Handle TLSOpts.GetCertificate in webhook

This change rewrites some of the webhook server logic to better handle
the user setting a custom configuration for the TLS options by providing
a custom GetCertificate function. When that's present, we won't start
a certwatcher routine. The change also updates the documentation to
better clarify when each of the options are effective.

Signed-off-by: Vince Prignano <[email protected]>

Author: vincepri

pkg/webhook/server.go

@@ -60,9 +60,13 @@ type Server struct {
 	CertDir string
 
 	// CertName is the server certificate name. Defaults to tls.crt.
+	//
+	// Note: This option should only be set when TLSOpts does not override GetCertificate.
 	CertName string
 
 	// KeyName is the server key name. Defaults to tls.key.
+	//
+	// Note: This option should only be set when TLSOpts does not override GetCertificate.
 	KeyName string
 
 	// ClientCAName is the CA certificate name which server used to verify remote(client)'s certificate.
@@ -169,32 +173,40 @@ func (s *Server) Start(ctx context.Context) error {
 	baseHookLog := log.WithName("webhooks")
 	baseHookLog.Info("Starting webhook server")
 
-	certPath := filepath.Join(s.CertDir, s.CertName)
-	keyPath := filepath.Join(s.CertDir, s.KeyName)
-
-	certWatcher, err := certwatcher.New(certPath, keyPath)
-	if err != nil {
-		return err
-	}
-
-	go func() {
-		if err := certWatcher.Start(ctx); err != nil {
-			log.Error(err, "certificate watcher error")
-		}
-	}()
-
 	tlsMinVersion, err := tlsVersion(s.TLSMinVersion)
 	if err != nil {
 		return err
 	}
 
 	cfg := &tls.Config{ //nolint:gosec
-		NextProtos:     []string{"h2"},
-		GetCertificate: certWatcher.GetCertificate,
-		MinVersion:     tlsMinVersion,
+		NextProtos: []string{"h2"},
+		MinVersion: tlsMinVersion,
+	}
+	// fallback TLS config ready, will now mutate if passer wants full control over it
+	for _, op := range s.TLSOpts {
+		op(cfg)
+	}
+
+	if cfg.GetCertificate == nil {
+		certPath := filepath.Join(s.CertDir, s.CertName)
+		keyPath := filepath.Join(s.CertDir, s.KeyName)
+
+		// Create the certificate watcher and
+		// set the config's GetCertificate on the TLSConfig
+		certWatcher, err := certwatcher.New(certPath, keyPath)
+		if err != nil {
+			return err
+		}
+		cfg.GetCertificate = certWatcher.GetCertificate
+
+		go func() {
+			if err := certWatcher.Start(ctx); err != nil {
+				log.Error(err, "certificate watcher error")
+			}
+		}()
 	}
 
-	// load CA to verify client certificate
+	// Load CA to verify client certificate, if configured.
 	if s.ClientCAName != "" {
 		certPool := x509.NewCertPool()
 		clientCABytes, err := os.ReadFile(filepath.Join(s.CertDir, s.ClientCAName))
@@ -211,11 +223,6 @@ func (s *Server) Start(ctx context.Context) error {
 		cfg.ClientAuth = tls.RequireAndVerifyClientCert
 	}
 
-	// fallback TLS config ready, will now mutate if passer wants full control over it
-	for _, op := range s.TLSOpts {
-		op(cfg)
-	}
-
 	listener, err := tls.Listen("tcp", net.JoinHostPort(s.Host, strconv.Itoa(s.Port)), cfg)
 	if err != nil {
 		return err

pkg/webhook/server_test.go

@@ -23,6 +23,8 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"path"
+	"reflect"
 
 	. "github.com/onsi/ginkgo/v2"
 	. "github.com/onsi/gomega"
@@ -181,7 +183,7 @@ var _ = Describe("Webhook Server", func() {
 		}
 		server.Register("/somepath", &testHandler{})
 		doneCh := genericStartServer(func(ctx context.Context) {
-			Expect(server.Start(ctx))
+			Expect(server.Start(ctx)).To(Succeed())
 		})
 
 		Eventually(func() ([]byte, error) {
@@ -199,6 +201,53 @@ var _ = Describe("Webhook Server", func() {
 		ctxCancel()
 		Eventually(doneCh, "4s").Should(BeClosed())
 	})
+
+	It("should prefer GetCertificate through TLSOpts", func() {
+		var finalCfg *tls.Config
+		finalCert, err := tls.LoadX509KeyPair(
+			path.Join(servingOpts.LocalServingCertDir, "tls.crt"),
+			path.Join(servingOpts.LocalServingCertDir, "tls.key"),
+		)
+		Expect(err).NotTo(HaveOccurred())
+		finalGetCertificate := func(_ *tls.ClientHelloInfo) (*tls.Certificate, error) { //nolint:unparam
+			return &finalCert, nil
+		}
+		server = &webhook.Server{
+			Host:          servingOpts.LocalServingHost,
+			Port:          servingOpts.LocalServingPort,
+			CertDir:       servingOpts.LocalServingCertDir,
+			TLSMinVersion: "1.2",
+			TLSOpts: []func(*tls.Config){
+				func(cfg *tls.Config) {
+					cfg.GetCertificate = finalGetCertificate
+					// save cfg after changes to test against
+					finalCfg = cfg
+				},
+			},
+		}
+		server.Register("/somepath", &testHandler{})
+		doneCh := genericStartServer(func(ctx context.Context) {
+			Expect(server.Start(ctx)).To(Succeed())
+		})
+
+		Eventually(func() ([]byte, error) {
+			resp, err := client.Get(fmt.Sprintf("https://%s/somepath", testHostPort))
+			Expect(err).NotTo(HaveOccurred())
+			defer resp.Body.Close()
+			return io.ReadAll(resp.Body)
+		}).Should(Equal([]byte("gadzooks!")))
+		Expect(finalCfg.MinVersion).To(Equal(uint16(tls.VersionTLS12)))
+		// We can't compare the functions directly, but we can compare their pointers
+		if reflect.ValueOf(finalCfg.GetCertificate).Pointer() != reflect.ValueOf(finalGetCertificate).Pointer() {
+			Fail("GetCertificate was not set properly, or overwritten")
+		}
+		cert, err := finalCfg.GetCertificate(nil)
+		Expect(err).NotTo(HaveOccurred())
+		Expect(cert).To(BeEquivalentTo(&finalCert))
+
+		ctxCancel()
+		Eventually(doneCh, "4s").Should(BeClosed())
+	})
 })
 
 type testHandler struct {