feat: allow setting a default decode hook

Signed-off-by: Mark Sagi-Kazar <mark.sagikazar@gmail.com>

viper.go

@@ -183,6 +183,8 @@ type Viper struct {
 	encoderRegistry EncoderRegistry
 	decoderRegistry DecoderRegistry
 
+	decodeHook mapstructure.DecodeHookFunc
+
 	experimentalFinder     bool
 	experimentalBindStruct bool
 }
@@ -251,6 +253,17 @@ func EnvKeyReplacer(r StringReplacer) Option {
 	})
 }
 
+// WithDecodeHook sets a default decode hook for mapstructure.
+func WithDecodeHook(h mapstructure.DecodeHookFunc) Option {
+	return optionFunc(func(v *Viper) {
+		if h == nil {
+			return
+		}
+
+		v.decodeHook = h
+	})
+}
+
 // NewWithOptions creates a new Viper instance.
 func NewWithOptions(opts ...Option) *Viper {
 	v := New()
@@ -896,7 +909,7 @@ func UnmarshalKey(key string, rawVal any, opts ...DecoderConfigOption) error {
 }
 
 func (v *Viper) UnmarshalKey(key string, rawVal any, opts ...DecoderConfigOption) error {
-	return decode(v.Get(key), defaultDecoderConfig(rawVal, opts...))
+	return decode(v.Get(key), v.defaultDecoderConfig(rawVal, opts...))
 }
 
 // Unmarshal unmarshals the config into a Struct. Make sure that the tags
@@ -919,13 +932,13 @@ func (v *Viper) Unmarshal(rawVal any, opts ...DecoderConfigOption) error {
 	}
 
 	// TODO: struct keys should be enough?
-	return decode(v.getSettings(keys), defaultDecoderConfig(rawVal, opts...))
+	return decode(v.getSettings(keys), v.defaultDecoderConfig(rawVal, opts...))
 }
 
 func (v *Viper) decodeStructKeys(input any, opts ...DecoderConfigOption) ([]string, error) {
 	var structKeyMap map[string]any
 
-	err := decode(input, defaultDecoderConfig(&structKeyMap, opts...))
+	err := decode(input, v.defaultDecoderConfig(&structKeyMap, opts...))
 	if err != nil {
 		return nil, err
 	}
@@ -942,15 +955,20 @@ func (v *Viper) decodeStructKeys(input any, opts ...DecoderConfigOption) ([]stri
 
 // defaultDecoderConfig returns default mapstructure.DecoderConfig with support
 // of time.Duration values & string slices.
-func defaultDecoderConfig(output any, opts ...DecoderConfigOption) *mapstructure.DecoderConfig {
-	c := &mapstructure.DecoderConfig{
-		Metadata:         nil,
-		WeaklyTypedInput: true,
-		DecodeHook: mapstructure.ComposeDecodeHookFunc(
+func (v *Viper) defaultDecoderConfig(output any, opts ...DecoderConfigOption) *mapstructure.DecoderConfig {
+	decodeHook := v.decodeHook
+	if decodeHook == nil {
+		decodeHook = mapstructure.ComposeDecodeHookFunc(
 			mapstructure.StringToTimeDurationHookFunc(),
 			// mapstructure.StringToSliceHookFunc(","),
 			stringToWeakSliceHookFunc(","),
-		),
+		)
+	}
+
+	c := &mapstructure.DecoderConfig{
+		Metadata:         nil,
+		WeaklyTypedInput: true,
+		DecodeHook:       decodeHook,
 	}
 
 	for _, opt := range opts {
@@ -1001,7 +1019,7 @@ func UnmarshalExact(rawVal any, opts ...DecoderConfigOption) error {
 }
 
 func (v *Viper) UnmarshalExact(rawVal any, opts ...DecoderConfigOption) error {
-	config := defaultDecoderConfig(rawVal, opts...)
+	config := v.defaultDecoderConfig(rawVal, opts...)
 	config.ErrorUnused = true
 
 	keys := v.AllKeys()

viper_test.go

@@ -894,6 +894,41 @@ func TestUnmarshal(t *testing.T) {
 	)
 }
 
+func TestUnmarshalWithDefaultDecodeHook(t *testing.T) {
+	opt := mapstructure.ComposeDecodeHookFunc(
+		mapstructure.StringToTimeDurationHookFunc(),
+		mapstructure.StringToSliceHookFunc(","),
+		// Custom Decode Hook Function
+		func(rf reflect.Kind, rt reflect.Kind, data any) (any, error) {
+			if rf != reflect.String || rt != reflect.Map {
+				return data, nil
+			}
+			m := map[string]string{}
+			raw := data.(string)
+			if raw == "" {
+				return m, nil
+			}
+			err := json.Unmarshal([]byte(raw), &m)
+			return m, err
+		},
+	)
+
+	v := NewWithOptions(WithDecodeHook(opt))
+	v.Set("credentials", "{\"foo\":\"bar\"}")
+
+	type config struct {
+		Credentials map[string]string
+	}
+
+	var C config
+
+	require.NoError(t, v.Unmarshal(&C), "unable to decode into struct")
+
+	assert.Equal(t, &config{
+		Credentials: map[string]string{"foo": "bar"},
+	}, &C)
+}
+
 func TestUnmarshalWithDecoderOptions(t *testing.T) {
 	v := New()
 	v.Set("credentials", "{\"foo\":\"bar\"}")