Detect Windows junctions with GetFileAttributesEx

[jakebailey]

This should be faster than doing a realpath.

[Copilot]

Pull Request Overview

This PR refactors Windows junction and symlink detection by replacing the Realpath-based approach with a dedicated IsSymlinkOrJunction function. The change improves efficiency by using direct Windows API calls to check for reparse points instead of resolving full paths.

• Adds platform-specific isSymlinkOrJunction implementations for Windows and Unix-like systems
• Updates internal.Common struct to use IsSymlinkOrJunction instead of Realpath field
• Simplifies junction detection logic in GetAccessibleEntries

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
internal/vfs/osvfs/symlink_windows.go	New file implementing Windows-specific junction/symlink detection using FILE_ATTRIBUTE_REPARSE_POINT
internal/vfs/osvfs/symlink_other.go	New file with nil function stub for Unix-like systems where detection is unnecessary
internal/vfs/osvfs/os.go	Updates osVFS initialization to use IsSymlinkOrJunction instead of Realpath
internal/vfs/internal/internal.go	Replaces Realpath field with IsSymlinkOrJunction in Common struct and simplifies detection logic in GetAccessibleEntries

[DanielRosenwasser]

Any specific differences you've seen in perf?

[jakebailey]

Haven't tested yet, just cleaning up some TODOs with my name on it. On any project I have it probably won't be very noticable...

[jakebailey]

If something is a junction, maybe we don't even need to stat it and just say that it's a dir? (I don't believe junctions/reparse points can be files, in which case I could rename funcs too)

[RyanCavanaugh]

We might want to prepend \\?\ to automatically handle long filepaths? https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getfileattributesexa

[jakebailey]

Ah, crap, converting those is a whole thing that is hard and not available to us outside stdlib. I'd need to do something like I did in realpath

func realpath(path string) (string, error) {
	var h windows.Handle
	if len(path) < 248 {
		var err error
		h, err = openMetadata(path)
		if err != nil {
			return "", err
		}
		defer windows.CloseHandle(h) //nolint:errcheck
	} else {
		// For long paths, defer to os.Open to run the path through fixLongPath.
		f, err := os.Open(path)
		if err != nil {
			return "", err
		}
		defer f.Close()

		// Works on directories too since https://go.dev/cl/405275.
		h = windows.Handle(f.Fd())
	}

Guess I'll wait on things until I figure that out.

[RyanCavanaugh]

I wouldn't block this on it

[jakebailey]

It's pretty easy to hit this on the use cases where it matters, e.g. pnpm ones, so I should definitely check it in afraid