pkg/system: compile volume-path regex once, and update GoDoc

Ideally, we would construct this lazily, but adding a function and a
sync.Once felt like a bit "too much".

Also updated the GoDoc for some functions to better describe what they do.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/system/filesys_windows.go

@@ -9,19 +9,22 @@ import (
 	"golang.org/x/sys/windows"
 )
 
-const (
-	// SddlAdministratorsLocalSystem is local administrators plus NT AUTHORITY\System
-	SddlAdministratorsLocalSystem = "D:P(A;OICI;GA;;;BA)(A;OICI;GA;;;SY)"
-)
+// SddlAdministratorsLocalSystem is local administrators plus NT AUTHORITY\System.
+const SddlAdministratorsLocalSystem = "D:P(A;OICI;GA;;;BA)(A;OICI;GA;;;SY)"
+
+// volumePath is a regular expression to check if a path is a Windows
+// volume path (e.g., "\\?\Volume{4c1b02c1-d990-11dc-99ae-806e6f6e6963}".
+var volumePath = regexp.MustCompile(`^\\\\\?\\Volume{[a-z0-9-]+}$`)
 
-// MkdirAllWithACL is a wrapper for MkdirAll that creates a directory
-// with an appropriate SDDL defined ACL.
-func MkdirAllWithACL(path string, perm os.FileMode, sddl string) error {
+// MkdirAllWithACL is a custom version of os.MkdirAll modified for use on Windows
+// so that it is both volume path aware, and can create a directory with
+// an appropriate SDDL defined ACL.
+func MkdirAllWithACL(path string, _ os.FileMode, sddl string) error {
 	return mkdirall(path, true, sddl)
 }
 
-// MkdirAll implementation that is volume path aware for Windows. It can be used
-// as a drop-in replacement for os.MkdirAll()
+// MkdirAll is a custom version of os.MkdirAll that is volume path aware for
+// Windows. It can be used as a drop-in replacement for os.MkdirAll.
 func MkdirAll(path string, _ os.FileMode) error {
 	return mkdirall(path, false, "")
 }
@@ -30,7 +33,7 @@ func MkdirAll(path string, _ os.FileMode) error {
 // so that it is both volume path aware, and can create a directory with
 // a DACL.
 func mkdirall(path string, applyACL bool, sddl string) error {
-	if re := regexp.MustCompile(`^\\\\\?\\Volume{[a-z0-9-]+}$`); re.MatchString(path) {
+	if volumePath.MatchString(path) {
 		return nil
 	}