vfs: answer for unowned paths under reserved root

[pipobscure]

The module loader manufactures paths under the reserved VFS root that no layer owns: resolving a mount point as a directory first probes the sibling names <mount>.js, <mount>.json and <mount>.node, and a package.json walk-up passes the parents of the mount point. The lookup declined those because their layer segment is not a plain id, so they fell through to the native loader and the real file system.

On POSIX that is harmless (ENOTDIR under /dev/null), but on Windows the root sits under \\.\nul, and \\.\nul\<anything> opens the NUL device: libuv reports it as a character device and a read returns nothing. The loader therefore picked \\.\nul\vfs\<id>.js as an existing file, and the native walk-up above it then read the device as an empty package.json and failed with ERR_INVALID_PACKAGE_CONFIG for \\.\nul\package.json. Any require() of a mount point hits this on Windows.

Distinguish "under the root but unowned" from "outside the root" in the lookup and have every loader override report the former as not found: stat gives ENOENT, reads and realpath throw ENOENT, the package.json lookups return their "no package.json" results, and upward walks stop at the reserved root. Paths outside the root still go to the native loader as before.

NOTE: This was only validated agains a pretty bad Windows VM on my end. So if someone that has access to a better Windows environment could double check this work, I'd appreciate it.

[mcollina]

lgtm

[mcollina]

let’s see what CI thinks

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.18%. Comparing base (2d22505) to head (fc612c8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/vfs/setup.js	94.59%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65814      +/-   ##
==========================================
- Coverage   90.19%   90.18%   -0.01%     
==========================================
  Files         770      770              
  Lines      264410   264448      +38     
  Branches    50243    50248       +5     
==========================================
+ Hits       238479   238491      +12     
- Misses      16926    16974      +48     
+ Partials     9005     8983      -22     

Files with missing lines	Coverage Δ
lib/internal/vfs/setup.js	87.79% <94.59%> (+0.27%)	⬆️

... and 37 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77035/