dumps([['','AB','C'],5,0])))
check('offset-progress fixture 3', json.loads(json.dumps(solve({'segments':[],'budgets':[0,1]}))), json.loads(json.dumps([['',''],0,0])))
check('offset-progress fixture 4', json.loads(json.dumps(solve({'segments':['ABCDE'],'budgets':[2,1]}))), json.loads(json.dumps([['AB','C'],0,3])))
check('offset-progress fixture 5', json.loads(json.dumps(solve({'segments':["p"*N,'X'],'budgets':[N,1]}))), json.loads(json.dumps([["p"*N,'X'],2,0])))
check('offset-progress fixture 6', json.loads(json.dumps(solve({'segments':['AB','CDE'],'budgets':[4]}))), json.loads(json.dumps([['ABCD'],1,2])))
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
offset-progress fixture 0[['--', 'b-', '-bb', '-bb'], 0, 1][['--', 'bH', 'BOD', 'Y\r\n'], 4, 0]Failed
offset-progress fixture 1[['A', '', 'B', 'C', 'C'], 1, 1][['A', '', 'B', 'C', ''], 2, 0]Failed
offset-progress fixture 2[['', 'AB', 'C'], 3, 1][['', 'AB', 'C'], 5, 0]Failed
offset-progress fixture 3[['', ''], 0, 0][['', ''], 0, 0]Passed
offset-progress fixture 4[['AB', 'C'], 0, 1][['AB', 'C'], 0, 3]Failed
offset-progress fixture 5[['p', 'X'], 2, 0][['p', 'X'], 2, 0]Passed
offset-progress fixture 6[['ABCD'], 1, 2][['ABCD'], 1, 2]Passed

SHA-256 / a6739bb5d9d1184fb11352c981e46e4169fc2a95da38558fb579992cadea7bbb

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(x):
    segments=x['segments']
    index=0
    offset=0
    out=[]
    for budget in x['budgets']:
        sent=''
        while index<len(segments):
            segment=segments[index]
            if offset>=len(segment):
                index+=1
                offset=0
                continue
            if budget==0: break
            amount=min(budget,len(segment)-offset)
            sent+=segment[offset:offset+amount]
            offset+=amount if offset==0 else amount+1
            offset=min(offset,len(segment))
            budget-=amount
        out.append(sent)
    return [out,index,offset]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('offset-progress fixture 0', json.loads(json.dumps(solve({'segments':['--b','H','BODY','\r\n'],'budgets':[2,2,3,3]}))), json.loads(json.dumps([['--','bH','BOD','Y\r\n'],4,0])))
check('offset-progress fixture 1', json.loads(json.dumps(solve({'segments':['A','BC'],'budgets':[1,0,1,1,1]}))), json.loads(json.dumps([['A','','B','C',''],2,0])))
check('offset-progress fixture 2', json.loads(json.dumps(solve({'segments':['','A','','BC',''],'budgets':[0,2,1]}))), json.loads(json.dumps([['','AB','C'],5,0])))
check('offset-progress fixture 3', json.loads(json.dumps(solve({'segments':[],'budgets':[0,1]}))), json.loads(json.dumps([['',''],0,0])))
check('offset-progress fixture 4', json.loads(json.dumps(solve({'segments':['ABCDE'],'budgets':[2,1]}))), json.loads(json.dumps([['AB','C'],0,3])))
check('offset-progress fixture 5', json.loads(json.dumps(solve({'segments':["p"*N,'X'],'budgets':[N,1]}))), json.loads(json.dumps([["p"*N,'X'],2,0])))
check('offset-progress fixture 6', json.loads(json.dumps(solve({'segments':['AB','CDE'],'budgets':[4]}))), json.loads(json.dumps([['ABCD'],1,2])))
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
offset-progress fixture 0[['--', 'bH', 'BOD', 'Y\r\n'], 4, 0][['--', 'bH', 'BOD', 'Y\r\n'], 4, 0]Passed
offset-progress fixture 1[['A', '', 'B', 'C', ''], 2, 0][['A', '', 'B', 'C', ''], 2, 0]Passed
offset-progress fixture 2[['', 'AB', 'C'], 5, 0][['', 'AB', 'C'], 5, 0]Passed
offset-progress fixture 3[['', ''], 0, 0][['', ''], 0, 0]Passed
offset-progress fixture 4[['AB', 'C'], 0, 4][['AB', 'C'], 0, 3]Failed
offset-progress fixture 5[['p', 'X'], 2, 0][['p', 'X'], 2, 0]Passed
offset-progress fixture 6[['ABCD'], 1, 2][['ABCD'], 1, 2]Passed

SHA-256 / b7337e7e0d05d86789474e1ffd984746bdcd70996169db72640eb8fe95f02072

3 / The verified repair

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

N = 1
observations = []
def solve(x):
    segments=x['segments']
    index=0
    offset=0
    out=[]
    for budget in x['budgets']:
        sent=''
        while index<len(segments):
            segment=segments[index]
            if offset>=len(segment):
                index+=1
                offset=0
                continue
            if budget==0: break
            amount=min(budget,len(segment)-offset)
            sent+=segment[offset:offset+amount]
            offset+=amount
            budget-=amount
        out.append(sent)
    return [out,index,offset]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('offset-progress fixture 0', json.loads(json.dumps(solve({'segments':['--b','H','BODY','\r\n'],'budgets':[2,2,3,3]}))), json.loads(json.dumps([['--','bH','BOD','Y\r\n'],4,0])))
check('offset-progress fixture 1', json.loads(json.dumps(solve({'segments':['A','BC'],'budgets':[1,0,1,1,1]}))), json.loads(json.dumps([['A','','B','C',''],2,0])))
check('offset-progress fixture 2', json.loads(json.dumps(solve({'segments':['','A','','BC',''],'budgets':[0,2,1]}))), json.loads(json.dumps([['','AB','C'],5,0])))
check('offset-progress fixture 3', json.loads(json.dumps(solve({'segments':[],'budgets':[0,1]}))), json.loads(json.dumps([['',''],0,0])))
check('offset-progress fixture 4', json.loads(json.dumps(solve({'segments':['ABCDE'],'budgets':[2,1]}))), json.loads(json.dumps([['AB','C'],0,3])))
check('offset-progress fixture 5', json.loads(json.dumps(solve({'segments':["p"*N,'X'],'budgets':[N,1]}))), json.loads(json.dumps([["p"*N,'X'],2,0])))
check('offset-progress fixture 6', json.loads(json.dumps(solve({'segments':['AB','CDE'],'budgets':[4]}))), json.loads(json.dumps([['ABCD'],1,2])))
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
offset-progress fixture 0[['--', 'bH', 'BOD', 'Y\r\n'], 4, 0][['--', 'bH', 'BOD', 'Y\r\n'], 4, 0]Passed
offset-progress fixture 1[['A', '', 'B', 'C', ''], 2, 0][['A', '', 'B', 'C', ''], 2, 0]Passed
offset-progress fixture 2[['', 'AB', 'C'], 5, 0][['', 'AB', 'C'], 5, 0]Passed
offset-progress fixture 3[['', ''], 0, 0][['', ''], 0, 0]Passed
offset-progress fixture 4[['AB', 'C'], 0, 3][['AB', 'C'], 0, 3]Passed
offset-progress fixture 5[['p', 'X'], 2, 0][['p', 'X'], 2, 0]Passed
offset-progress fixture 6[['ABCD'], 1, 2][['ABCD'], 1, 2]Passed

SHA-256 / 90b44ffbc96f9b2307f7dad6f0bab59e25bf93bd6aebf482b6a41498dce6f587

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 2026-09-29T14:41:22.662327+00:00.

Case digest / 09313e7d5de7f8ffa0af45ec7fce58c3bae75097aeaac91ecc5b1f427d8c8f0d