kubeadm: update the CRI socket detection logic

- Throw an error if there is more than one known socket on the host.
- Remove the special handling for docker+containerd.
- Remove the local instances of constants for endpoints for
Windows / Unix and use the defaultKnownCRISockets variable
which is populated from OS specific constants.
- Update error message in detectCRISocketImpl to have more
details.
- Make detectCRISocketImpl accept a list of "known" sockets
- Update unit tests for detectCRISocketImpl and make them
use generic paths such as "unix:///foo/bar.sock".

cmd/kubeadm/app/util/runtime/runtime.go

@@ -27,6 +27,13 @@ import (
 	"k8s.io/kubernetes/cmd/kubeadm/app/constants"
 )
 
+// defaultKnownCRISockets holds the set of known CRI endpoints
+var defaultKnownCRISockets = []string{
+	constants.CRISocketContainerd,
+	constants.CRISocketCRIO,
+	constants.CRISocketDocker,
+}
+
 // ContainerRuntime is an interface for working with container runtimes
 type ContainerRuntime interface {
 	Socket() string
@@ -115,21 +122,8 @@ func (runtime *CRIRuntime) ImageExists(image string) (bool, error) {
 }
 
 // detectCRISocketImpl is separated out only for test purposes, DON'T call it directly, use DetectCRISocket instead
-func detectCRISocketImpl(isSocket func(string) bool) (string, error) {
+func detectCRISocketImpl(isSocket func(string) bool, knownCRISockets []string) (string, error) {
 	foundCRISockets := []string{}
-	knownCRISockets := []string{
-		// Docker and containerd sockets are special cased below, hence not to be included here
-		"unix:///var/run/crio/crio.sock",
-	}
-
-	if isSocket(dockerSocket) {
-		// the path in dockerSocket is not CRI compatible, hence we should replace it with a CRI compatible socket
-		foundCRISockets = append(foundCRISockets, constants.DefaultCRISocket)
-	} else if isSocket(containerdSocket) {
-		// Docker 18.09 gets bundled together with containerd, thus having both dockerSocket and containerdSocket present.
-		// For compatibility reasons, we use the containerd socket only if Docker is not detected.
-		foundCRISockets = append(foundCRISockets, containerdSocket)
-	}
 
 	for _, socket := range knownCRISockets {
 		if isSocket(socket) {
@@ -139,18 +133,20 @@ func detectCRISocketImpl(isSocket func(string) bool) (string, error) {
 
 	switch len(foundCRISockets) {
 	case 0:
-		// Fall back to Docker if no CRI is detected, we can error out later on if we need it
+		// Fall back to the default socket if no CRI is detected, we can error out later on if we need it
 		return constants.DefaultCRISocket, nil
 	case 1:
 		// Precisely one CRI found, use that
 		return foundCRISockets[0], nil
 	default:
 		// Multiple CRIs installed?
-		return "", errors.Errorf("Found multiple CRI sockets, please use --cri-socket to select one: %s", strings.Join(foundCRISockets, ", "))
+		return "", errors.Errorf("Found multiple CRI endpoints on the host. Please define which one do you wish "+
+			"to use by setting the 'criSocket' field in the kubeadm configuration file: %s",
+			strings.Join(foundCRISockets, ", "))
 	}
 }
 
 // DetectCRISocket uses a list of known CRI sockets to detect one. If more than one or none is discovered, an error is returned.
 func DetectCRISocket() (string, error) {
-	return detectCRISocketImpl(isExistingSocket)
+	return detectCRISocketImpl(isExistingSocket, defaultKnownCRISockets)
 }

cmd/kubeadm/app/util/runtime/runtime_test.go

@@ -380,32 +380,15 @@ func TestDetectCRISocketImpl(t *testing.T) {
 		},
 		{
 			name:            "One valid CRI socket leads to success",
-			existingSockets: []string{"unix:///var/run/crio/crio.sock"},
+			existingSockets: []string{"unix:///foo/bar.sock"},
 			expectedError:   false,
-			expectedSocket:  "unix:///var/run/crio/crio.sock",
-		},
-		{
-			name: "CRI and Docker sockets lead to an error",
-			existingSockets: []string{
-				"unix:///var/run/docker.sock",
-				"unix:///var/run/crio/crio.sock",
-			},
-			expectedError: true,
-		},
-		{
-			name: "Docker and containerd lead to Docker being used",
-			existingSockets: []string{
-				"unix:///var/run/docker.sock",
-				"unix:///run/containerd/containerd.sock",
-			},
-			expectedError:  false,
-			expectedSocket: constants.DefaultCRISocket,
+			expectedSocket:  "unix:///foo/bar.sock",
 		},
 		{
 			name: "Multiple CRI sockets lead to an error",
 			existingSockets: []string{
-				"unix:///var/run/crio/crio.sock",
-				"unix:///run/containerd/containerd.sock",
+				"unix:///foo/bar.sock",
+				"unix:///foo/baz.sock",
 			},
 			expectedError: true,
 		},
@@ -419,9 +402,9 @@ func TestDetectCRISocketImpl(t *testing.T) {
 						return true
 					}
 				}
-
 				return false
-			})
+			}, test.existingSockets)
+
 			if (err != nil) != test.expectedError {
 				t.Fatalf("detectCRISocketImpl returned unexpected result\n\tExpected error: %t\n\tGot error: %t", test.expectedError, err != nil)
 			}

cmd/kubeadm/app/util/runtime/runtime_unix.go

@@ -24,11 +24,6 @@ import (
 	"net/url"
 )
 
-const (
-	dockerSocket     = "unix:///var/run/docker.sock" // The Docker socket is not CRI compatible
-	containerdSocket = "unix:///run/containerd/containerd.sock"
-)
-
 // isExistingSocket checks if path exists and is domain socket
 func isExistingSocket(path string) bool {
 	u, err := url.Parse(path)

cmd/kubeadm/app/util/runtime/runtime_windows.go

@@ -25,11 +25,6 @@ import (
 	winio "github.com/Microsoft/go-winio"
 )
 
-const (
-	dockerSocket     = "npipe:////./pipe/docker_engine"         // The Docker socket is not CRI compatible
-	containerdSocket = "npipe:////./pipe/containerd-containerd" // Proposed containerd named pipe for Windows
-)
-
 // isExistingSocket checks if path exists and is domain socket
 func isExistingSocket(path string) bool {
 	u, err := url.Parse(path)