perf(kubernetes): optimize AutoPath slice allocation (#7323)

Pre-allocate slice capacity in AutoPath to eliminate unnecessary
memory reallocations. This avoids slice growth when appending
search domains and sentinel value.

Benchmark shows significant performance improvement:
- Before: 538.6 ns/op, 560 B/op, 13 allocs/op
- After:  436.8 ns/op, 336 B/op, 11 allocs/op
- Result: 19% faster, 40% less memory, 15% fewer allocations

The optimization benefits Kubernetes clusters using autopath for
server-side search path completion.

Adds benchmark test to measure AutoPath performance.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/kubernetes/autopath.go

@@ -33,15 +33,12 @@ func (k *Kubernetes) AutoPath(state request.Request) []string {
 		return nil
 	}
 
-	search := make([]string, 3)
+	totalSize := 3 + len(k.autoPathSearch) + 1 // +1 for sentinel
+	search := make([]string, 0, totalSize)
 	if zone == "." {
-		search[0] = pod.Namespace + ".svc."
-		search[1] = "svc."
-		search[2] = "."
+		search = append(search, pod.Namespace+".svc.", "svc.", ".")
 	} else {
-		search[0] = pod.Namespace + ".svc." + zone
-		search[1] = "svc." + zone
-		search[2] = zone
+		search = append(search, pod.Namespace+".svc."+zone, "svc."+zone, zone)
 	}
 
 	search = append(search, k.autoPathSearch...)

plugin/kubernetes/autopath_test.go

@@ -0,0 +1,77 @@
+package kubernetes
+
+import (
+	"context"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/kubernetes/object"
+	"github.com/coredns/coredns/plugin/test"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	api "k8s.io/api/core/v1"
+)
+
+// Mock data for benchmarks
+var (
+	mockPod = &object.Pod{
+		Namespace: "test-namespace",
+	}
+	mockAutoPathSearch = []string{"example.com", "internal.example.com", "cluster.local"}
+)
+
+// Mock API connector for testing
+type mockAPIConnector struct{}
+
+func (m *mockAPIConnector) PodIndex(ip string) []*object.Pod {
+	return []*object.Pod{mockPod}
+}
+
+// Minimal implementation of other required methods
+func (m *mockAPIConnector) ServiceList() []*object.Service                     { return nil }
+func (m *mockAPIConnector) EndpointsList() []*object.Endpoints                 { return nil }
+func (m *mockAPIConnector) ServiceImportList() []*object.ServiceImport         { return nil }
+func (m *mockAPIConnector) SvcIndex(s string) []*object.Service                { return nil }
+func (m *mockAPIConnector) SvcIndexReverse(s string) []*object.Service         { return nil }
+func (m *mockAPIConnector) SvcExtIndexReverse(s string) []*object.Service      { return nil }
+func (m *mockAPIConnector) SvcImportIndex(s string) []*object.ServiceImport    { return nil }
+func (m *mockAPIConnector) EpIndex(s string) []*object.Endpoints               { return nil }
+func (m *mockAPIConnector) EpIndexReverse(s string) []*object.Endpoints        { return nil }
+func (m *mockAPIConnector) McEpIndex(s string) []*object.MultiClusterEndpoints { return nil }
+func (m *mockAPIConnector) GetNodeByName(ctx context.Context, name string) (*api.Node, error) {
+	return nil, nil
+}
+func (m *mockAPIConnector) GetNamespaceByName(name string) (*object.Namespace, error) {
+	return nil, nil
+}
+func (m *mockAPIConnector) Run()                        {}
+func (m *mockAPIConnector) HasSynced() bool             { return true }
+func (m *mockAPIConnector) Stop() error                 { return nil }
+func (m *mockAPIConnector) Modified(ModifiedMode) int64 { return 0 }
+
+func BenchmarkAutoPath(b *testing.B) {
+	k := &Kubernetes{
+		Zones:          []string{"cluster.local."},
+		autoPathSearch: mockAutoPathSearch,
+		podMode:        podModeVerified,
+		opts: dnsControlOpts{
+			initPodCache: true,
+		},
+		APIConn: &mockAPIConnector{},
+	}
+
+	// Create a mock DNS request
+	req := &dns.Msg{}
+	req.SetQuestion("test.cluster.local.", dns.TypeA)
+
+	// Create a request state with a mock ResponseWriter
+	state := request.Request{W: &test.ResponseWriter{}, Req: req}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		result := k.AutoPath(state)
+		_ = result
+	}
+}