feat: Introduce ErrYAMLMultidoc

Return ErrYAMLMultidoc if the YAML source contains multiple documents.

README.md

@@ -57,6 +57,7 @@ an error annotated with line and column in the YAML file if necessary.
 	- 🚫 Forbids assigning non-string values to Go types that implement
 	the [`encoding.TextUnmarshaler`](https://pkg.go.dev/encoding#TextUnmarshaler) interface.
 	- 🚫 Forbids empty array items ([see rationale](#why-are-empty-array-items-forbidden)).
+	- 🚫 Forbids multi-document files.
 - Features:
 	- 🪄 If any type within your configuration struct implements the `Validate` interface,
 	then its validation method will be called using reflection

yamagiconf.go

@@ -32,6 +32,7 @@ var (
 	ErrValidation           = errors.New("validation")
 	ErrValidateTagViolation = errors.New("violates validation rule")
 
+	ErrYAMLMultidoc        = errors.New("multi-document YAML files are not supported")
 	ErrYAMLEmptyFile       = errors.New("empty file")
 	ErrYAMLMalformed       = errors.New("malformed YAML")
 	ErrYAMLInlineNonAnon   = errors.New("inline yaml on non-embedded field")
@@ -127,8 +128,19 @@ func Load[T any, S string | []byte](yamlSource S, config *T) error {
 	}
 
 	var rootNode yaml.Node
-	if err := newDecoderYAML(yamlSource).Decode(&rootNode); err != nil {
-		return fmt.Errorf("decoding yaml structure: %w", err)
+	{
+		dec := newDecoderYAML(yamlSource)
+		if err := dec.Decode(&rootNode); err != nil {
+			return fmt.Errorf("decoding yaml structure: %w", err)
+		}
+
+		// Check if multi-doc
+		var n yaml.Node
+		if err = dec.Decode(&n); err == nil {
+			return fmt.Errorf("at %d:%d: %w", n.Line, n.Column, ErrYAMLMultidoc)
+		} else if !errors.Is(err, io.EOF) {
+			return fmt.Errorf("%w: %w", ErrYAMLMultidoc, err)
+		}
 	}
 
 	configType := reflect.TypeOf(config).Elem()

yamagiconf_test.go

@@ -1446,6 +1446,40 @@ func TestLoadErr(t *testing.T) {
 		require.NotZero(t, c)
 	})
 
+	t.Run("multidocument", func(t *testing.T) {
+		_, err := LoadSrc[TestConfig](`# Second doc contains different data.
+---
+foo: 1
+---
+foo: 2
+`)
+		require.ErrorIs(t, err, yamagiconf.ErrYAMLMultidoc)
+		require.Equal(t, "at 4:1: "+yamagiconf.ErrYAMLMultidoc.Error(), err.Error())
+	})
+
+	t.Run("multidocument_second_doc_not_to_spec", func(t *testing.T) {
+		_, err := LoadSrc[TestConfig](`# Second doc can't be decoded to TestConfig.
+---
+foo: 1
+---
+bar: 2
+`)
+		require.ErrorIs(t, err, yamagiconf.ErrYAMLMultidoc)
+		require.Equal(t, "at 4:1: "+yamagiconf.ErrYAMLMultidoc.Error(), err.Error())
+	})
+
+	t.Run("multidocument_error_in_second_file", func(t *testing.T) {
+		_, err := LoadSrc[TestConfig](`# Second doc contains a syntax error.
+---
+foo: 1
+---
+:
+`)
+		require.ErrorIs(t, err, yamagiconf.ErrYAMLMultidoc)
+		require.Equal(t, yamagiconf.ErrYAMLMultidoc.Error()+
+			": yaml: line 4: did not find expected key", err.Error())
+	})
+
 	t.Run("unsupported_boolean_literal", func(t *testing.T) {
 		type TestConfig struct {
 			Boolean bool `yaml:"boolean"`