Lazy init useradd and remove init()

This should not have been in init() as it causes these lookups to happen
in all reexecs of the Docker binary. The only time it needs to be
resolved is when a user is added, which is extremely rare.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

idtools/usergroupadd_linux.go

@@ -8,6 +8,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"sync"
 )
 
 // add a user and/or group to Linux /etc/passwd, /etc/group using standard
@@ -16,6 +17,7 @@ import (
 // useradd -r -s /bin/false <username>
 
 var (
+	once        sync.Once
 	userCommand string
 
 	cmdTemplates = map[string]string{
@@ -31,15 +33,6 @@ var (
 	userMod           = "usermod"
 )
 
-func init() {
-	// set up which commands are used for adding users/groups dependent on distro
-	if _, err := resolveBinary("adduser"); err == nil {
-		userCommand = "adduser"
-	} else if _, err := resolveBinary("useradd"); err == nil {
-		userCommand = "useradd"
-	}
-}
-
 func resolveBinary(binname string) (string, error) {
 	binaryPath, err := exec.LookPath(binname)
 	if err != nil {
@@ -94,7 +87,14 @@ func AddNamespaceRangesUser(name string) (int, int, error) {
 }
 
 func addUser(userName string) error {
-
+	once.Do(func() {
+		// set up which commands are used for adding users/groups dependent on distro
+		if _, err := resolveBinary("adduser"); err == nil {
+			userCommand = "adduser"
+		} else if _, err := resolveBinary("useradd"); err == nil {
+			userCommand = "useradd"
+		}
+	})
 	if userCommand == "" {
 		return fmt.Errorf("Cannot add user; no useradd/adduser binary found")
 	}