Adding new PodSpec field called setHostnameAsFQDN and SetHostnameAsFQDN gate

These changes add a new field, called setHostnameAsFQDN, to the PodSpec. This
field is a bool that will be used to indicate whether we would like
FQDN be set as hostname or not.

This is PART1 of the changes to enable KEP #1797 and addresses #91036

pkg/api/pod/util.go

@@ -438,6 +438,12 @@ func dropDisabledFields(
 		// does not specify any values for these fields.
 		podSpec.PreemptionPolicy = nil
 	}
+
+	if !utilfeature.DefaultFeatureGate.Enabled(features.SetHostnameAsFQDN) && !setHostnameAsFQDNInUse(oldPodSpec) {
+		// Set SetHostnameAsFQDN to nil only if feature is disabled and it is not used
+		podSpec.SetHostnameAsFQDN = nil
+	}
+
 }
 
 // dropDisabledRunAsGroupField removes disabled fields from PodSpec related
@@ -720,3 +726,11 @@ func multiplePodIPsInUse(podStatus *api.PodStatus) bool {
 	}
 	return false
 }
+
+// setHostnameAsFQDNInUse returns true if any pod's spec defines setHostnameAsFQDN field.
+func setHostnameAsFQDNInUse(podSpec *api.PodSpec) bool {
+	if podSpec == nil || podSpec.SetHostnameAsFQDN == nil {
+		return false
+	}
+	return *podSpec.SetHostnameAsFQDN
+}

pkg/apis/core/types.go

@@ -2693,6 +2693,12 @@ type PodSpec struct {
 	// If not specified, the pod will not have a domainname at all.
 	// +optional
 	Subdomain string
+	// If true the pod's hostname will be configured as the pod's FQDN, rather than the leaf name (the default).
+	// In Linux containers, this means setting the FQDN in the hostname field of the kernel (the nodename field of struct utsname).
+	// In Windows containers, this means setting the registry value of hostname for the registry key HKEY_LOCAL_MACHINE\\SYSTEM\\CurrentControlSet\\Services\\Tcpip\\Parameters to FQDN.
+	// If a pod does not have FQDN, this has no effect.
+	// +optional
+	SetHostnameAsFQDN *bool
 	// If specified, the pod's scheduling constraints
 	// +optional
 	Affinity *Affinity

pkg/features/kube_features.go

@@ -592,6 +592,13 @@ const (
 	//
 	// Enables usage of any object for volume data source in PVCs
 	AnyVolumeDataSource featuregate.Feature = "AnyVolumeDataSource"
+
+	// owner: @javidiaz
+	// alpha: v1.19
+	//
+	// Allow setting the Fully Qualified Domain Name (FQDN) in the hostname of a Pod. If a Pod does not
+	// have FQDN, this feature has no effect.
+	SetHostnameAsFQDN featuregate.Feature = "SetHostnameAsFQDN"
 )
 
 func init() {
@@ -683,6 +690,7 @@ var defaultKubernetesFeatureGates = map[featuregate.Feature]featuregate.FeatureS
 	ExternalPolicyForExternalIP:                    {Default: true, PreRelease: featuregate.GA}, // remove in 1.20
 	AnyVolumeDataSource:                            {Default: false, PreRelease: featuregate.Alpha},
 	DefaultPodTopologySpread:                       {Default: false, PreRelease: featuregate.Alpha},
+	SetHostnameAsFQDN:                              {Default: false, PreRelease: featuregate.Alpha},
 
 	// inherited features from generic apiserver, relisted here to get a conflict if it is changed
 	// unintentionally on either side:

staging/src/k8s.io/api/core/v1/types.go

@@ -3055,6 +3055,13 @@ type PodSpec struct {
 	// +listMapKey=topologyKey
 	// +listMapKey=whenUnsatisfiable
 	TopologySpreadConstraints []TopologySpreadConstraint `json:"topologySpreadConstraints,omitempty" patchStrategy:"merge" patchMergeKey:"topologyKey" protobuf:"bytes,33,opt,name=topologySpreadConstraints"`
+	// If true the pod's hostname will be configured as the pod's FQDN, rather than the leaf name (the default).
+	// In Linux containers, this means setting the FQDN in the hostname field of the kernel (the nodename field of struct utsname).
+	// In Windows containers, this means setting the registry value of hostname for the registry key HKEY_LOCAL_MACHINE\\SYSTEM\\CurrentControlSet\\Services\\Tcpip\\Parameters to FQDN.
+	// If a pod does not have FQDN, this has no effect.
+	// Default to false.
+	// +optional
+	SetHostnameAsFQDN *bool `json:"setHostnameAsFQDN,omitempty" protobuf:"varint,35,opt,name=setHostnameAsFQDN"`
 }
 
 type UnsatisfiableConstraintAction string