Fix go plugins not loaded when beat starts (elastic#4799)

CHANGELOG.asciidoc

@@ -31,6 +31,8 @@ https://github.com/elastic/beats/compare/v6.0.0-beta1...master[Check the HEAD di
 
 *Affecting all Beats*
 
+- Fix go plugins not loaded when beat starts {pull}4799[4799]
+
 *Auditbeat*
 
 *Filebeat*

libbeat/cmd/root.go

@@ -68,6 +68,9 @@ func GenRootCmdWithRunFlags(name, version string, beatCreator beat.Creator, runF
 	rootCmd.PersistentFlags().AddGoFlag(flag.CommandLine.Lookup("path.logs"))
 	rootCmd.PersistentFlags().AddGoFlag(flag.CommandLine.Lookup("path.home"))
 	rootCmd.PersistentFlags().AddGoFlag(flag.CommandLine.Lookup("strict.perms"))
+	if f := flag.CommandLine.Lookup("plugin"); f != nil {
+		rootCmd.PersistentFlags().AddGoFlag(f)
+	}
 
 	// Inherit root flags from run command
 	// TODO deprecate when root command no longer executes run (7.0)

libbeat/plugin/cli.go

@@ -18,8 +18,12 @@ func (p *pluginList) String() string {
 }
 
 func (p *pluginList) Set(v string) error {
-	// TODO: check file exists
-
+	for _, path := range p.paths {
+		if path == v {
+			logp.Warn("%s is already a registered plugin")
+			return nil
+		}
+	}
 	p.paths = append(p.paths, v)
 	return nil
 }