libct/cap: switch to lazy init

A map which is created in func init is only used by capSlice, which is
only used by New, which is only used by runc init. Switch to lazy init
to slightly save on startup time.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/capabilities/capabilities.go

@@ -5,6 +5,7 @@ package capabilities
 import (
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/sirupsen/logrus"
@@ -14,25 +15,25 @@ import (
 const allCapabilityTypes = capability.CAPS | capability.BOUNDING | capability.AMBIENT
 
 var (
-	capabilityMap map[string]capability.Cap
-	capTypes      = []capability.CapType{
+	capTypes = []capability.CapType{
 		capability.BOUNDING,
 		capability.PERMITTED,
 		capability.INHERITABLE,
 		capability.EFFECTIVE,
 		capability.AMBIENT,
 	}
-)
 
-func init() {
-	capabilityMap = make(map[string]capability.Cap, capability.CAP_LAST_CAP+1)
-	for _, c := range capability.List() {
-		if c > capability.CAP_LAST_CAP {
-			continue
+	capMap = sync.OnceValue(func() map[string]capability.Cap {
+		cm := make(map[string]capability.Cap, capability.CAP_LAST_CAP+1)
+		for _, c := range capability.List() {
+			if c > capability.CAP_LAST_CAP {
+				continue
+			}
+			cm["CAP_"+strings.ToUpper(c.String())] = c
 		}
-		capabilityMap["CAP_"+strings.ToUpper(c.String())] = c
-	}
-}
+		return cm
+	})
+)
 
 // KnownCapabilities returns the list of the known capabilities.
 // Used by `runc features`.
@@ -78,9 +79,10 @@ func New(capConfig *configs.Capabilities) (*Caps, error) {
 // equivalent, and returns them as a slice. Unknown or unavailable capabilities
 // are not returned, but appended to unknownCaps.
 func capSlice(caps []string, unknownCaps map[string]struct{}) []capability.Cap {
+	cm := capMap()
 	out := make([]capability.Cap, 0, len(caps))
 	for _, c := range caps {
-		if v, ok := capabilityMap[c]; !ok {
+		if v, ok := cm[c]; !ok {
 			unknownCaps[c] = struct{}{}
 		} else {
 			out = append(out, v)