FA-27731 / HTTP ranges / Open access

A delta downloader reuses matching local blocks and fetches the rest with batched ranges: the final remote block length is the remaining representation bytes · case 01

A delta downloader reuses matching local blocks and fetches the rest with batched ranges: the final remote block length is the remaining representation bytes.

Verified by executionVariant 1 · 13 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The delta-block-fetch-final-block-length decision uses T=x['total'] B=x['block'] remote=x['remote'] if B<=0 or T<0 or len(remote)!=-(-T//B): return None where={} for j,d in enumerate(x['local']): if (j+1)B<=x['local_total']: where.setdefault(d,j) copies=[] runs=[] for i,d in enumerate(remote): n=B if d in where and n==B: copies.append([iB,where[d]B,n]) continue if runs and runs[-1][1]==iB-1: runs[-1][1]=iB+n-1 else: runs.append([iB,i*B+n-1]) R=x['max_ranges'] headers=[] for k in range(0,len(runs),R): headers.append('bytes='+','.join('%d-%d'%(a,b) for a,b in runs[k:k+R])) return [copies,headers].

THE FAILURE

The delta-block-fetch-final-block-length decision uses T=x['total'] B=x['block'] remote=x['remote'] if B<=0 or T<0 or len(remote)!=-(-T//B): return None where={} for j,d in enumerate(x['local']): if (j+1)*B<=x['local_total']: where.setdefault(d,j) copies=[] runs=[] for i,d in enumerate(remote): n=B if d in where and n==B: copies.append([i*B,where[d]*B,n]) continue if runs and runs[-1][1]==i*B-1: runs[-1][1]=i*B+n-1 else: runs.append([i*B,i*B+n-1]) R=x['max_ranges'] headers=[] for k in range(0,len(runs),R): headers.append('bytes='+','.join('%d-%d'%(a,b) for a,b in runs[k:k+R])) return [copies,headers].

Unsuccessful approach: The partial repair uses T=x['total'] B=x['block'] remote=x['remote'] if B<=0 or T<0 or len(remote)!=-(-T//B): return None where={} for j,d in enumerate(x['local']): if (j+1)*B<=x['local_total']: where.setdefault(d,j) copies=[] runs=[] for i,d in enumerate(remote): n=T%B if i==len(remote)-1 else B if d in where and n==B: copies.append([i*B,where[d]*B,n]) continue if runs and runs[-1][1]==i*B-1: runs[-1][1]=i*B+n-1 else: runs.append([i*B,i*B+n-1]) R=x['max_ranges'] headers=[] for k in range(0,len(runs),R): headers.append('bytes='+','.join('%d-%d'%(a,b) for a,b in runs[k:k+R])) return [copies,headers], which still violates the stated contract.

Case contract

x has remote total length, block size, one remote digest per block (the last block may be short), local digests per local block with the local file length, and a per-request range limit R>=1. Reject nonpositive block size, negative total, or a manifest whose count is not ceil(total/block). Only complete local blocks may be copied; a full remote block whose digest occurs at any local position is copied from the first such complete local block as [dest-offset,source-offset,length]. The short final remote block is always fetched. Missing bytes form maximal adjacent inclusive runs, sent in consecutive batches of at most R ranges. Return [copies,Range values].

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):
    T=x['total']
    B=x['block']
    remote=x['remote']
    if B<=0 or T<0 or len(remote)!=-(-T//B): return None
    where={}
    for j,d in enumerate(x['local']):
        if (j+1)*B<=x['local_total']: where.setdefault(d,j)
    copies=[]
    runs=[]
    for i,d in enumerate(remote):
        n=B
        if d in where and n==B:
            copies.append([i*B,where[d]*B,n])
            continue
        if runs and runs[-1][1]==i*B-1: runs[-1][1]=i*B+n-1
        else: runs.append([i*B,i*B+n-1])
    R=x['max_ranges']
    headers=[]
    for k in range(0,len(runs),R):
        headers.append('bytes='+','.join('%d-%d'%(a,b) for a,b in runs[k:k+R]))
    return [copies,headers]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('final-block-length fixture 0', json.loads(json.dumps(solve({'block':4,'local':['b','x'],'local_total':8,'max_ranges':2,'remote':['a','b','c'],'total':10}))), json.loads(json.dumps([[[4,0,4]],['bytes=0-3,8-9']])))
check('final-block-length fixture 1', json.loads(json.dumps(solve({'block':4,'local':['z','z','a'],'local_total':12,'max_ranges':2,'remote':['a','b'],'total':8}))), json.loads(json.dumps([[[0,8,4]],['bytes=4-7']])))
check('final-block-length fixture 2', json.loads(json.dumps(solve({'block':4,'local':['t','a','b'],'local_total':12,'max_ranges':2,'remote':['a','b','t'],'total':10}))), json.loads(json.dumps([[[0,4,4],[4,8,4]],['bytes=8-9']])))
check('final-block-length fixture 3', json.loads(json.dumps(solve({'block':4,'local':[],'local_total':0,'max_ranges':2,'remote':['a','b'],'total':8}))), json.loads(json.dumps([[],['bytes=0-7']])))
check('final-block-length fixture 4', json.loads(json.dumps(solve({'block':4,'local':['k'],'local_total':4,'max_ranges':2,'remote':['a','k','b','k','c','k'],'total':24}))), json.loads(json.dumps([[[4,0,4],[12,0,4],[20,0,4]],['bytes=0-3,8-11','bytes=16-19']])))
check('final-block-length fixture 5', json.loads(json.dumps(solve({'block':4,'local':[],'local_total':0,'max_ranges':2,'remote':['a','b'],'total':10}))), json.loads(json.dumps(None)))
check('final-block-length fixture 6', json.loads(json.dumps(solve({'block':4,'local':[],'local_total':0,'max_ranges':2,'remote':['a','b','c'],'total':8}))), json.loads(json.dumps(None)))
check('final-block-length fixture 7', json.loads(json.dumps(solve({'block':4,'local':['s'],'local_total':2,'max_ranges':2,'remote':['a','s'],'total':8}))), json.loads(json.dumps([[],['bytes=0-7']])))
check('final-block-length fixture 8', json.loads(json.dumps(solve({'block':4,'local':['a'],'local_total':4,'max_ranges':2,'remote':['a','b'],'total':8}))), json.loads(json.dumps([[[0,0,4]],['bytes=4-7']])))
check('final-block-length fixture 9', json.loads(json.dumps(solve({'block':4,'local':[],'local_total':0,'max_ranges':2,'remote':[],'total':0}))), json.loads(json.dumps([[],[]])))
check('final-block-length fixture 10', json.loads(json.dumps(solve({'block':0,'local':[],'local_total':0,'max_ranges':2,'remote':['a','b'],'total':8}))), json.loads(json.dumps(None)))
check('final-block-length fixture 11', json.loads(json.dumps(solve({'block':4,'local':[],'local_total':0,'max_ranges':1,'remote':['a','b','c'],'total':12}))), json.loads(json.dumps([[],['bytes=0-11']])))
check('final-block-length fixture 12', json.loads(json.dumps(solve({'block':4,'local':['h'],'local_total':4,'max_ranges':2,'remote':["h"]*N,'total':4*N}))), json.loads(json.dumps([[[4*i,0,4] for i in range(N)],[]])))
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
final-block-length fixture 0	`[[[4, 0, 4]], ['bytes=0-3,8-11']]`	`[[[4, 0, 4]], ['bytes=0-3,8-9']]`	Failed
final-block-length fixture 1	`[[[0, 8, 4]], ['bytes=4-7']]`	`[[[0, 8, 4]], ['bytes=4-7']]`	Passed
final-block-length fixture 2	`[[[0, 4, 4], [4, 8, 4], [8, 0, 4]], []]`	`[[[0, 4, 4], [4, 8, 4]], ['bytes=8-9']]`	Failed
final-block-length fixture 3	`[[], ['bytes=0-7']]`	`[[], ['bytes=0-7']]`	Passed
final-block-length fixture 4	`[[[4, 0, 4], [12, 0, 4], [20, 0, 4]], ['bytes=0-3,8-11', 'bytes=16-19']]`	`[[[4, 0, 4], [12, 0, 4], [20, 0, 4]], ['bytes=0-3,8-11', 'bytes=16-19']]`	Passed
final-block-length fixture 5	`None`	`None`	Passed
final-block-length fixture 6	`None`	`None`	Passed
final-block-length fixture 7	`[[], ['bytes=0-7']]`	`[[], ['bytes=0-7']]`	Passed
final-block-length fixture 8	`[[[0, 0, 4]], ['bytes=4-7']]`	`[[[0, 0, 4]], ['bytes=4-7']]`	Passed
final-block-length fixture 9	`[[], []]`	`[[], []]`	Passed
final-block-length fixture 10	`None`	`None`	Passed
final-block-length fixture 11	`[[], ['bytes=0-11']]`	`[[], ['bytes=0-11']]`	Passed
final-block-length fixture 12	`[[[0, 0, 4]], []]`	`[[[0, 0, 4]], []]`	Passed

SHA-256 / bdabb23c703026f18f304442efa0350762cc4f4b8bbbd989515aa37ae24a8ddd

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(x):
    T=x['total']
    B=x['block']
    remote=x['remote']
    if B<=0 or T<0 or len(remote)!=-(-T//B): return None
    where={}
    for j,d in enumerate(x['local']):
        if (j+1)*B<=x['local_total']: where.setdefault(d,j)
    copies=[]
    runs=[]
    for i,d in enumerate(remote):
        n=T%B if i==len(remote)-1 else B
        if d in where and n==B:
            copies.append([i*B,where[d]*B,n])
            continue
        if runs and runs[-1][1]==i*B-1: runs[-1][1]=i*B+n-1
        else: runs.append([i*B,i*B+n-1])
    R=x['max_ranges']
    headers=[]
    for k in range(0,len(runs),R):
        headers.append('bytes='+','.join('%d-%d'%(a,b) for a,b in runs[k:k+R]))
    return [copies,headers]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('final-block-length fixture 0', json.loads(json.dumps(solve({'block':4,'local':['b','x'],'local_total':8,'max_ranges':2,'remote':['a','b','c'],'total':10}))), json.loads(json.dumps([[[4,0,4]],['bytes=0-3,8-9']])))
check('final-block-length fixture 1', json.loads(json.dumps(solve({'block':4,'local':['z','z','a'],'local_total':12,'max_ranges':2,'remote':['a','b'],'total':8}))), json.loads(json.dumps([[[0,8,4]],['bytes=4-7']])))
check('final-block-length fixture 2', json.loads(json.dumps(solve({'block':4,'local':['t','a','b'],'local_total':12,'max_ranges':2,'remote':['a','b','t'],'total':10}))), json.loads(json.dumps([[[0,4,4],[4,8,4]],['bytes=8-9']])))
check('final-block-length fixture 3', json.loads(json.dumps(solve({'block':4,'local':[],'local_total':0,'max_ranges':2,'remote':['a','b'],'total':8}))), json.loads(json.dumps([[],['bytes=0-7']])))
check('final-block-length fixture 4', json.loads(json.dumps(solve({'block':4,'local':['k'],'local_total':4,'max_ranges':2,'remote':['a','k','b','k','c','k'],'total':24}))), json.loads(json.dumps([[[4,0,4],[12,0,4],[20,0,4]],['bytes=0-3,8-11','bytes=16-19']])))
check('final-block-length fixture 5', json.loads(json.dumps(solve({'block':4,'local':[],'local_total':0,'max_ranges':2,'remote':['a','b'],'total':10}))), json.loads(json.dumps(None)))
check('final-block-length fixture 6', json.loads(json.dumps(solve({'block':4,'local':[],'local_total':0,'max_ranges':2,'remote':['a','b','c'],'total':8}))), json.loads(json.dumps(None)))
check('final-block-length fixture 7', json.loads(json.dumps(solve({'block':4,'local':['s'],'local_total':2,'max_ranges':2,'remote':['a','s'],'total':8}))), json.loads(json.dumps([[],['bytes=0-7']])))
check('final-block-length fixture 8', json.loads(json.dumps(solve({'block':4,'local':['a'],'local_total':4,'max_ranges':2,'remote':['a','b'],'total':8}))), json.loads(json.dumps([[[0,0,4]],['bytes=4-7']])))
check('final-block-length fixture 9', json.loads(json.dumps(solve({'block':4,'local':[],'local_total':0,'max_ranges':2,'remote':[],'total':0}))), json.loads(json.dumps([[],[]])))
check('final-block-length fixture 10', json.loads(json.dumps(solve({'block':0,'local':[],'local_total':0,'max_ranges':2,'remote':['a','b'],'total':8}))), json.loads(json.dumps(None)))
check('final-block-length fixture 11', json.loads(json.dumps(solve({'block':4,'local':[],'local_total':0,'max_ranges':1,'remote':['a','b','c'],'total':12}))), json.loads(json.dumps([[],['bytes=0-11']])))
check('final-block-length fixture 12', json.loads(json.dumps(solve({'block':4,'local':['h'],'local_total':4,'max_ranges':2,'remote':["h"]*N,'total':4*N}))), json.loads(json.dumps([[[4*i,0,4] for i in range(N)],[]])))
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
final-block-length fixture 0	`[[[4, 0, 4]], ['bytes=0-3,8-9']]`	`[[[4, 0, 4]], ['bytes=0-3,8-9']]`	Passed
final-block-length fixture 1	`[[[0, 8, 4]], ['bytes=4-3']]`	`[[[0, 8, 4]], ['bytes=4-7']]`	Failed
final-block-length fixture 2	`[[[0, 4, 4], [4, 8, 4]], ['bytes=8-9']]`	`[[[0, 4, 4], [4, 8, 4]], ['bytes=8-9']]`	Passed
final-block-length fixture 3	`[[], ['bytes=0-3']]`	`[[], ['bytes=0-7']]`	Failed
final-block-length fixture 4	`[[[4, 0, 4], [12, 0, 4]], ['bytes=0-3,8-11', 'bytes=16-19']]`	`[[[4, 0, 4], [12, 0, 4], [20, 0, 4]], ['bytes=0-3,8-11', 'bytes=16-19']]`	Failed
final-block-length fixture 5	`None`	`None`	Passed
final-block-length fixture 6	`None`	`None`	Passed
final-block-length fixture 7	`[[], ['bytes=0-3']]`	`[[], ['bytes=0-7']]`	Failed
final-block-length fixture 8	`[[[0, 0, 4]], ['bytes=4-3']]`	`[[[0, 0, 4]], ['bytes=4-7']]`	Failed
final-block-length fixture 9	`[[], []]`	`[[], []]`	Passed
final-block-length fixture 10	`None`	`None`	Passed
final-block-length fixture 11	`[[], ['bytes=0-7']]`	`[[], ['bytes=0-11']]`	Failed
final-block-length fixture 12	`[[], ['bytes=0--1']]`	`[[[0, 0, 4]], []]`	Failed

SHA-256 / 3b98e20db77e08d74fff00d8e6aa999ae830dc5c7f12734b0fecf4f760c06604

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 13 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.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

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 2026-09-29T14:41:27.677491+00:00.

Case digest / 42fc54fe17e29bffb1f079e7cde1609610eb6085801150a7036dc900d2084af3