From b655224c01bdc85dd8d755f21dbe32699258a626 Mon Sep 17 00:00:00 2001
From: Gabriel Aszalos <gabriel.aszalos@gmail.com>
Date: Thu, 10 Sep 2020 13:08:26 +0300
Subject: [PATCH] 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      | 31 +++++++++++++++++++------------
 viper_test.go | 14 +++++++++++++-
 2 files changed, 32 insertions(+), 13 deletions(-)

diff --git a/viper.go b/viper.go
index 8889f87..82a88d7 100644
--- a/viper.go
+++ b/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
 }
 
@@ -1122,10 +1119,12 @@ func (v *Viper) find(lcaseKey string, flagDefault bool) interface{} {
 			return nil
 		}
 	}
-	envkey, exists := v.env[lcaseKey]
+	envkeys, exists := v.env[lcaseKey]
 	if exists {
-		if val, ok := v.getEnv(envkey); ok {
-			return val
+		for _, envkey := range envkeys {
+			if val, ok := v.getEnv(envkey); ok {
+				return val
+			}
 		}
 	}
 	if nested && v.isPathShadowedInFlatMap(path, v.env) != "" {
@@ -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, "")
diff --git a/viper_test.go b/viper_test.go
index c38c665..28c7f13 100644
--- a/viper_test.go
+++ b/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()