pkg/systemd: use sync.Once for systemd detection

This brings over the enhancement from a506630e57.

We don't expect the systemd state to change while containerd is running,
so we can use a `sync.Once` for this, to prevent stat'ing each time.

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

pkg/systemd/util.go

@@ -32,16 +32,27 @@
 
 package systemd
 
-import "os"
+import (
+	"os"
+	"sync"
+)
+
+var (
+	runningSystemd bool
+	detectSystemd  sync.Once
+)
 
 // IsRunningSystemd checks whether the host was booted with systemd as its init
 // system. This functions similarly to systemd's `sd_booted(3)`: internally, it
 // checks whether /run/systemd/system/ exists and is a directory.
 // https://github.com/coreos/go-systemd/blob/d843340ab4bd3815fda02e648f9b09ae2dc722a7/util/util.go#L68-L78
 func IsRunningSystemd() bool {
-	fi, err := os.Lstat("/run/systemd/system")
-	if err != nil {
-		return false
-	}
-	return fi.IsDir()
+	detectSystemd.Do(func() {
+		fi, err := os.Lstat("/run/systemd/system")
+		if err != nil {
+			return
+		}
+		runningSystemd = fi.IsDir()
+	})
+	return runningSystemd
 }