FA-27151 / HTTP ranges / Open access

A multipart sender resumes framing and payload segments across transport credit windows: zero transport credit cannot emit a pending delimiter byte · case 01

A multipart sender resumes framing and payload segments across transport credit windows: zero transport credit cannot emit a pending delimiter byte.

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

ROOT CAUSE

The multipart-write-cursor-zero-credit 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: budget=1 amount=min(budget,len(segment)-offset) sent+=segment[offset:offset+amount] offset+=amount 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 and sent: break if budget==0: budget=1 amount=min(budget,len(segment)-offset) sent+=segment[offset:offset+amount] offset+=amount 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: budget=1
            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('zero-credit 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('zero-credit 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('zero-credit fixture 2', json.loads(json.dumps(solve({'segments':['','A','','BC',''],'budgets':[0,2,1]}))), json.loads(json.dumps([['','AB','C'],5,0])))
check('zero-credit fixture 3', json.loads(json.dumps(solve({'segments':[],'budgets':[0,1]}))), json.loads(json.dumps([['',''],0,0])))
check('zero-credit fixture 4', json.loads(json.dumps(solve({'segments':['ABCDE'],'budgets':[2,1]}))), json.loads(json.dumps([['AB','C'],0,3])))
check('zero-credit fixture 5', json.loads(json.dumps(solve({'segments':["p"*N,'X'],'budgets':[N,1]}))), json.loads(json.dumps([["p"*N,'X'],2,0])))
check('zero-credit 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 fixture	Actual	Expected	Outcome
zero-credit fixture 0	`[['--bHBODY\r\n', '', '', ''], 4, 0]`	`[['--', 'bH', 'BOD', 'Y\r\n'], 4, 0]`	Failed
zero-credit fixture 1	`[['ABC', '', '', '', ''], 2, 0]`	`[['A', '', 'B', 'C', ''], 2, 0]`	Failed
zero-credit fixture 2	`[['ABC', '', ''], 5, 0]`	`[['', 'AB', 'C'], 5, 0]`	Failed
zero-credit fixture 3	`[['', ''], 0, 0]`	`[['', ''], 0, 0]`	Passed
zero-credit fixture 4	`[['ABCDE', ''], 1, 0]`	`[['AB', 'C'], 0, 3]`	Failed
zero-credit fixture 5	`[['pX', ''], 2, 0]`	`[['p', 'X'], 2, 0]`	Failed
zero-credit fixture 6	`[['ABCDE'], 2, 0]`	`[['ABCD'], 1, 2]`	Failed

SHA-256 / 3f9cb56746e2c652084b87cb854fb5b95c49e40a72b9f0610d3982204172feb6

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 and sent: break
            if budget==0: budget=1
            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('zero-credit 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('zero-credit 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('zero-credit fixture 2', json.loads(json.dumps(solve({'segments':['','A','','BC',''],'budgets':[0,2,1]}))), json.loads(json.dumps([['','AB','C'],5,0])))
check('zero-credit fixture 3', json.loads(json.dumps(solve({'segments':[],'budgets':[0,1]}))), json.loads(json.dumps([['',''],0,0])))
check('zero-credit fixture 4', json.loads(json.dumps(solve({'segments':['ABCDE'],'budgets':[2,1]}))), json.loads(json.dumps([['AB','C'],0,3])))
check('zero-credit fixture 5', json.loads(json.dumps(solve({'segments':["p"*N,'X'],'budgets':[N,1]}))), json.loads(json.dumps([["p"*N,'X'],2,0])))
check('zero-credit 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 fixture	Actual	Expected	Outcome
zero-credit fixture 0	`[['--', 'bH', 'BOD', 'Y\r\n'], 4, 0]`	`[['--', 'bH', 'BOD', 'Y\r\n'], 4, 0]`	Passed
zero-credit fixture 1	`[['A', 'B', 'C', '', ''], 2, 0]`	`[['A', '', 'B', 'C', ''], 2, 0]`	Failed
zero-credit fixture 2	`[['A', 'BC', ''], 5, 0]`	`[['', 'AB', 'C'], 5, 0]`	Failed
zero-credit fixture 3	`[['', ''], 0, 0]`	`[['', ''], 0, 0]`	Passed
zero-credit fixture 4	`[['AB', 'C'], 0, 3]`	`[['AB', 'C'], 0, 3]`	Passed
zero-credit fixture 5	`[['p', 'X'], 2, 0]`	`[['p', 'X'], 2, 0]`	Passed
zero-credit fixture 6	`[['ABCD'], 1, 2]`	`[['ABCD'], 1, 2]`	Passed

SHA-256 / 1edbfc53a24625e312483c83ce9307477447156c1773085b5557c9f4e6acc2ae

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('zero-credit 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('zero-credit 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('zero-credit fixture 2', json.loads(json.dumps(solve({'segments':['','A','','BC',''],'budgets':[0,2,1]}))), json.loads(json.dumps([['','AB','C'],5,0])))
check('zero-credit fixture 3', json.loads(json.dumps(solve({'segments':[],'budgets':[0,1]}))), json.loads(json.dumps([['',''],0,0])))
check('zero-credit fixture 4', json.loads(json.dumps(solve({'segments':['ABCDE'],'budgets':[2,1]}))), json.loads(json.dumps([['AB','C'],0,3])))
check('zero-credit fixture 5', json.loads(json.dumps(solve({'segments':["p"*N,'X'],'budgets':[N,1]}))), json.loads(json.dumps([["p"*N,'X'],2,0])))
check('zero-credit 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 fixture	Actual	Expected	Outcome
zero-credit fixture 0	`[['--', 'bH', 'BOD', 'Y\r\n'], 4, 0]`	`[['--', 'bH', 'BOD', 'Y\r\n'], 4, 0]`	Passed
zero-credit fixture 1	`[['A', '', 'B', 'C', ''], 2, 0]`	`[['A', '', 'B', 'C', ''], 2, 0]`	Passed
zero-credit fixture 2	`[['', 'AB', 'C'], 5, 0]`	`[['', 'AB', 'C'], 5, 0]`	Passed
zero-credit fixture 3	`[['', ''], 0, 0]`	`[['', ''], 0, 0]`	Passed
zero-credit fixture 4	`[['AB', 'C'], 0, 3]`	`[['AB', 'C'], 0, 3]`	Passed
zero-credit fixture 5	`[['p', 'X'], 2, 0]`	`[['p', 'X'], 2, 0]`	Passed
zero-credit fixture 6	`[['ABCD'], 1, 2]`	`[['ABCD'], 1, 2]`	Passed

SHA-256 / 6d0021779c7db0de4e3403b26ccd4a46981dc5ecf7f7f8e92567a310064ea542

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 2026-09-29T14:41:22.616766+00:00.

Case digest / 2f859c4614ef5f38534dc6f6292ace2cf8f740dd8fe3ae16287798c206f622c3