Update fallback for OTEL config flag

cmd/amazon-cloudwatch-agent/amazon-cloudwatch-agent.go

@@ -494,7 +494,7 @@ func main() {
 	flag.Var(&fOtelConfigs, configprovider.OtelConfigFlagName, "YAML configuration files to run OTel pipeline")
 	flag.Parse()
 	if len(fOtelConfigs) == 0 {
-		_ = fOtelConfigs.Set(paths.YamlConfigPath)
+		_ = fOtelConfigs.Set(getFallbackOtelConfig(*fTomlConfig, paths.YamlConfigPath))
 	}
 	args := flag.Args()
 	sectionFilters, inputFilters, outputFilters := []string{}, []string{}, []string{}
@@ -743,3 +743,28 @@ func checkRightForBinariesFileWithInputPlugins(inputPlugins []string) (string, e
 
 	return "", nil
 }
+
+// getFallbackOtelConfig returns the first fallback YAML file that exists. It checks files in the following order:
+//  1. Default YAML path
+//  2. Default YAML in the provided TOML directory
+//  3. YAML with the same name as the provided TOML
+func getFallbackOtelConfig(tomlPath, defaultYamlPath string) string {
+	candidatePaths := []string{defaultYamlPath}
+	if tomlPath != "" {
+		tomlDir := filepath.Dir(tomlPath)
+		samePathYAML := strings.TrimSuffix(tomlPath, filepath.Ext(tomlPath)) + ".yaml"
+		candidatePaths = append(candidatePaths,
+			filepath.Join(tomlDir, paths.YAML),
+			samePathYAML,
+		)
+	}
+	fallbackPath := defaultYamlPath
+	for _, candidatePath := range candidatePaths {
+		_, err := os.Stat(candidatePath)
+		if err == nil {
+			fallbackPath = candidatePath
+			break
+		}
+	}
+	return fallbackPath
+}

cmd/amazon-cloudwatch-agent/amazon-cloudwatch-agent_test.go

@@ -20,6 +20,7 @@ import (
 	"github.com/aws/amazon-cloudwatch-agent/cfg/envconfig"
 	"github.com/aws/amazon-cloudwatch-agent/internal/merge/confmap"
 	"github.com/aws/amazon-cloudwatch-agent/logger"
+	"github.com/aws/amazon-cloudwatch-agent/tool/paths"
 )
 
 func Test_getCollectorParams(t *testing.T) {
@@ -154,6 +155,52 @@ service:
 	}
 }
 
+func TestFallbackOtelConfig(t *testing.T) {
+	defaultYamlRelativePath := filepath.Join("default", paths.YAML)
+	testCases := map[string]struct {
+		tomlRelativePath string
+		filesToCreate    []string
+		want             string
+	}{
+		"WithoutAnyFiles": {
+			tomlRelativePath: filepath.Join("config", "config.toml"),
+			want:             defaultYamlRelativePath,
+		},
+		"WithDefaultYamlPath": {
+			tomlRelativePath: filepath.Join("config", "config.toml"),
+			filesToCreate:    []string{defaultYamlRelativePath, filepath.Join("config", paths.YAML)},
+			want:             defaultYamlRelativePath,
+		},
+		"WithDefaultYamlInTomlDir": {
+			tomlRelativePath: filepath.Join("config", "config.toml"),
+			filesToCreate:    []string{filepath.Join("config", paths.YAML), filepath.Join("config", "config.yaml")},
+			want:             filepath.Join("config", paths.YAML),
+		},
+		"WithSameNameAsToml": {
+			tomlRelativePath: filepath.Join("config", "config.toml"),
+			filesToCreate:    []string{filepath.Join("config", "config.yaml")},
+			want:             filepath.Join("config", "config.yaml"),
+		},
+		"WithoutTomlPath": {
+			tomlRelativePath: "",
+			filesToCreate:    []string{filepath.Join("config", "config.yaml")},
+			want:             defaultYamlRelativePath,
+		},
+	}
+	for name, testCase := range testCases {
+		t.Run(name, func(t *testing.T) {
+			tmpDir := t.TempDir()
+			for _, fileToCreate := range testCase.filesToCreate {
+				path := filepath.Join(tmpDir, fileToCreate)
+				require.NoError(t, os.MkdirAll(filepath.Dir(path), 0755))
+				require.NoError(t, os.WriteFile(path, nil, 0600))
+			}
+			got := getFallbackOtelConfig(filepath.Join(tmpDir, testCase.tomlRelativePath), filepath.Join(tmpDir, defaultYamlRelativePath))
+			assert.Equal(t, filepath.Join(tmpDir, testCase.want), got)
+		})
+	}
+}
+
 func mustLoadFromFile(t *testing.T, path string) *confmap.Conf {
 	conf, err := confmap.NewFileLoader(path).Load()
 	require.NoError(t, err)

service/configprovider/flags.go

@@ -4,6 +4,7 @@
 package configprovider
 
 import (
+	"flag"
 	"fmt"
 )
 
@@ -13,6 +14,8 @@ const (
 
 type OtelConfigFlags []string
 
+var _ flag.Value = (*OtelConfigFlags)(nil)
+
 func (o *OtelConfigFlags) String() string {
 	return fmt.Sprint(*o)
 }