Make FQDN lookup function more testable

providers/shared/fqdn.go

@@ -32,6 +32,10 @@ func FQDN() (string, error) {
 		return "", fmt.Errorf("could not get hostname to look for FQDN: %w", err)
 	}
 
+	return fqdn(hostname)
+}
+
+func fqdn(hostname string) (string, error) {
 	var errs error
 	cname, err := net.LookupCNAME(hostname)
 	if err != nil {

providers/shared/fqdn_test.go

@@ -0,0 +1,72 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you 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 shared
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestFQDN(t *testing.T) {
+	tests := map[string]struct {
+		osHostname   string
+		expectedFQDN string
+		expectedErr  error
+	}{
+		"long_real_hostname": {
+			osHostname:   "elastic.co",
+			expectedFQDN: "elastic.co",
+			expectedErr:  nil,
+		},
+		"long_nonexistent_hostname": {
+			osHostname:   "foo.bar.elastic.co",
+			expectedFQDN: "",
+			expectedErr:  makeError("foo.bar.elastic.co"),
+		},
+		"short_nonexistent_hostname": {
+			osHostname:   "foobarbaz",
+			expectedFQDN: "",
+			expectedErr:  makeError("foobarbaz"),
+		},
+	}
+
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			actualFQDN, err := fqdn(test.osHostname)
+			require.Equal(t, test.expectedFQDN, actualFQDN)
+
+			if test.expectedErr == nil {
+				require.Nil(t, err)
+			} else {
+				require.Equal(t, test.expectedErr.Error(), err.Error())
+			}
+		})
+	}
+}
+
+func makeError(osHostname string) error {
+	return fmt.Errorf(
+		"could not get FQDN, all methods failed: "+
+			"failed looking up CNAME: lookup %s: no such host: "+
+			"failed looking up IP: lookup %s: no such host",
+		osHostname,
+		osHostname,
+	)
+}