write a canonical header for machine-generated files

Signed-off-by: Denys Smirnov <denys@sourced.tech>

internal/code-generator/generator/generator.go

@@ -1,8 +1,12 @@
 package generator
 
 import (
+	"bytes"
 	"go/format"
+	"io"
 	"io/ioutil"
+	"path/filepath"
+	"text/template"
 )
 
 // File is the function's type that generate source file from a file to be parsed, linguist's samples dir and a template.
@@ -13,10 +17,45 @@ func formatedWrite(outPath string, source []byte) error {
 	if err != nil {
 		return err
 	}
-
 	if err := ioutil.WriteFile(outPath, formatedSource, 0666); err != nil {
 		return err
 	}
-
 	return nil
 }
+
+func executeTemplate(w io.Writer, name, path, commit string, fmap template.FuncMap, data interface{}) error {
+	getCommit := func() string {
+		return commit
+	}
+
+	buf := bytes.NewBuffer(nil)
+
+	const headerTmpl = "header.go.tmpl"
+
+	headerPath := filepath.Join(filepath.Dir(path), headerTmpl)
+
+	h := template.Must(template.New(headerTmpl).Funcs(template.FuncMap{
+		"getCommit": getCommit,
+	}).ParseFiles(headerPath))
+
+	if err := h.Execute(buf, data); err != nil {
+		return err
+	}
+
+	if fmap == nil {
+		fmap = make(template.FuncMap)
+	}
+	fmap["getCommit"] = getCommit
+
+	t := template.Must(template.New(name).Funcs(fmap).ParseFiles(path))
+	if err := t.Execute(buf, data); err != nil {
+		return err
+	}
+
+	src, err := format.Source(buf.Bytes())
+	if err != nil {
+		return err
+	}
+	_, err = w.Write(src)
+	return err
+}