Give precedence to monitoring reporter hosts over output hosts (#17991)

* Give precedence to monitoring reporter hosts over output hosts

* Add CHANGELOG entry

* Removing debugging statements

* No delete

* Helper function

* Make new config if nil

* Formatting code

CHANGELOG.next.asciidoc

@@ -105,6 +105,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix Elasticsearch license endpoint URL referenced in error message. {issue}17880[17880] {pull}18030[18030]
 - Fix panic when assigning a key to a `nil` value in an event. {pull}18143[18143]
 - Change `decode_json_fields` processor, to merge parsed json objects with existing objects in the event instead of fully replacing them. {pull}17958[17958]
+- Gives monitoring reporter hosts, if configured, total precedence over corresponding output hosts. {issue}17937[17937] {pull}17991[17991]
 
 *Auditbeat*
 

libbeat/monitoring/report/report.go

@@ -21,6 +21,8 @@ import (
 	"errors"
 	"fmt"
 
+	errw "github.com/pkg/errors"
+
 	"github.com/elastic/beats/v7/libbeat/beat"
 	"github.com/elastic/beats/v7/libbeat/common"
 )
@@ -59,6 +61,10 @@ type Reporter interface {
 
 type ReporterFactory func(beat.Info, Settings, *common.Config) (Reporter, error)
 
+type hostsCfg struct {
+	Hosts []string `config:"hosts"`
+}
+
 var (
 	defaultConfig = config{}
 
@@ -111,9 +117,7 @@ func getReporterConfig(
 		// merge reporter config with output config if both are present
 		if outCfg := outputs.Config(); outputs.Name() == name && outCfg != nil {
 			// require monitoring to not configure any hosts if output is configured:
-			hosts := struct {
-				Hosts []string `config:"hosts"`
-			}{}
+			hosts := hostsCfg{}
 			rc.Unpack(&hosts)
 
 			if settings.Format == FormatXPackMonitoringBulk && len(hosts.Hosts) > 0 {
@@ -127,6 +131,13 @@ func getReporterConfig(
 			if err != nil {
 				return "", nil, err
 			}
+
+			// Make sure hosts from reporter configuration get precedence over hosts
+			// from output configuration
+			if err := mergeHosts(merged, outCfg, rc); err != nil {
+				return "", nil, err
+			}
+
 			rc = merged
 		}
 
@@ -155,3 +166,44 @@ func collectSubObject(cfg *common.Config) *common.Config {
 	}
 	return out
 }
+
+func mergeHosts(merged, outCfg, reporterCfg *common.Config) error {
+	if merged == nil {
+		merged = common.NewConfig()
+	}
+
+	outputHosts := hostsCfg{}
+	if outCfg != nil {
+		if err := outCfg.Unpack(&outputHosts); err != nil {
+			return errw.Wrap(err, "unable to parse hosts from output config")
+		}
+	}
+
+	reporterHosts := hostsCfg{}
+	if reporterCfg != nil {
+		if err := reporterCfg.Unpack(&reporterHosts); err != nil {
+			return errw.Wrap(err, "unable to parse hosts from reporter config")
+		}
+	}
+
+	if len(outputHosts.Hosts) == 0 && len(reporterHosts.Hosts) == 0 {
+		return nil
+	}
+
+	// Give precedence to reporter hosts over output hosts
+	var newHostsCfg *common.Config
+	var err error
+	if len(reporterHosts.Hosts) > 0 {
+		newHostsCfg, err = common.NewConfigFrom(reporterHosts.Hosts)
+	} else {
+		newHostsCfg, err = common.NewConfigFrom(outputHosts.Hosts)
+	}
+	if err != nil {
+		return errw.Wrap(err, "unable to make config from new hosts")
+	}
+
+	if err := merged.SetChild("hosts", -1, newHostsCfg); err != nil {
+		return errw.Wrap(err, "unable to set new hosts into merged config")
+	}
+	return nil
+}

libbeat/monitoring/report/report_test.go

@@ -0,0 +1,78 @@
+// 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 report
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/elastic/beats/v7/libbeat/common"
+)
+
+func TestMergeHosts(t *testing.T) {
+	tests := map[string]struct {
+		outCfg      *common.Config
+		reporterCfg *common.Config
+		expectedCfg *common.Config
+	}{
+		"no_hosts": {
+			expectedCfg: newConfigWithHosts(),
+		},
+		"only_reporter_hosts": {
+			reporterCfg: newConfigWithHosts("r1", "r2"),
+			expectedCfg: newConfigWithHosts("r1", "r2"),
+		},
+		"only_output_hosts": {
+			outCfg:      newConfigWithHosts("o1", "o2"),
+			expectedCfg: newConfigWithHosts("o1", "o2"),
+		},
+		"equal_hosts": {
+			outCfg:      newConfigWithHosts("o1", "o2"),
+			reporterCfg: newConfigWithHosts("r1", "r2"),
+			expectedCfg: newConfigWithHosts("r1", "r2"),
+		},
+		"more_output_hosts": {
+			outCfg:      newConfigWithHosts("o1", "o2"),
+			reporterCfg: newConfigWithHosts("r1"),
+			expectedCfg: newConfigWithHosts("r1"),
+		},
+		"more_reporter_hosts": {
+			outCfg:      newConfigWithHosts("o1"),
+			reporterCfg: newConfigWithHosts("r1", "r2"),
+			expectedCfg: newConfigWithHosts("r1", "r2"),
+		},
+	}
+
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			mergedCfg := common.MustNewConfigFrom(map[string]interface{}{})
+			err := mergeHosts(mergedCfg, test.outCfg, test.reporterCfg)
+			require.NoError(t, err)
+
+			require.Equal(t, test.expectedCfg, mergedCfg)
+		})
+	}
+}
+
+func newConfigWithHosts(hosts ...string) *common.Config {
+	if len(hosts) == 0 {
+		return common.MustNewConfigFrom(map[string][]string{})
+	}
+	return common.MustNewConfigFrom(map[string][]string{"hosts": hosts})
+}