Replace repeated subscripts with single TrimPrefix (#1176)

This stood out to me while reading the code: [7:] is skipping "unix://",
so why not do that?

Doing so reveals a bug in the last line changed, where chmod(2) failure
would print just the prefix, not everything but it... easy to miss, but
now this kind of bug can no longer happen.

Author: klemensn

src/admin/admin.go

@@ -238,27 +238,28 @@ func (a *AdminSocket) listen() {
 	if err == nil {
 		switch strings.ToLower(u.Scheme) {
 		case "unix":
-			if _, err := os.Stat(listenaddr[7:]); err == nil {
-				a.log.Debugln("Admin socket", listenaddr[7:], "already exists, trying to clean up")
-				if _, err := net.DialTimeout("unix", listenaddr[7:], time.Second*2); err == nil || err.(net.Error).Timeout() {
-					a.log.Errorln("Admin socket", listenaddr[7:], "already exists and is in use by another process")
+			file := strings.TrimPrefix(listenaddr, "unix://")
+			if _, err := os.Stat(file); err == nil {
+				a.log.Debugln("Admin socket", file, "already exists, trying to clean up")
+				if _, err := net.DialTimeout("unix", file, time.Second*2); err == nil || err.(net.Error).Timeout() {
+					a.log.Errorln("Admin socket", file, "already exists and is in use by another process")
 					os.Exit(1)
 				} else {
-					if err := os.Remove(listenaddr[7:]); err == nil {
-						a.log.Debugln(listenaddr[7:], "was cleaned up")
+					if err := os.Remove(file); err == nil {
+						a.log.Debugln(file, "was cleaned up")
 					} else {
-						a.log.Errorln(listenaddr[7:], "already exists and was not cleaned up:", err)
+						a.log.Errorln(file, "already exists and was not cleaned up:", err)
 						os.Exit(1)
 					}
 				}
 			}
-			a.listener, err = net.Listen("unix", listenaddr[7:])
+			a.listener, err = net.Listen("unix", file)
 			if err == nil {
-				switch listenaddr[7:8] {
+				switch file[:1] {
 				case "@": // maybe abstract namespace
 				default:
-					if err := os.Chmod(listenaddr[7:], 0660); err != nil {
-						a.log.Warnln("WARNING:", listenaddr[:7], "may have unsafe permissions!")
+					if err := os.Chmod(file, 0660); err != nil {
+						a.log.Warnln("WARNING:", file, "may have unsafe permissions!")
 					}
 				}
 			}