Add Fallback flag to ClusterSPIFFEIDs

api/v1alpha1/clusterspiffeid_types.go

@@ -79,6 +79,10 @@ type ClusterSPIFFEIDSpec struct {
 	// +kubebuilder:validation:Optional
 	ClassName string `json:"className,omitempty"`
 
+	// Apply this ID only if there are no other matching non fallback ClusterSPIFFEIDs.
+	// +kubebuilder:validation:Optional
+	Fallback bool `json:"fallback,omitempty"`
+
 	// Set the entry hint
 	// +kubebuilder:validation:Optional
 	Hint string `json:"hint,omitempty"`

config/crd/bases/spire.spiffe.io_clusterspiffeids.yaml

@@ -52,6 +52,11 @@ spec:
               className:
                 description: Set which Controller Class will act on this object
                 type: string
+              fallback:
+                description: |-
+                  Apply this ID only if there are no other matching non fallback
+                  ClusterSPIFFEIDs
+                type: boolean
               dnsNameTemplates:
                 description: |-
                   DNSNameTemplate represents templates for extra DNS names that are

docs/clusterspiffeid-crd.md

@@ -27,6 +27,7 @@ The definition can be found [here](../api/v1alpha1/clusterspiffeid_types.go).
 | `admin`                     | OPTIONAL | Indicates whether the target workload is an admin workload (i.e. can access SPIRE administrative APIs) |
 | `downstream`                | OPTIONAL | Indicates that the entry describes a downstream SPIRE server. |
 | `autoPopulateDNSNames`      | OPTIONAL | Indicates whether or not to auto populate service DNS names. |
+| `fallback`                  | OPTIONAL | Apply this ID only if there are no other matching non fallback ClusterSPIFFEIDs. |
 
 ## ClusterSPIFFEIDStatus
 

pkg/spireentry/reconciler.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"io"
 	"regexp"
+	"slices"
 	"sort"
 	"strings"
 	"text/template"
@@ -358,6 +359,17 @@ func (r *entryReconciler) addClusterStaticEntryEntriesState(ctx context.Context,
 
 func (r *entryReconciler) addClusterSPIFFEIDEntriesState(ctx context.Context, state entriesState, clusterSPIFFEIDs []*ClusterSPIFFEID) {
 	log := log.FromContext(ctx)
+	podsWithNonFallbackApplied := make(map[types.UID]struct{})
+	// Process all the fallback clusterSPIFFEIDs last.
+	slices.SortStableFunc(clusterSPIFFEIDs, func(x, y *ClusterSPIFFEID) int {
+		if x.Spec.Fallback == y.Spec.Fallback {
+			return 0
+		}
+		if x.Spec.Fallback {
+			return 1
+		}
+		return -1
+	})
 	for _, clusterSPIFFEID := range clusterSPIFFEIDs {
 		log := log.WithValues(clusterSPIFFEIDLogKey, objectName(clusterSPIFFEID))
 
@@ -399,6 +411,9 @@ func (r *entryReconciler) addClusterSPIFFEIDEntriesState(ctx context.Context, st
 			clusterSPIFFEID.NextStatus.Stats.PodsSelected += len(pods)
 			for i := range pods {
 				log := log.WithValues(podLogKey, objectName(&pods[i]))
+				if _, ok := podsWithNonFallbackApplied[pods[i].UID]; ok && clusterSPIFFEID.Spec.Fallback {
+					continue
+				}
 
 				entry, err := r.renderPodEntry(ctx, spec, &pods[i])
 				switch {
@@ -409,6 +424,9 @@ func (r *entryReconciler) addClusterSPIFFEIDEntriesState(ctx context.Context, st
 					// renderPodEntry will return a nil entry if requisite k8s
 					// objects disappeared from underneath.
 					state.AddDeclared(*entry, clusterSPIFFEID)
+					if !clusterSPIFFEID.Spec.Fallback {
+						podsWithNonFallbackApplied[pods[i].UID] = struct{}{}
+					}
 				}
 			}
 		}