feat(check): support copy-on-write sandboxing on Linux#156
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`select_sandbox()` gated the real CoW sandbox to macOS and routed every other target — including Linux — to `UnsupportedSandbox`, which always errored. `multi check` therefore could not create a sandbox on Linux at all. Add a native Linux implementation, `ReflinkSandbox`. Linux has no whole-tree clone syscall like macOS's `clonefile`, so it recreates the directory structure under a temp dir and clones each regular file with the `FICLONE` ioctl (a reflink — a metadata-only, copy-on-write share of the file's extents). `FICLONE` only works on CoW filesystems (Btrfs, XFS with reflink=1, bcachefs); on others (ext4, tmpfs) it fails and we fall back to a plain byte copy, so the sandbox is always an independent clone — just without the CoW space savings. Permissions (incl. exec bits) are preserved on both paths, and symlinks are recreated verbatim. Wire it into the `cfg` selection (macOS → clonefile, Linux → reflink, other → stub) and widen the independence test to run on Linux too. Update CHECKS.md to require sandboxing on both macOS and Linux. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Problem
select_sandbox()gated the real copy-on-write sandbox totarget_os = "macos"and routed everything else — including Linux — toUnsupportedSandbox, which always errors. Somulti checkcould not create a sandbox on Linux at all; every run failed before an agent could start.Fix
A native Linux CoW sandbox,
ReflinkSandbox(src/checks/sandbox/linux.rs):clonefile(2), so it walks the tree, recreates directories/symlinks, and clones each regular file with theFICLONEioctl — a reflink (metadata-only, copy-on-write share of the file's extents).FICLONEonly works on CoW filesystems (Btrfs, XFSreflink=1, bcachefs). On ext4/tmpfs/overlay it fails, so we fall back to a plain byte copy — the sandbox is always a fully independent clone, just without the space savings.Wiring (
src/checks/sandbox/mod.rs):cfgselection is now macOS →clonefile, Linux → reflink, any other target → the erroring stub. The independence test now runs on Linux too.CHECKS.md: the sandbox requirement is now Cross-Platform and Platform-Gated — it fails if a real sandbox is created on only one of macOS/Linux.Validation
cargo make clippy+cargo make test(119 pass).macos.rsis not built): compiled the exact reflink logic in a Linux container — clippy-clean; fallback copy path verified on overlay fs;FICLONEsuccess path verified on a real btrfs loopback (reflink OKper file). Independence, nested dirs, symlink preservation, exec-bit preservation, and RAII teardown all pass on both paths.Note
Reflinks only engage when
$TMPDIRand the working tree share the same CoW filesystem (same cross-volume constraint macOSclonefilehas). Otherwise every file takes the correct-but-not-CoW copy fallback. A future tweak — placing the temp dir as a sibling of the source tree — would guarantee reflinks; left out of this PR to keep parity with the macOS implementation.🤖 Generated with Claude Code