Move GetHostname() from k/k/pkg/util/node to k/component-helpers/node/util

In fact, this actually uses pkg/util/node's GetHostname() but takes
the unit tests from cmd/kubeadm/app/util's private fork of that
function since they were more extensive. (Of course the fact that
kubeadm had a private fork of this function is a strong argument for
moving it to component-helpers.)

staging/src/k8s.io/component-helpers/node/util/hostname.go

@@ -0,0 +1,46 @@
+/*
+Copyright 2015 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package util
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+// GetHostname returns OS's hostname if 'hostnameOverride' is empty; otherwise, it returns
+// 'hostnameOverride'. In either case, the value is canonicalized (trimmed and
+// lowercased).
+func GetHostname(hostnameOverride string) (string, error) {
+	hostName := hostnameOverride
+	if len(hostName) == 0 {
+		nodeName, err := os.Hostname()
+		if err != nil {
+			return "", fmt.Errorf("couldn't determine hostname: %w", err)
+		}
+		hostName = nodeName
+	}
+
+	// Trim whitespaces first to avoid getting an empty hostname
+	// For linux, the hostname is read from file /proc/sys/kernel/hostname directly
+	hostName = strings.TrimSpace(hostName)
+	if len(hostName) == 0 {
+		return "", fmt.Errorf("empty hostname is invalid")
+	}
+
+	return strings.ToLower(hostName), nil
+}

staging/src/k8s.io/component-helpers/node/util/hostname_test.go

@@ -0,0 +1,84 @@
+/*
+Copyright 2019 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package util
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestGetHostname(t *testing.T) {
+	hostname, err := os.Hostname()
+
+	testCases := []struct {
+		desc        string
+		hostname    string
+		result      string
+		expectedErr error
+	}{
+		{
+			desc:        "overridden hostname",
+			hostname:    "overridden",
+			result:      "overridden",
+			expectedErr: nil,
+		},
+		{
+			desc:        "overridden hostname uppercase",
+			hostname:    "OVERRIDDEN",
+			result:      "overridden",
+			expectedErr: nil,
+		},
+		{
+			desc:        "hostname contains spaces",
+			hostname:    " OVERRIDDEN ",
+			result:      "overridden",
+			expectedErr: nil,
+		},
+		{
+			desc:        "hostname contains only spaces",
+			hostname:    " ",
+			result:      "",
+			expectedErr: fmt.Errorf("empty hostname is invalid"),
+		},
+		{
+			desc:        "empty parameter",
+			hostname:    "",
+			result:      strings.ToLower(hostname),
+			expectedErr: err,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			result, err := GetHostname(tc.hostname)
+
+			if err != nil && tc.expectedErr == nil {
+				t.Errorf("unexpected error: %v", err)
+			}
+
+			if err == nil && tc.expectedErr != nil {
+				t.Errorf("expected error %v, got nil", tc.expectedErr)
+			}
+
+			if tc.result != result {
+				t.Errorf("unexpected result: %s, expected: %s", result, tc.result)
+			}
+		})
+	}
+}