Add yamlgen/cfggen (#79)

Signed-off-by: Saswata Mukherjee <saswataminsta@yahoo.com>

Author: saswatamcode

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/charmbracelet/glamour v0.3.0
 	github.com/efficientgo/tools/core v0.0.0-20210609125236-d73259166f20
 	github.com/efficientgo/tools/extkingpin v0.0.0-20210609125236-d73259166f20
+	github.com/fatih/structtag v1.2.0
 	github.com/felixge/fgprof v0.9.1
 	github.com/go-kit/kit v0.10.0
 	github.com/gobwas/glob v0.2.3

go.sum

@@ -193,6 +193,8 @@ github.com/envoyproxy/protoc-gen-validate v0.1.0/go.mod h1:iSmxcyjqTsJpI2R4NaDN7
 github.com/evanw/esbuild v0.6.5/go.mod h1:mptxmSXIzBIKKCe4jo9A5SToEd1G+AKZ9JmY85dYRJ0=
 github.com/fatih/color v1.7.0 h1:DkWD4oS2D8LGGgTQ6IvwJJXSL5Vp2ffcQg58nFV38Ys=
 github.com/fatih/color v1.7.0/go.mod h1:Zm6kSWBoL9eyXnKyktHP6abPY2pDugNf5KwzbycvMj4=
+github.com/fatih/structtag v1.2.0 h1:/OdNE99OxoI/PqaW/SuSK9uxxT3f/tcSZgon/ssNSx4=
+github.com/fatih/structtag v1.2.0/go.mod h1:mBJUNpUnHmRKrKlQQlmCrh5PuhftFbNv8Ys4/aAZl94=
 github.com/felixge/fgprof v0.9.1 h1:E6FUJ2Mlv043ipLOCFqo8+cHo9MhQ203E2cdEK/isEs=
 github.com/felixge/fgprof v0.9.1/go.mod h1:7/HK6JFtFaARhIljgP2IV8rJLIoHDoOYoUphsnGvqxE=
 github.com/fortytw2/leaktest v1.2.0/go.mod h1:jDsjWgpAGjm2CA7WthBh/CdZYEPF31XHquHwclZch5g=

pkg/yamlgen/cfggen.go

@@ -0,0 +1,65 @@
+// Copyright (c) Bartłomiej Płotka @bwplotka
+// Licensed under the Apache License 2.0.
+
+// Taken from Thanos project.
+//
+// Copyright (c) The Thanos Authors.
+// Licensed under the Apache License 2.0.
+
+package yamlgen
+
+import (
+	"io"
+	"reflect"
+
+	"github.com/fatih/structtag"
+	"github.com/pkg/errors"
+	"gopkg.in/yaml.v3"
+)
+
+func Generate(obj interface{}, w io.Writer) error {
+	// We forbid omitempty option. This is for simplification for doc generation.
+	if err := checkForOmitEmptyTagOption(obj); err != nil {
+		return errors.Wrap(err, "invalid type")
+	}
+	return yaml.NewEncoder(w).Encode(obj)
+}
+
+func checkForOmitEmptyTagOption(obj interface{}) error {
+	return checkForOmitEmptyTagOptionRec(reflect.ValueOf(obj))
+}
+
+func checkForOmitEmptyTagOptionRec(v reflect.Value) error {
+	switch v.Kind() {
+	case reflect.Struct:
+		for i := 0; i < v.NumField(); i++ {
+			tags, err := structtag.Parse(string(v.Type().Field(i).Tag))
+			if err != nil {
+				return errors.Wrapf(err, "%s: failed to parse tag %q", v.Type().Field(i).Name, v.Type().Field(i).Tag)
+			}
+
+			tag, err := tags.Get("yaml")
+			if err != nil {
+				return errors.Wrapf(err, "%s: failed to get tag %q", v.Type().Field(i).Name, v.Type().Field(i).Tag)
+			}
+
+			for _, opts := range tag.Options {
+				if opts == "omitempty" {
+					return errors.Errorf("omitempty is forbidden for config, but spotted on field '%s'", v.Type().Field(i).Name)
+				}
+			}
+
+			if err := checkForOmitEmptyTagOptionRec(v.Field(i)); err != nil {
+				return errors.Wrapf(err, "%s", v.Type().Field(i).Name)
+			}
+		}
+
+	case reflect.Ptr:
+		return errors.New("nil pointers are not allowed in configuration")
+
+	case reflect.Interface:
+		return checkForOmitEmptyTagOptionRec(v.Elem())
+	}
+
+	return nil
+}