WIP/not merge: Introduce extra interface for introspection...

In elastic#4378 we briefly discussed how to test the parsing of config
parameters. The way ucfg works is that the yaml/json is unpacked
into the struct and  with the current approach at least two issues
can occur:

1.) One can have entries in the config file that are not consumed
2.) One can have members in the struct that are not filled

To allow testing proper parsing of a valid configuration file and
help users to show what has been passed I introduce a new config
option.

Instead of forcing this new method for the existing Beater interface,
add a new introspection interface...

TODO: Is this a sound approach? Shall a list of interfaces be returned?
Print them to a file? What about the b.Config? How to add this as a
system test?

TODO: Don't use go-yaml/yaml but the one used by ucfg?

libbeat/beat/beat.go

@@ -60,6 +60,7 @@ import (
 	svc "github.com/elastic/beats/libbeat/service"
 	"github.com/elastic/beats/libbeat/template"
 	"github.com/elastic/beats/libbeat/version"
+	"github.com/go-yaml/yaml"
 
 	// Register default processors.
 	_ "github.com/elastic/beats/libbeat/processors/actions"
@@ -92,6 +93,13 @@ type Beater interface {
 	Stop()
 }
 
+// Additional interface to provide introspection. E.g. for system tests
+// or other forms of runtime introspection
+type BeaterIntrospection interface {
+	// Provide the internal Config structure
+	Config() interface{}
+}
+
 // Creator initializes and configures a new Beater instance used to execute
 // the beat its run-loop.
 type Creator func(*Beat, *common.Config) (Beater, error)
@@ -246,6 +254,15 @@ func (b *Beat) launch(bt Creator) error {
 		defer reporter.Stop()
 	}
 
+	// Provide yaml presentation of what was parsed
+	if cfgfile.DumpConfig() {
+		if val, ok := beater.(BeaterIntrospection); ok {
+			dat, _ := yaml.Marshal(val.Config())
+			fmt.Print(string(dat))
+		}
+		return GracefulExit
+	}
+
 	// If -configtest was specified, exit now prior to run.
 	if cfgfile.IsTestConfig() {
 		fmt.Println("Config OK")

libbeat/cfgfile/cfgfile.go

@@ -18,6 +18,7 @@ var (
 	configfiles = flagArgList("c", "beat.yml", "Configuration file, relative to path.config")
 	overwrites  = common.NewFlagConfig(nil, nil, "E", "Configuration overwrite")
 	testConfig  = flag.Bool("configtest", false, "Test configuration and exit.")
+	dumpConfig  = flag.Bool("dumpconfig", false, "Dump parsed configuration.")
 
 	// Additional default settings, that must be available for variable expansion
 	defaults = mustNewConfigFrom(map[string]interface{}{
@@ -171,3 +172,8 @@ func GetPathConfig() string {
 func IsTestConfig() bool {
 	return *testConfig
 }
+
+// DumpConfig returns where or not to print the parsed configuration
+func DumpConfig() bool {
+	return *dumpConfig
+}

packetbeat/beater/packetbeat.go

@@ -282,3 +282,7 @@ func (pb *packetbeat) icmpConfig() (*common.Config, error) {
 
 	return icmp, nil
 }
+
+func (pb *packetbeat) Config() interface{} {
+	return pb.config
+}