Expose output disk queue settings when running under agent (#40857)

* Expose output disk queue settings when running under agent

libbeat/outputs/util.go

@@ -25,6 +25,7 @@ import (
 	"github.com/elastic/beats/v7/libbeat/publisher/queue/diskqueue"
 	"github.com/elastic/beats/v7/libbeat/publisher/queue/memqueue"
 	"github.com/elastic/elastic-agent-libs/config"
+	"github.com/elastic/elastic-agent-libs/logp"
 )
 
 // Fail helper can be used by output factories, to create a failure response when
@@ -47,7 +48,8 @@ func Success(cfg config.Namespace, batchSize, retry int, encoderFactory queue.En
 			q = memqueue.FactoryForSettings(settings)
 		case diskqueue.QueueType:
 			if management.UnderAgent() {
-				return Group{}, fmt.Errorf("disk queue not supported under agent")
+				logger := logp.NewLogger("output")
+				logger.Warn("Disk queue configuration found while running under agent: this configuration is unsupported and in technical preview.")
 			}
 			settings, err := diskqueue.SettingsForUserConfig(cfg.Config())
 			if err != nil {

libbeat/outputs/util_test.go

@@ -0,0 +1,104 @@
+// 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 outputs
+
+import (
+	"fmt"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/zap/zapcore"
+
+	"github.com/elastic/beats/v7/libbeat/management"
+	"github.com/elastic/beats/v7/libbeat/publisher/queue"
+	"github.com/elastic/elastic-agent-libs/config"
+	"github.com/elastic/elastic-agent-libs/logp"
+)
+
+func TestDiskQueueUnderAgent(t *testing.T) {
+
+	type args struct {
+		cfg            string
+		batchSize      int
+		retry          int
+		encoderFactory queue.EncoderFactory
+		clients        []Client
+	}
+	tests := []struct {
+		name    string
+		args    args
+		want    Group
+		wantErr bool
+	}{
+		{
+			name: "Happy path",
+			args: args{
+				cfg: `
+                    disk:
+                        max_size: 100MB
+                        path: %s
+                `,
+				clients:   []Client{},
+				batchSize: 10,
+				retry:     3,
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			previousUnderAgent := management.UnderAgent()
+			t.Cleanup(func() {
+				management.SetUnderAgent(previousUnderAgent)
+			})
+
+			tempDir := t.TempDir()
+
+			queueConfig := config.Namespace{}
+			conf, err := config.NewConfigFrom(fmt.Sprintf(tt.args.cfg, tempDir))
+			require.NoError(t, err, "error parsing queue config")
+			err = queueConfig.Unpack(conf)
+			require.NoError(t, err, "error unpacking queue config")
+
+			management.SetUnderAgent(true)
+
+			actualGroup, err := Success(queueConfig, tt.args.batchSize, tt.args.retry, tt.args.encoderFactory, tt.args.clients...)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Success() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+
+			if tt.wantErr {
+				// if an error was expected, we need no more assertions: return
+				return
+			}
+
+			require.NotNil(t, actualGroup)
+			require.NotNil(t, actualGroup.QueueFactory)
+
+			testlogger, _ := logp.NewInMemory("test-diskqueue", zapcore.EncoderConfig{})
+
+			actualQueue, err := actualGroup.QueueFactory(testlogger, nil, 1, nil)
+			require.NoError(t, err)
+			require.NotNil(t, actualQueue)
+			// assert that the file exists in the path we specified
+			assert.FileExists(t, filepath.Join(tempDir, "state.dat"))
+		})
+	}
+}