cmd/root: Shuffle some code around and sprinkle some debug logs

Having a separate convenience function reduces the indentation levels by
at least one, and sometimes two, and makes it easy to have more detailed
debug logs.

This will make the subsequent commit easier to read.

#1246

src/cmd/root.go

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2019 – 2022 Red Hat Inc.
+ * Copyright © 2019 – 2023 Red Hat Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -139,13 +139,8 @@ func preRun(cmd *cobra.Command, args []string) error {
 	if !utils.IsInsideContainer() {
 		logrus.Debugf("Running on a cgroups v%d host", cgroupsVersion)
 
-		if currentUser.Uid != "0" {
-			logrus.Debugf("Looking for sub-GID and sub-UID ranges for user %s", currentUser.Username)
-
-			if _, err := utils.ValidateSubIDRanges(currentUser); err != nil {
-				logrus.Debugf("Looking for sub-GID and sub-UID ranges: %s", err)
-				return newSubIDError()
-			}
+		if _, err := validateSubIDRanges(cmd, args, currentUser); err != nil {
+			return err
 		}
 	}
 
@@ -392,3 +387,24 @@ func setUpLoggers() error {
 
 	return nil
 }
+
+func validateSubIDRanges(cmd *cobra.Command, args []string, user *user.User) (bool, error) {
+	logrus.Debugf("Looking for sub-GID and sub-UID ranges for user %s", user.Username)
+
+	if user.Uid == "0" {
+		logrus.Debugf("Look-up not needed: user %s doesn't need them", user.Username)
+		return true, nil
+	}
+
+	if utils.IsInsideContainer() {
+		logrus.Debug("Look-up not needed: running inside a container")
+		return true, nil
+	}
+
+	if _, err := utils.ValidateSubIDRanges(user); err != nil {
+		logrus.Debugf("Looking for sub-GID and sub-UID ranges: %s", err)
+		return false, newSubIDError()
+	}
+
+	return true, nil
+}