Fix make assets error in metricbeat

On elastic#13188 a check is added on global fields generator to check that the
generated fields file is valid. This check was done by reading the
result from disk, but this file is not available when output is stdout.

Modify the check so it is done over the buffered result before writing
to file or stdout.

libbeat/generator/fields/fields.go

@@ -20,6 +20,7 @@ package fields
 import (
 	"bufio"
 	"bytes"
+	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -104,28 +105,13 @@ func isLibbeat(beatPath string) bool {
 	return filepath.Base(beatPath) == "libbeat"
 }
 
-func writeGeneratedFieldsYml(fieldFiles []*YmlFile, output string) error {
+func writeGeneratedFieldsYml(fieldFiles []*YmlFile, output io.Writer) error {
 	data, err := GenerateFieldsYml(fieldFiles)
 	if err != nil {
 		return err
 	}
 
-	if output == "-" {
-		fw := bufio.NewWriter(os.Stdout)
-		_, err = fw.Write(data)
-		if err != nil {
-			return err
-		}
-		return fw.Flush()
-	}
-
-	f, err := os.Create(output)
-	if err != nil {
-		return err
-	}
-	defer f.Close()
-
-	fw := bufio.NewWriter(f)
+	fw := bufio.NewWriter(output)
 	_, err = fw.Write(data)
 	if err != nil {
 		return err
@@ -164,7 +150,7 @@ func writeIndentedLine(buf *bytes.Buffer, line string, indent int) error {
 }
 
 // Generate collects fields.yml files and concatenates them into one global file.
-func Generate(esBeatsPath, beatPath string, files []*YmlFile, output string) error {
+func Generate(esBeatsPath, beatPath string, files []*YmlFile, output io.Writer) error {
 	files, err := collectCommonFiles(esBeatsPath, beatPath, files)
 	if err != nil {
 		return err

libbeat/scripts/cmd/global_fields/main.go

@@ -18,8 +18,10 @@
 package main
 
 import (
+	"bytes"
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 
@@ -90,21 +92,29 @@ func main() {
 		fieldsFiles = append(fieldsFiles, fieldsFile...)
 	}
 
-	err = fields.Generate(esBeatsPath, beatPath, fieldsFiles, output)
+	var buffer bytes.Buffer
+	err = fields.Generate(esBeatsPath, beatPath, fieldsFiles, &buffer)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "Cannot generate global fields.yml file for %s: %+v\n", name, err)
 		os.Exit(3)
 	}
 
-	_, err = mapping.LoadFieldsYaml(output)
+	_, err = mapping.LoadFields(buffer.Bytes())
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "Generated global fields.yml file for %s is invalid: %+v\n", name, err)
 		os.Exit(3)
 	}
 
-	outputPath, _ := filepath.Abs(output)
+	if output == "-" {
+		fmt.Print(buffer.String())
+		return
+	}
+
+	err = ioutil.WriteFile(output, buffer.Bytes(), 0644)
 	if err != nil {
-		outputPath = output
+		fmt.Fprintf(os.Stderr, "Cannot write global fields.yml file for %s: %v", name, err)
 	}
+
+	outputPath, _ := filepath.Abs(output)
 	fmt.Fprintf(os.Stderr, "Generated fields.yml for %s to %s\n", name, outputPath)
 }