Adding Kubelet changes to enable SetHostnameAsFQDN feature

These changes allow to set FQDN as hostname of pods for pods
that set the new PodSpec field setHostnameAsFQDN to true. The PodSpec
new field was added in related PR.

This is PART2 (last) of the changes to enable KEP #1797 and addresses #91036

pkg/kubelet/util/util.go

@@ -17,6 +17,8 @@ limitations under the License.
 package util
 
 import (
+	"fmt"
+
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 )
 
@@ -25,3 +27,19 @@ import (
 func FromApiserverCache(opts *metav1.GetOptions) {
 	opts.ResourceVersion = "0"
 }
+
+// GetNodenameForKernel gets hostname value to set in the hostname field (the nodename field of struct utsname) of the pod.
+func GetNodenameForKernel(hostname string, hostDomainName string, setHostnameAsFQDN *bool) (string, error) {
+	kernelHostname := hostname
+	// FQDN has to be 64 chars to fit in the Linux nodename kernel field (specification 64 chars and the null terminating char).
+	const fqdnMaxLen = 64
+	if len(hostDomainName) > 0 && setHostnameAsFQDN != nil && *setHostnameAsFQDN == true {
+		fqdn := fmt.Sprintf("%s.%s", hostname, hostDomainName)
+		// FQDN has to be shorter than hostnameMaxLen characters.
+		if len(fqdn) > fqdnMaxLen {
+			return "", fmt.Errorf("Failed to construct FQDN from pod hostname and cluster domain, FQDN %s is too long (%d characters is the max, %d characters requested)", fqdn, fqdnMaxLen, len(fqdn))
+		}
+		kernelHostname = fqdn
+	}
+	return kernelHostname, nil
+}