Initialize Paths before loading the keystore (elastic#10706)

* Initialize Paths before loading the keystore

The paths were incorrectly initialized meaning that instead of creating
the keystore in the data directory it was created next to the binary.

The problem was the call to `paths.InitPaths()` was done after loading
the keystore, this was causing a chicken and egg situation and
`paths.Resolve(path.Data, "hello")` was returning "hello" instead of
`data/hello`.

To solve that situation we do a partial extract of the configuration,
just enough to initialize the paths and we move on to the keystore and
the complete unpack.

CHANGELOG.next.asciidoc

@@ -29,6 +29,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Move output.elasticsearch.ilm settings to setup.ilm. {pull}10347[10347]
 - ILM will be available by default if Elasticsearch > 7.0 is used. {pull}10347[10347]
 - Allow Central Management to send events back to kibana. {issue}9382[9382]
+- Initialize the Paths before the keystore and save the keystore into `data/{beatname}.keystore`. {pull}10706[10706]
 
 *Auditbeat*
 

libbeat/cmd/instance/beat.go

@@ -519,6 +519,10 @@ func (b *Beat) configure(settings Settings) error {
 		return fmt.Errorf("error loading config file: %v", err)
 	}
 
+	if err := initPaths(cfg); err != nil {
+		return err
+	}
+
 	// We have to initialize the keystore before any unpack or merging the cloud
 	// options.
 	store, err := LoadKeystore(cfg, b.Info.Beat)
@@ -551,11 +555,6 @@ func (b *Beat) configure(settings Settings) error {
 		b.Info.Name = name
 	}
 
-	err = paths.InitPaths(&b.Config.Path)
-	if err != nil {
-		return fmt.Errorf("error setting default paths: %v", err)
-	}
-
 	if err := configure.Logging(b.Info.Beat, b.Config.Logging); err != nil {
 		return fmt.Errorf("error initializing logging: %v", err)
 	}
@@ -908,3 +907,22 @@ func initKibanaConfig(beatConfig beatConfig) (*common.Config, error) {
 	}
 	return kibanaConfig, nil
 }
+
+func initPaths(cfg *common.Config) error {
+	// To Fix the chicken-egg problem with the Keystore and the loading of the configuration
+	// files we are doing a partial unpack of the configuration file and only take into consideration
+	// the paths field. After we will unpack the complete configuration and keystore reference
+	// will be correctly replaced.
+	partialConfig := struct {
+		Path paths.Path `config:"path"`
+	}{}
+
+	if err := cfg.Unpack(&partialConfig); err != nil {
+		return fmt.Errorf("error extracting default paths: %+v", err)
+	}
+
+	if err := paths.InitPaths(&partialConfig.Path); err != nil {
+		return fmt.Errorf("error setting default paths: %+v", err)
+	}
+	return nil
+}