Decode: unstable/Unmarshal interface (#940)

Co-authored-by: Pavlos Karakalidis <pkarakal@pkarakal.com>
Co-authored-by: Thomas Pelletier <thomas@pelletier.codes>

unmarshaler.go

@@ -35,6 +35,9 @@ type Decoder struct {
 
 	// global settings
 	strict bool
+
+	// toggles unmarshaler interface
+	unmarshalerInterface bool
 }
 
 // NewDecoder creates a new Decoder that will read from r.
@@ -54,6 +57,24 @@ func (d *Decoder) DisallowUnknownFields() *Decoder {
 	return d
 }
 
+// EnableUnmarshalerInterface allows to enable unmarshaler interface.
+//
+// With this feature enabled, types implementing the unstable/Unmarshaler
+// interface can be decoded from any structure of the document. It allows types
+// that don't have a straightfoward TOML representation to provide their own
+// decoding logic.
+//
+// Currently, types can only decode from a single value. Tables and array tables
+// are not supported.
+//
+// *Unstable:* This method does not follow the compatibility guarantees of
+// semver. It can be changed or removed without a new major version being
+// issued.
+func (d *Decoder) EnableUnmarshalerInterface() *Decoder {
+	d.unmarshalerInterface = true
+	return d
+}
+
 // Decode the whole content of r into v.
 //
 // By default, values in the document that don't exist in the target Go value
@@ -108,6 +129,7 @@ func (d *Decoder) Decode(v interface{}) error {
 		strict: strict{
 			Enabled: d.strict,
 		},
+		unmarshalerInterface: d.unmarshalerInterface,
 	}
 
 	return dec.FromParser(v)
@@ -143,6 +165,9 @@ type decoder struct {
 	// Strict mode
 	strict strict
 
+	// Flag that enables/disables unmarshaler interface.
+	unmarshalerInterface bool
+
 	// Current context for the error.
 	errorContext *errorContext
 }
@@ -648,6 +673,14 @@ func (d *decoder) handleValue(value *unstable.Node, v reflect.Value) error {
 		v = initAndDereferencePointer(v)
 	}
 
+	if d.unmarshalerInterface {
+		if v.CanAddr() && v.Addr().CanInterface() {
+			if outi, ok := v.Addr().Interface().(unstable.Unmarshaler); ok {
+				return outi.UnmarshalTOML(value)
+			}
+		}
+	}
+
 	ok, err := d.tryTextUnmarshaler(value, v)
 	if ok || err != nil {
 		return err

unmarshaler_test.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/pelletier/go-toml/v2"
+	"github.com/pelletier/go-toml/v2/unstable"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -3772,3 +3773,95 @@ func TestUnmarshal_Nil(t *testing.T) {
 		})
 	}
 }
+
+type CustomUnmarshalerKey struct {
+	A int64
+}
+
+func (k *CustomUnmarshalerKey) UnmarshalTOML(value *unstable.Node) error {
+	item, err := strconv.ParseInt(string(value.Data), 10, 64)
+	if err != nil {
+		return fmt.Errorf("error converting to int64, %v", err)
+	}
+	k.A = item
+	return nil
+
+}
+
+func TestUnmarshal_CustomUnmarshaler(t *testing.T) {
+	type MyConfig struct {
+		Unmarshalers []CustomUnmarshalerKey `toml:"unmarshalers"`
+		Foo          *string                `toml:"foo,omitempty"`
+	}
+
+	examples := []struct {
+		desc                        string
+		disableUnmarshalerInterface bool
+		input                       string
+		expected                    MyConfig
+		err                         bool
+	}{
+		{
+			desc:     "empty",
+			input:    ``,
+			expected: MyConfig{Unmarshalers: []CustomUnmarshalerKey{}, Foo: nil},
+		},
+		{
+			desc:  "simple",
+			input: `unmarshalers = [1,2,3]`,
+			expected: MyConfig{
+				Unmarshalers: []CustomUnmarshalerKey{
+					{A: 1},
+					{A: 2},
+					{A: 3},
+				},
+				Foo: nil,
+			},
+		},
+		{
+			desc: "unmarshal string and custom unmarshaler",
+			input: `unmarshalers = [1,2,3]
+foo = "bar"`,
+			expected: MyConfig{
+				Unmarshalers: []CustomUnmarshalerKey{
+					{A: 1},
+					{A: 2},
+					{A: 3},
+				},
+				Foo: func(v string) *string {
+					return &v
+				}("bar"),
+			},
+		},
+		{
+			desc:                        "simple example, but unmarshaler interface disabled",
+			disableUnmarshalerInterface: true,
+			input:                       `unmarshalers = [1,2,3]`,
+			err:                         true,
+		},
+	}
+
+	for _, ex := range examples {
+		e := ex
+		t.Run(e.desc, func(t *testing.T) {
+			foo := MyConfig{}
+
+			decoder := toml.NewDecoder(bytes.NewReader([]byte(e.input)))
+			if !ex.disableUnmarshalerInterface {
+				decoder.EnableUnmarshalerInterface()
+			}
+			err := decoder.Decode(&foo)
+
+			if e.err {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+				require.Equal(t, len(foo.Unmarshalers), len(e.expected.Unmarshalers))
+				for i := 0; i < len(foo.Unmarshalers); i++ {
+					require.Equal(t, foo.Unmarshalers[i], e.expected.Unmarshalers[i])
+				}
+				require.Equal(t, foo.Foo, e.expected.Foo)
+			}
+		})
+	}
+}

unstable/unmarshaler.go

@@ -0,0 +1,7 @@
+package unstable
+
+// The Unmarshaler interface may be implemented by types to customize their
+// behavior when being unmarshaled from a TOML document.
+type Unmarshaler interface {
+	UnmarshalTOML(value *Node) error
+}