Allow BindEnv to register multiple environment variables.

This change modifies BindEnv to permit a list of environment variable
names in order to support multiple env. vars. for the same config key.
When this form is used, env. keys take precedence in the written order.

Closes #971

viper.go

@@ -205,7 +205,7 @@ type Viper struct {
 	defaults       map[string]interface{}
 	kvstore        map[string]interface{}
 	pflags         map[string]FlagValue
-	env            map[string]string
+	env            map[string][]string
 	aliases        map[string]string
 	typeByDefValue bool
 
@@ -228,7 +228,7 @@ func New() *Viper {
 	v.defaults = make(map[string]interface{})
 	v.kvstore = make(map[string]interface{})
 	v.pflags = make(map[string]FlagValue)
-	v.env = make(map[string]string)
+	v.env = make(map[string][]string)
 	v.aliases = make(map[string]string)
 	v.typeByDefValue = false
 
@@ -1029,21 +1029,18 @@ func (v *Viper) BindFlagValue(key string, flag FlagValue) error {
 func BindEnv(input ...string) error { return v.BindEnv(input...) }
 
 func (v *Viper) BindEnv(input ...string) error {
-	var key, envkey string
 	if len(input) == 0 {
 		return fmt.Errorf("missing key to bind to")
 	}
 
-	key = strings.ToLower(input[0])
+	key := strings.ToLower(input[0])
 
 	if len(input) == 1 {
-		envkey = v.mergeWithEnvPrefix(key)
+		v.env[key] = append(v.env[key], v.mergeWithEnvPrefix(key))
 	} else {
-		envkey = input[1]
+		v.env[key] = append(v.env[key], input[1:]...)
 	}
 
-	v.env[key] = envkey
-
 	return nil
 }
 
@@ -1122,12 +1119,14 @@ func (v *Viper) find(lcaseKey string, flagDefault bool) interface{} {
 			return nil
 		}
 	}
-	envkey, exists := v.env[lcaseKey]
+	envkeys, exists := v.env[lcaseKey]
 	if exists {
+		for _, envkey := range envkeys {
 			if val, ok := v.getEnv(envkey); ok {
 				return val
 			}
 		}
+	}
 	if nested && v.isPathShadowedInFlatMap(path, v.env) != "" {
 		return nil
 	}
@@ -1711,6 +1710,14 @@ func castToMapStringInterface(
 	return tgt
 }
 
+func castMapStringSliceToMapInterface(src map[string][]string) map[string]interface{} {
+	tgt := map[string]interface{}{}
+	for k, v := range src {
+		tgt[k] = v
+	}
+	return tgt
+}
+
 func castMapStringToMapInterface(src map[string]string) map[string]interface{} {
 	tgt := map[string]interface{}{}
 	for k, v := range src {
@@ -1883,7 +1890,7 @@ func (v *Viper) AllKeys() []string {
 	m = v.flattenAndMergeMap(m, castMapStringToMapInterface(v.aliases), "")
 	m = v.flattenAndMergeMap(m, v.override, "")
 	m = v.mergeFlatMap(m, castMapFlagToMapInterface(v.pflags))
-	m = v.mergeFlatMap(m, castMapStringToMapInterface(v.env))
+	m = v.mergeFlatMap(m, castMapStringSliceToMapInterface(v.env))
 	m = v.flattenAndMergeMap(m, v.config, "")
 	m = v.flattenAndMergeMap(m, v.kvstore, "")
 	m = v.flattenAndMergeMap(m, v.defaults, "")

viper_test.go

@@ -486,10 +486,11 @@ func TestEnv(t *testing.T) {
 	initJSON()
 
 	BindEnv("id")
-	BindEnv("f", "FOOD")
+	BindEnv("f", "FOOD", "OLD_FOOD")
 
 	os.Setenv("ID", "13")
 	os.Setenv("FOOD", "apple")
+	os.Setenv("OLD_FOOD", "banana")
 	os.Setenv("NAME", "crunk")
 
 	assert.Equal(t, "13", Get("id"))
@@ -501,6 +502,17 @@ func TestEnv(t *testing.T) {
 	assert.Equal(t, "crunk", Get("name"))
 }
 
+func TestMultipleEnv(t *testing.T) {
+	initJSON()
+
+	BindEnv("f", "FOOD", "OLD_FOOD")
+
+	os.Unsetenv("FOOD")
+	os.Setenv("OLD_FOOD", "banana")
+
+	assert.Equal(t, "banana", Get("f"))
+}
+
 func TestEmptyEnv(t *testing.T) {
 	initJSON()