[robobun]

Fixes #44875

Problem

• Inside a Windows AppContainer, fs.promises.realpath, fs.realpath.native and fs.realpathSync.native fail with EPERM: operation not permitted, realpath 'C:\...' for every path, including directories the container can read and write. chokidar catches the error, climbs to dirname() forever at the drive root, and a sandboxed Bun binary burns one or two cores idle.
• The cause: realpath_inner (src/runtime/node/node_fs.rs:7575) called uv_fs_realpath. libuv queries GetFinalPathNameByHandleW(VOLUME_NAME_DOS) through the raw kernel32 import. The DOS translation opens the mount manager, which a lowbox token is denied. Bun's own wrapper (src/sys/windows/mod.rs:849) already has the fallback from windows: run Bun inside an AppContainer (lowbox token) #33119, but libuv bypasses it.

Fix

• The Windows sys::realpath body (src/sys/lib.rs:4436) now does what libuv did (attribute-only CreateFileW, then the final name by handle) through windows::GetFinalPathNameByHandle. On a lowbox token, that wrapper rebuilds X:\... from the NT name for the system volume.
• realpath_inner calls sys::realpath on Windows. The equal-input shortcut is unchanged. The result is returned as is, as fs.realpath: a backslash is part of the file name on POSIX, and each variant reads its path as Node does #45423 made it.
• Errors keep their Win32 code: GetFinalPathNameByHandleError::FileNotFound is now Failed(Win32Error), mapped with Error::from_win32. ENOENT, ENOTDIR, EPERM and ENAMETOOLONG come out as before. get_fd_path and get_fd_path_w no longer collapse every failure to EBADF.
• Verified: test/js/bun/windows/appcontainer.test.ts now asserts that the three native variants return the path the host sees, and that fs.realpathSync stays at EPERM. cargo check --workspace passes on x86_64 and aarch64 windows-msvc. test/js/node/fs/fs.test.ts -t realpath passes on Linux. The AppContainer test ran on the Windows x64 and aarch64 CI lanes (1 pass, 9 assertions each). It skips on other platforms, so the Linux gate cannot prove it.

Background

• An AppContainer (lowbox) token cannot open \\.\MountPointManager, so VOLUME_NAME_DOS fails with ERROR_ACCESS_DENIED. VOLUME_NAME_NT still works. windows: run Bun inside an AppContainer (lowbox token) #33119 added lowbox_dos_name_fallback, which composes <system drive>: + the NT tail, for Bun's resolver and bun install, and left user-facing fs.realpath at Node parity as a known limitation.
• This is the shape the libuv-removal branch Remove libuv on Windows #42819 already uses for sys::realpath (attribute-only open, the wrapper, Failed(Win32Error)). Landing it first makes that hunk a no-op on rebase. The branch is rebased on fs.realpath: a backslash is part of the file name on POSIX, and each variant reads its path as Node does #45423, which removed the emulated trailing-backslash trim from these lines.
• Considered an EPERM retry around the libuv call: libuv would still issue the denied query on every call, and the retry adds a second open with GENERIC_READ that fails for paths the token can open attribute-only but not read.

Downsides

• Inside a container, the native variants answer only for paths on the system volume. A path on another volume still returns EPERM, as before.
• fs.realpathSync and callback fs.realpath on Windows are the Node-ported component walk. They lstat every ancestor, including the drive root the container cannot read, and still fail with EPERM. chokidar 4 uses fs/promises, which this fixes. chokidar 3 uses callback fs.realpath and still loops.
• The seven internal sys::realpath callers (bun install bin linking, patched packages, the compile cache, bun lint, the TypeScript host) now open the path with no access bits instead of GENERIC_READ, and get the real errno instead of EBADF. None of them matches on EBADF. patchPackage.rs:799 matches on ENOENT, which the Win32 mapping preserves. Per call this removes one WCHAR allocation, one CRT fd and one kernel32 call. Not measured: no Windows machine here.

Notes

• The issue also asked to tolerate a denied drive root in the JS walk. That alone does not make fs.realpathSync succeed: the walk then fails at lstat 'C:\Users', and there is a second walk in callback fs.realpath. Both stay at Node parity.
• libuv's fs__realpath is CreateFileW(path, 0, 0, NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL) followed by GetFinalPathNameByHandleW(FILE_NAME_NORMALIZED | VOLUME_NAME_DOS) and a strip of the \\?\ or \\?\UNC\ prefix. The wrapper strips the same prefixes. Errors map through Bun's Win32 to errno table (Error::from_win32), the one every other bun_sys call on Windows uses. It differs from libuv's in two rows: ERROR_DIRECTORY gives ENOTDIR (libuv: ENOENT) and ERROR_FILENAME_EXCED_RANGE gives ENAMETOOLONG (libuv: ENOENT).
• lowbox_dos_name_fallback returns Failed(ACCESS_DENIED) when it cannot answer, so a non-system volume surfaces the original denial.
• Userland workaround until this ships: BUN_OPTIONS="--preload ./realpath-shim.js" with a shim that wraps the realpath variants, catches EPERM, and walks with lstat/readlink while skipping the drive root.
• Self-reviewed: 4 concerns raised, 4 addressed (limitation documented, get_fd_path mapping, rebased on fs.realpath: a backslash is part of the file name on POSIX, and each variant reads its path as Node does #45423 after it merged, test guards the system-drive assumption).

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no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/windows/appcontainer.test.ts

[robobun]

^{Updated 3:15 PM PT - Oct 19th, 2026}

✅ @robobun, your commit 8702d3e5955cbf679d8ce1ed640310360f25ca5f passed in Build #128977! 🎉

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bunx bun-pr 45436

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Walkthrough

Windows realpath now uses handle-based final-name resolution. NodeFS calls sys::realpath, which maps lookup errors and closes the handle after resolution. AppContainer tests cover native realpath variants and retain the expected EPERM result for the Node-ported synchronous variant.

Changes

Windows AppContainer realpath

Layer / File(s)	Summary
Preserve final-name lookup errors src/sys/windows/mod.rs, src/sys/lib.rs	Final-name lookup errors now retain Win32 error information. Lowbox fallback failures and fd path lookups map those errors, with ENAMETOOLONG handling for long paths.
Resolve Windows realpaths by handle src/sys/lib.rs, src/runtime/node/node_fs.rs, test/js/bun/windows/appcontainer.test.ts	Windows sys::realpath opens paths with attribute-only access and resolves their final names. NodeFS calls this implementation. AppContainer tests check native realpath results against host resolution when the fixture is on the system drive; the Node-ported synchronous variant is still expected to return EPERM.

Suggested reviewers: jarred-sumner, dylan-conway

Priority: ➖ Normal

Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 8702d

The sharing behavior was already present in Windows realpath. No PR-introduced issue established by the supplied evidence remains to block merging.

Pre-merge checks: 3 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Linked Issues check		Issue #44875 requires both native and non-native Windows fs.realpath behavior for readable AppContainer paths. The PR routes the native variants through sys::realpath and tests their system-volume…	Update the Windows non-native realpath walk to tolerate EPERM for the AppContainer drive root. Add AppContainer assertions that fs.realpathSync and callback fs.realpath resolve a readable system-volume path.

Passed checks: 3 passed

Check name	Status	Explanation
Title check		The title clearly identifies the main change: routing Windows Node.js realpath handling through bun_sys so it works in an AppContainer.
Description check		The description explains the problem, implementation, limitations, linked issue, and verification. It provides the required information for what the PR does and how it was verified, although it uses d…
Out of Scope Changes check		The realpath routing, system-volume DOS-name fallback, Win32 error preservation, and AppContainer regression coverage directly support issue #44875. No unrelated change is demonstrated.

Full details: Description check

Explanation

The description explains the problem, implementation, limitations, linked issue, and verification. It provides the required information for what the PR does and how it was verified, although it uses different section headings from the template.

Full details: Linked Issues check

Explanation

Issue #44875 requires both native and non-native Windows fs.realpath behavior for readable AppContainer paths. The PR routes the native variants through sys::realpath and tests their system-volume result. The non-native fs.realpathSync and callback fs.realpath walk still fails with EPERM at the drive root. The test intentionally asserts that failure, so the non-native requirement remains unmet.

Resolution

Update the Windows non-native realpath walk to tolerate EPERM for the AppContainer drive root. Add AppContainer assertions that fs.realpathSync and callback fs.realpath resolve a readable system-volume path.

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[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @src/sys/lib.rs:
- Line 4455: Update the CreateFileW call used by realpath to pass the existing
FILE_SHARE mask as its dwShareMode instead of zero, allowing shared access while
preserving the current path-resolution flow.

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[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline findings, I also checked the handle lifetime in the new Windows sys::realpath (the CloseHandle scopeguard is armed right after CreateFileW succeeds, before the fallible GetFinalPathNameByHandle, and every early return drops it), the two other match sites on GetFinalPathNameByHandleError in src/sys/lib.rs (~6693 and ~6718 use Err(_), so removing FileNotFound leaves no stale arm), and the pool-buffer borrows (wpath shadows but does not drop the pool guard; the returned slice borrows the caller's PathBuffer, not wide).

Extended reasoning...

The change replaces the libuv-backed Windows realpath in src/runtime/node/node_fs.rs with a bun_sys implementation (attribute-only CreateFileW plus the crate's GetFinalPathNameByHandle wrapper) and widens the wrapper's error enum to carry the real Win32 code; it touches no auth, injection or data-exposure surface. The ruled-out note records the memory-safety and enum-exhaustiveness checks that were examined and found sound, while the inline findings on errno mapping and the test's system-drive assumption remain for a human to weigh.

[claude]

I re-reviewed the latest push and found no new bugs; since the previous round the AppContainer test now branches on whether the temp directory sits on the system drive, which covers the CI-layout case raised earlier. A human look is still worthwhile because the Windows-only code cannot be compiled or exercised on this host.

What was reviewed:

• New windows_impl::realpath body in src/sys/lib.rs: every w::/bun_paths symbol it uses resolves in crate::windows and string_paths, the CreateFileW handle is released by the scopeguard on every exit, and the flags match libuv's fs__realpath.
• GetFinalPathNameByHandleError::FileNotFound → Failed(Win32Error): the remaining matches in the repo use NameTooLong/Err(_) only (src/sys/lib.rs:6688, :6713), so no cfg(windows) site is left referencing the removed variant.
• get_fd_path's new .with_fd(fd) carries a debug assert on Fd::INVALID; the call sites I checked all pass handles obtained from a successful open.
• node_fs.rs still uses UvFsReq, uv::, and c_char elsewhere, so the removed block leaves no unused imports.

Extended reasoning...

The PR replaces the libuv uv_fs_realpath call behind Windows fs.realpath.native/realpathSync.native/fs.promises.realpath with a bun_sys implementation (attribute-only CreateFileW + the in-tree GetFinalPathNameByHandle wrapper that has the AppContainer system-volume fallback), changes the wrapper's error enum to carry the Win32 code, and stops get_fd_path/get_fd_path_w from collapsing failures to EBADF/ENOENT. It touches no auth, crypto, or injection surface; the sensitive part is errno-contract drift for the roughly seven internal sys::realpath callers and the many get_fd_path callers, which the author has documented. Deciding against approve: all four files are #[cfg(windows)]-gated and cannot be type-checked or run here, the only test is a Windows-CI-only AppContainer harness, and two earlier optional findings were closed by author self-resolution rather than code changes.

[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.