compile()/check()/lint() on a string fail when the base directory is a filesystem root ("cannot resolve path /: file not found: /")

[dean0x]

Summary

Any string-source compile / check / lint whose base directory resolves to a filesystem root fails with mds::io:

cannot resolve path /: file not found: /

Found by the new Alpine load test in PR #370 (#340): inside docker run node:22-alpine node /w/probe.cjs … the musl addon dlopened correctly and require.resolve landed on the right binary, but compile('Hello {{n}}!', { vars: { n: 'alpine' } }) threw. The container has no WORKDIR, so process.cwd() is /.

Trigger

The base directory is a filesystem root, either implicitly (process.cwd() / std::env::current_dir() == / — e.g. a Docker container with no WORKDIR) or explicitly (basePath: "/"). Reproduced on macOS with the prebuilt darwin addon and with the prebuilt CLI; no musl, read-only mount, bind mount, symlink or realpath involvement — it is pure std::path logic evaluated before any I/O, so Windows drive roots (C:\) hit it too.

cwd=/                        compile -> THREW cannot resolve path /: file not found: /   (code mds::io)
cwd=/Users/dean/Sandbox/mdl  compile -> {"kind":"markdown","output":"Hello x!\n",...}
cwd=/private/tmp             compile -> OK
basePath "/tmp" from cwd /   -> OK
basePath "/"    from cwd /   -> THREW (same error)
check / lint from cwd /      -> THREW (same error)
$ cd / && echo 'Hello {{n}}!' | mds build - --set n=x
mds::io
  × cannot resolve path /: file not found: /

A read-only cwd is fine; a direct child of / is fine. Only a root fails.

Root cause

• crates/mds-core/src/lib.rs:456-462 (resolve_base_dir): the None arm returns std::env::current_dir() verbatim (/).
• crates/mds-core/src/resolver.rs:626 and :652 pass it to self.fs.canonicalize(base_dir).
• crates/mds-core/src/fs.rs:538-556 (NativeFs::canonicalize) delegates to check_symlink, whose first statement in check_symlink_named (crates/mds-core/src/fs.rs:408-410) is

  let file_name = path.file_name().ok_or_else(|| MdsError::file_not_found(shown.to_string()))?;

  Path::new("/").file_name() is None, so it returns FileNotFound("/") before any syscall; fs.rs:549-553 re-wraps it as mds::io → cannot resolve path /: file not found: /.
• effective_parent (fs.rs:310-316) is never reached; its doc covers the PF-006 bare-name case but not a root.

Blast radius

Affected (all reach resolve_base_dir → NativeFs::canonicalize):

• @mdscript/mds-napi string compile / check / lint — crates/mds-napi/src/lib.rs:681, :749, :984 (reproduced).
• CLI stdin paths mds build -, mds check -, mds lint - — crates/mds-cli/src/build.rs:804, :1268, crates/mds-cli/src/main.rs:294, crates/mds-cli/src/lint.rs:844 (reproduced). File-path compiles are unaffected (a file path has a file_name()).
• Python markdown_script.compile / check / lint — crates/mds-python/src/lib.rs:1407, :1493, :1560 (same call site; not executed locally).
• @mdscript/mds on the native backend (forwards no basePath by default; packages/mds/src/types.ts:114 documents the cwd default).

Not affected: WASM (compile_virtual_with_deps_opts, VirtualFs) and every compile_virtual* entry point.

No existing test covers a root base directory: crates/mds-napi/__test__/index.spec.mjs:215 only chdirs to FIXTURES; crates/mds-core/src/fs.rs:1345-1372 covers bare / ./ / subdir / absolute paths but never a root; the '/some/path' uses in the napi spec are rejection tests that never reach the fs.

Suggested fix

Handle the root case in check_symlink_named (or in NativeFs::canonicalize, since base-dir resolution is a directory resolve, not an import resolve): when path.parent() is None, a root can never itself be a symlink, so std::fs::canonicalize(path) plus an existence / is_dir check is sufficient and the file_name() split is unnecessary. Tests: NativeFs::canonicalize("/") succeeds; a napi and a Python string compile with basePath: "/" succeed; a CLI stdin build with cwd / succeeds (guard the CLI test for Windows drive roots).

Interim

PR #370's Alpine load tests run the container with -w /w (the read-only fixture directory), which is the shape a real user gets with any non-root working directory; the probe asserts its cwd so a future removal of -w fails loudly instead of resurfacing this error.

Related

• ci(release): load-test the linux-x64-musl addon on Alpine before upload #340 (the load test that surfaced this), PR ci(release): load-test both musl addons on node:22-alpine before publish (#340) #370
• PF-006 (bare relative filename symlink-check case, the sibling edge of the same function)