feat: add option for missing field

Add DisallowMissingField option to create error when a field of a struct
is missing.

Remark:
This solves the issue that the validator can't handle: when a int type
is absent in the yaml, the validator has no way to find out as it checks
against go structs.

decode.go

@@ -14,6 +14,7 @@ import (
 	"reflect"
 	"sort"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/goccy/go-yaml/ast"
@@ -39,6 +40,7 @@ type Decoder struct {
 	validator            StructValidator
 	disallowUnknownField bool
 	disallowDuplicateKey bool
+	disallowMissingField bool
 	useOrderedMap        bool
 	useJSONUnmarshaler   bool
 	parsedFile           *ast.File
@@ -538,6 +540,18 @@ func errUnknownField(msg string, tk *token.Token) *unknownFieldError {
 	return &unknownFieldError{err: errors.ErrSyntax(msg, tk)}
 }
 
+type missingRequiredfieldError struct {
+	err error
+}
+
+func (e *missingRequiredfieldError) Error() string {
+	return e.err.Error()
+}
+
+func errMissingRequiredField(msg string, tk *token.Token) *missingRequiredfieldError {
+	return &missingRequiredfieldError{err: errors.ErrSyntax(msg, tk)}
+}
+
 func errUnexpectedNodeType(actual, expected ast.NodeType, tk *token.Token) error {
 	return errors.ErrSyntax(fmt.Sprintf("%s was used where %s is expected", actual.YAMLName(), expected.YAMLName()), tk)
 }
@@ -1177,6 +1191,7 @@ func (d *Decoder) decodeStruct(ctx context.Context, dst reflect.Value, src ast.N
 		}
 	}
 
+	var missingFields []string
 	aliasName := d.getMergeAliasName(src)
 	var foundErr error
 
@@ -1243,6 +1258,9 @@ func (d *Decoder) decodeStruct(ctx context.Context, dst reflect.Value, src ast.N
 		}
 		v, exists := keyToNodeMap[structField.RenderName]
 		if !exists {
+			if d.disallowMissingField {
+				missingFields = append(missingFields, structField.RenderName)
+			}
 			continue
 		}
 		delete(unknownFields, structField.RenderName)
@@ -1273,6 +1291,10 @@ func (d *Decoder) decodeStruct(ctx context.Context, dst reflect.Value, src ast.N
 		return errors.Wrapf(foundErr, "failed to decode value")
 	}
 
+	if len(missingFields) != 0 && d.disallowMissingField && src.GetToken() != nil {
+		return errMissingRequiredField(fmt.Sprintf(`missing required field: "%s" `, strings.Join(missingFields, ",")), src.GetToken())
+	}
+
 	// Ignore unknown fields when parsing an inline struct (recognized by a nil token).
 	// Unknown fields are expected (they could be fields from the parent struct).
 	if len(unknownFields) != 0 && d.disallowUnknownField && src.GetToken() != nil {

decode_test.go

@@ -2890,3 +2890,40 @@ func TestSameNameInineStruct(t *testing.T) {
 		t.Fatalf("failed to decode")
 	}
 }
+
+func Test_DecoderMissingFieldOption(t *testing.T) {
+	yml := `
+b: 2
+`
+	expected := `
+[2:2] missing required field: "a,v" 
+>  2 | b: 2
+        ^
+`
+	t.Run("map", func(t *testing.T) {
+		var v map[string]string
+		err := yaml.NewDecoder(strings.NewReader(yml), yaml.DisallowMissingField()).Decode(&v)
+		if err != nil {
+			t.Fatal("decoding should success")
+		}
+		if len(v) != 1 && v["b"] != "2" {
+			t.Fatal("failed to decode, DisallowMissingField should have no impact on map")
+		}
+	})
+	t.Run("struct", func(t *testing.T) {
+		var v struct {
+			A int
+			B int
+			V string
+		}
+		err := yaml.NewDecoder(strings.NewReader(yml), yaml.DisallowMissingField()).Decode(&v)
+		if err == nil {
+			t.Fatal("decoding should fail")
+		}
+		actual := "\n" + err.Error()
+		if expected != actual {
+			fmt.Println(err)
+			t.Fatalf("expected:[%s] actual:[%s]", expected, actual)
+		}
+	})
+}

option.go

@@ -77,6 +77,15 @@ func DisallowDuplicateKey() DecodeOption {
 	}
 }
 
+// DisallowMissingField causes an error when mapping keys that are missing if the destination is a struct
+// This does no effect on dynamic types like map.
+func DisallowMissingField() DecodeOption {
+	return func(d *Decoder) error {
+		d.disallowMissingField = true
+		return nil
+	}
+}
+
 // UseOrderedMap can be interpreted as a map,
 // and uses MapSlice ( ordered map ) aggressively if there is no type specification
 func UseOrderedMap() DecodeOption {