Add a special-cased runtime handler for dockershim

pkg/kubelet/dockershim/docker_sandbox.go

@@ -96,7 +96,7 @@ func (ds *dockerService) RunPodSandbox(ctx context.Context, r *runtimeapi.RunPod
 	}
 
 	// Step 2: Create the sandbox container.
-	if r.GetRuntimeHandler() != "" {
+	if r.GetRuntimeHandler() != "" && r.GetRuntimeHandler() != runtimeName {
 		return nil, fmt.Errorf("RuntimeHandler %q not supported", r.GetRuntimeHandler())
 	}
 	createConfig, err := ds.makeSandboxDockerConfig(config, image)

test/e2e/common/runtimeclass.go

@@ -38,6 +38,9 @@ const (
 	// PreconfiguredRuntimeHandler is the name of the runtime handler that is expected to be
 	// preconfigured in the test environment.
 	PreconfiguredRuntimeHandler = "test-handler"
+	// DockerRuntimeHandler is a hardcoded runtime handler that is accepted by dockershim, and
+	// treated equivalently to a nil runtime handler.
+	DockerRuntimeHandler = "docker"
 )
 
 var _ = Describe("[sig-node] RuntimeClass", func() {
@@ -59,9 +62,12 @@ var _ = Describe("[sig-node] RuntimeClass", func() {
 	// This test requires that the PreconfiguredRuntimeHandler has already been set up on nodes.
 	It("should run a Pod requesting a RuntimeClass with a configured handler [NodeFeature:RuntimeHandler]", func() {
 		// The built-in docker runtime does not support configuring runtime handlers.
-		framework.SkipIfContainerRuntimeIs("docker")
+		handler := PreconfiguredRuntimeHandler
+		if framework.TestContext.ContainerRuntime == "docker" {
+			handler = DockerRuntimeHandler
+		}
 
-		rcName := createRuntimeClass(f, "preconfigured-handler", PreconfiguredRuntimeHandler)
+		rcName := createRuntimeClass(f, "preconfigured-handler", handler)
 		pod := createRuntimeClassPod(f, rcName)
 		expectPodSuccess(f, pod)
 	})