mount-utils: only detect systemd when needed

The detectSystemd() function adds quite some logging when systemd is not
detected. This is not critical when the k8s.io/mount-utils package is
used by external applications or CSI-drivers. In that case, it may not
be required to detect systemd at all. To reduce logging in cases where
systemd does not need to be detected, add a hasSystemd() helper function
that calls detectSystemd() only once.

staging/src/k8s.io/mount-utils/mount_linux.go

@@ -58,7 +58,7 @@ const (
 // kubelet is running in the host's root mount namespace.
 type Mounter struct {
 	mounterPath                string
-	withSystemd                bool
+	withSystemd                *bool
 	withSafeNotMountedBehavior bool
 }
 
@@ -70,11 +70,21 @@ var _ MounterForceUnmounter = &Mounter{}
 func New(mounterPath string) Interface {
 	return &Mounter{
 		mounterPath:                mounterPath,
-		withSystemd:                detectSystemd(),
 		withSafeNotMountedBehavior: detectSafeNotMountedBehavior(),
 	}
 }
 
+// hasSystemd validates that the withSystemd bool is set, if it is not,
+// detectSystemd will be called once for this Mounter instance.
+func (mounter *Mounter) hasSystemd() bool {
+	if mounter.withSystemd == nil {
+		withSystemd := detectSystemd()
+		mounter.withSystemd = &withSystemd
+	}
+
+	return *mounter.withSystemd
+}
+
 // Mount mounts source to target as fstype with given options. 'source' and 'fstype' must
 // be an empty string in case it's not required, e.g. for remount, or for auto filesystem
 // type, where kernel handles fstype for you. The mount 'options' is a list of options,
@@ -154,7 +164,7 @@ func (mounter *Mounter) doMount(mounterPath string, mountCmd string, source stri
 		mountCmd = mounterPath
 	}
 
-	if mounter.withSystemd && systemdMountRequired {
+	if mounter.hasSystemd() && systemdMountRequired {
 		// Try to run mount via systemd-run --scope. This will escape the
 		// service where kubelet runs and any fuse daemons will be started in a
 		// specific scope. kubelet service than can be restarted without killing

staging/src/k8s.io/mount-utils/mount_linux_test.go

@@ -525,6 +525,14 @@ func TestSensitiveMountOptions(t *testing.T) {
 	}
 }
 
+func TestHasSystemd(t *testing.T) {
+	mounter := &Mounter{}
+	_ = mounter.hasSystemd()
+	if mounter.withSystemd == nil {
+		t.Error("Failed to run detectSystemd()")
+	}
+}
+
 func mountArgsContainString(t *testing.T, mountArgs []string, wanted string) bool {
 	for _, mountArg := range mountArgs {
 		if mountArg == wanted {