os: support app store symbolic links

IO_REPARSE_TAG_APPEXECLINK are types of links that are created when
installing an application via the Windows App Store. The location
of these links is added to the Windows PATH and the system can
resolve them. This adds support for these links so go can resolve them.

Fixes golang#42919

src/internal/syscall/windows/reparse_windows.go

@@ -6,12 +6,14 @@ package windows
 
 import (
 	"syscall"
+	"unicode/utf16"
 	"unsafe"
 )
 
 const (
 	FSCTL_SET_REPARSE_POINT    = 0x000900A4
 	IO_REPARSE_TAG_MOUNT_POINT = 0xA0000003
+	IO_REPARSE_TAG_APPEXECLINK = 0x8000001B
 
 	SYMLINK_FLAG_RELATIVE = 1
 )
@@ -88,3 +90,30 @@ func (rb *MountPointReparseBuffer) Path() string {
 	n2 := (rb.SubstituteNameOffset + rb.SubstituteNameLength) / 2
 	return syscall.UTF16ToString((*[0xffff]uint16)(unsafe.Pointer(&rb.PathBuffer[0]))[n1:n2:n2])
 }
+
+// AppExecLinkReparseBuffer is described at
+// https://stackoverflow.com/a/65583702
+// It appears there is no official Microsoft
+// documentation for AppExecLinkReparseBuffer yet.
+type AppExecLinkReparseBuffer struct {
+	Version    uint32
+	StringList [1]uint16
+}
+
+// Path returns the path stored in rb.
+// The buffer contains 4 null terminated strings inside
+// StringList. The third string contains the path to the
+// executable
+func (rb *AppExecLinkReparseBuffer) Path() string {
+	for j, from, i, p := 0, 0, 0, (*[1 << 24]uint16)(unsafe.Pointer(&rb.StringList[0])); true; i++ {
+		if p[i] == 0 {
+			// return 3rd string
+			if j == 2 {
+				return string(utf16.Decode(p[from:i]))
+			}
+			j++
+			from = i + 1
+		}
+	}
+	return ""
+}

src/os/exec/lp_windows.go

@@ -48,6 +48,9 @@ func findExecutable(file string, exts []string) (string, error) {
 			return f, nil
 		}
 	}
+	if file, err := os.Readlink(file); err == nil {
+		return file, nil
+	}
 	return "", fs.ErrNotExist
 }
 

src/os/file_windows.go

@@ -482,6 +482,16 @@ func readlink(path string) (string, error) {
 		return normaliseLinkPath(s)
 	case windows.IO_REPARSE_TAG_MOUNT_POINT:
 		return normaliseLinkPath((*windows.MountPointReparseBuffer)(unsafe.Pointer(&rdb.DUMMYUNIONNAME)).Path())
+	case windows.IO_REPARSE_TAG_APPEXECLINK:
+		appExecLink := (*windows.AppExecLinkReparseBuffer)(unsafe.Pointer(&rdb.DUMMYUNIONNAME))
+		if appExecLink.Version != 3 {
+			return " ", errors.New("unknown AppExecLink version")
+		}
+		s := appExecLink.Path()
+		if s == "" {
+			return "", errors.New("unable to retrieve AppExecLink from buffer")
+		}
+		return normaliseLinkPath(s)
 	default:
 		// the path is not a symlink or junction but another type of reparse
 		// point