fix(node:fs): preserve symlink traversal in realpath

[steipete]

What does this PR do?

Fixes POSIX node:fs realpath handling when .. follows a symlink. The native implementation first normalized the input to an absolute path, so link/../target could resolve to a lexical collision beside link instead of following link and then traversing to its parent.

Pass the original NUL-terminated path to the OS so component traversal keeps POSIX semantics. The explicit input-length check preserves ENAMETOOLONG behavior, and Windows remains on its existing implementation.

This was found while validating OpenClaw under Bun; the matching application workaround is tracked in openclaw/openclaw#144700.

AI assistance: This change was developed and reviewed with Codex.

How did you verify your code works?

• The new collision regression failed on the original release binary and passed after the source change.
• Incremental release build completed successfully with LLVM 21.1.8 and the repository's pinned Rust toolchain.
• Focused realpath tests: 11 passed, 1 platform skip, 0 failed.
• cargo fmt --all -- --check, configured Prettier check, and git diff --check passed.
• Independent P0-P2 review found no actionable issues.

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📥 Commits

Reviewing files that changed from the base of the PR and between 81f97bb and 133b7b9.

📒 Files selected for processing (2)

• src/runtime/node/node_fs.rs
• test/js/node/fs/fs.test.ts

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Walkthrough

The POSIX realpath implementation now resolves original path components directly through the OS. A regression test covers symlink resolution followed by .. traversal across all supported API variants.

Changes

POSIX realpath resolution

Layer / File(s)	Summary
Direct POSIX path resolution src/runtime/node/node_fs.rs	The POSIX implementation removes the resolver filesystem dependency, validates path length, and passes the original path directly to the OS.
Symlink traversal regression coverage test/js/node/fs/fs.test.ts	The test verifies synchronous, native synchronous, promise, callback, and native callback realpath variants for a symlink followed by .. traversal.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to 133b7

The realpath fix preserves symlink traversal semantics and is covered across the affected API forms. No merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description includes both required sections. It clearly explains the POSIX realpath fix, preserves ENAMETOOLONG behavior, identifies Windows scope, and documents verification results.
Title check	✅ Passed	The title clearly and concisely describes the main change: preserving symlink traversal in node:fs realpath handling.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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