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Detect Windows junctions with GetFileAttributesEx #2013

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Detect Windows junctions with GetFileAttributesEx #2013

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dd027bb
Replace Realpath with more efficient GetFileAttributesEx
jakebailey Nov 4, 2025
84a0244
Nil it out, comment
jakebailey Nov 4, 2025
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14 changes: 7 additions & 7 deletions internal/vfs/internal/internal.go
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Original file line number Diff line number Diff line change
Expand Up @@ -13,8 +13,8 @@ import (
)

type Common struct {
RootFor func(root string) fs.FS
Realpath func(path string) string
RootFor func(root string) fs.FS
IsSymlinkOrJunction func(path string) bool
}

func RootLength(p string) int {
Expand Down Expand Up @@ -93,12 +93,12 @@ func (vfs *Common) GetAccessibleEntries(path string) (result vfs.Entries) {
continue
}

if entryType&fs.ModeIrregular != 0 && vfs.Realpath != nil {
// Could be a Windows junction. Try Realpath.
// TODO(jakebailey): use syscall.Win32FileAttributeData instead
if entryType&fs.ModeIrregular != 0 && vfs.IsSymlinkOrJunction != nil {
// Could be a Windows junction or other reparse point.
// Check using the OS-specific helper.
fullPath := path + "/" + entry.Name()
if realpath := vfs.Realpath(fullPath); fullPath != realpath {
if stat := vfs.Stat(realpath); stat != nil {
if vfs.IsSymlinkOrJunction(fullPath) {
if stat := vfs.Stat(fullPath); stat != nil {
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@jakebailey jakebailey Nov 4, 2025

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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)

addToResult(entry.Name(), stat.Mode())
}
}
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4 changes: 2 additions & 2 deletions internal/vfs/osvfs/os.go
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Expand Up @@ -21,8 +21,8 @@ func FS() vfs.FS {

var osVFS vfs.FS = &osFS{
common: internal.Common{
RootFor: os.DirFS,
Realpath: osFSRealpath,
RootFor: os.DirFS,
IsSymlinkOrJunction: isSymlinkOrJunction,
},
}

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7 changes: 7 additions & 0 deletions internal/vfs/osvfs/symlink_other.go
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
//go:build !windows

package osvfs

// On Unix-like systems, symlinks are already properly detected by the
// fs.ModeSymlink bit in the directory entry type, so this check is not needed.
var isSymlinkOrJunction func(path string) bool
27 changes: 27 additions & 0 deletions internal/vfs/osvfs/symlink_windows.go
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Original file line number Diff line number Diff line change
@@ -0,0 +1,27 @@
package osvfs

import (
"syscall"
"unsafe"
)

// isSymlinkOrJunction checks if the given path is a symlink or junction point
// on Windows by checking the FILE_ATTRIBUTE_REPARSE_POINT attribute.
func isSymlinkOrJunction(path string) bool {
pathUTF16, err := syscall.UTF16PtrFromString(path)
if err != nil {
return false
}

var data syscall.Win32FileAttributeData
err = syscall.GetFileAttributesEx(
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@RyanCavanaugh RyanCavanaugh Nov 4, 2025

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

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@jakebailey jakebailey Nov 4, 2025

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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.

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@RyanCavanaugh RyanCavanaugh Nov 4, 2025

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I wouldn't block this on it

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@jakebailey jakebailey Nov 4, 2025

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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

pathUTF16,
syscall.GetFileExInfoStandard,
(*byte)(unsafe.Pointer(&data)),
)
if err != nil {
return false
}

return data.FileAttributes&syscall.FILE_ATTRIBUTE_REPARSE_POINT != 0
}