fix(downloader): in-process SHA-256 + per-candidate integrity fallback#321
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Two defects found by mcpp's macos-14 fresh-install lane (mcpp PR #123): 1. sha256 verification shelled out to `sha256sum` — a GNU coreutils tool absent on stock macOS (only `shasum` exists). On hosts without coreutils every sha256-pinned download failed as 'SHA256 mismatch' regardless of payload integrity (macos-14 runner image; macos-15 happens to carry it transitively). Replaced both call sites with an in-process FIPS 180-4 implementation (xlings.libs.sha256) — no host tool dependency on any platform. 2. Integrity was checked AFTER the multi-candidate download loop: a mirror that wins the adaptive latency race but serves corrupted bytes failed the whole download with the remaining candidates (including the author URL) untried. download_file now takes an onVerify per-candidate acceptance hook — a rejected candidate is removed, penalized (mirror::adaptive), and the next URL is tried. Unit tests: NIST vectors + padding boundaries + file digests for sha256; candidate-fallback behavior via a transferOverride test seam.
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Context
mcpp's new macos-14 fresh-install CI lane (mcpp-community/mcpp PR #123) caught
mcpplibs.cmdline@0.0.1failing withSHA256 mismatchon every attempt — including with the author URL first and bytes proven intact via curl (shasummatched the declared sha256).Root causes
sha256sumshell-out: stock macOS has nosha256sum(coreutils; onlyshasumships).run_command_capturefails → treated as mismatch → every sha256-pinned download fails on such hosts, irrespective of content. The macos-14 runner image lacks it; macos-15 happens to have it transitively, masking the bug there.download_filereturned, so a fast-but-corrupt mirror (adaptive ordering may put mirrors before the author URL when bytes are sha-pinned) failed the whole download with the remaining candidates untried.Fix
xlings.libs.sha256: in-process FIPS 180-4 SHA-256 (streaming hasher +hex/hex_file); both downloader call sites switched — no host-tool dependency on any platform.download_filegains anonVerifyper-candidate acceptance hook: rejected candidate → file removed,onUrlAttemptFailedfires (host penalized viamirror::adaptive), next URL tried. Wired to the sha check; behavior unchanged when no hook is set.Tests
test_sha256.cpp: NIST vectors (empty/abc/two-block/million-a), padding boundaries (55/56/63/64 + split updates), file digest, missing file.DownloadVerify.*: rejected candidate falls through to next URL; all-rejected fails with file removed; no-hook keeps prior behavior (via atransferOverridetest seam — same pattern as the exportedStallDetector).Extract.ZipUtf8PathRoundTrip, which fails identically on clean main (local host zip/locale issue, green on CI).