Filebeat: Error on startup for unconfigured module

Since elastic#27526 and elastic#27762, Filebeat will have all filesets disabled by
default. To prevent user confusion, a warning message to alert the user
of a configured module without any enabled filesets was added. However,
due to Filebeat internals, this message will only appear for modules
configured from the command-line (--modules flag).

This updates the code to ensure it works also for modules configured via
modules.d directory and turns the warning into a hard-error that
prevents startup.

filebeat/beater/filebeat.go

@@ -111,22 +111,6 @@ func newBeater(b *beat.Beat, plugins PluginFactory, rawConfig *common.Config) (b
 	if err != nil {
 		return nil, err
 	}
-	if !moduleRegistry.Empty() {
-		logp.Info("Enabled modules/filesets: %s", moduleRegistry.InfoString())
-		for _, mod := range moduleRegistry.ModuleNames() {
-			if mod == "" {
-				continue
-			}
-			filesets, err := moduleRegistry.ModuleConfiguredFilesets(mod)
-			if err != nil {
-				logp.Err("Failed listing filesets for module %s", mod)
-				continue
-			}
-			if len(filesets) == 0 {
-				logp.Warn("Module %s is enabled but has no enabled filesets", mod)
-			}
-		}
-	}
 
 	moduleInputs, err := moduleRegistry.GetInputConfigs()
 	if err != nil {

filebeat/fileset/modules.go

@@ -100,6 +100,20 @@ func newModuleRegistry(modulesPath string,
 		}
 	}
 
+	logp.Info("Enabled modules/filesets: %s", reg.InfoString())
+	for _, mod := range reg.ModuleNames() {
+		if mod == "" {
+			continue
+		}
+		filesets, err := reg.ModuleConfiguredFilesets(mod)
+		if err != nil {
+			logp.Err("Failed listing filesets for module %s", mod)
+			continue
+		}
+		if len(filesets) == 0 {
+			return nil, errors.Errorf("module %s is configured but has no enabled filesets", mod)
+		}
+	}
 	return &reg, nil
 }