Detect Windows junctions with GetFileAttributesEx

[jakebailey]

Faster than doing a realpath + comparison.

goos: windows
goarch: amd64
pkg: github.com/microsoft/typescript-go/internal/vfs/osvfs
cpu: Intel(R) Core(TM) i9-10900K CPU @ 3.70GHz
                                    │   out.txt   │
                                    │   sec/op    │
Realpath/target-20                    76.62µ ± 1%
Realpath/link-20                      87.68µ ± 2%
IsSymlinkOrJunction/regular_file-20   28.60µ ± 2%
IsSymlinkOrJunction/junction-20       22.62µ ± 2%
IsSymlinkOrJunction/nonexistent-20    11.45µ ± 0%
geomean                               34.63µ

                                    │  out.txt   │
                                    │    B/op    │
Realpath/target-20                    944.0 ± 0%
Realpath/link-20                      944.0 ± 0%
IsSymlinkOrJunction/regular_file-20   192.0 ± 0%
IsSymlinkOrJunction/junction-20       176.0 ± 0%
IsSymlinkOrJunction/nonexistent-20    192.0 ± 0%
geomean                               356.8

                                    │  out.txt   │
                                    │ allocs/op  │
Realpath/target-20                    8.000 ± 0%
Realpath/link-20                      8.000 ± 0%
IsSymlinkOrJunction/regular_file-20   1.000 ± 0%
IsSymlinkOrJunction/junction-20       1.000 ± 0%
IsSymlinkOrJunction/nonexistent-20    1.000 ± 0%
geomean                               2.297

[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]

To actually test this, I switched us over to the Microsoft fork of Go, which disables the SEH bit that auto-handles longpaths. I can split that out into its own PR for clarity if desired.