From e5d7915cac87bf08aa571bd3e99d75ef797a3990 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 50fd66e..87a4440 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
 
@@ -993,21 +993,18 @@ func (v *Viper) BindFlagValue(key string, flag FlagValue) error {
 // EnvPrefix will be used when set when env name is not provided.
 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
 }
 
@@ -1086,10 +1083,12 @@ 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) != "" {
@@ -1658,6 +1657,14 @@ 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 {
@@ -1828,7 +1835,7 @@ 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 fe942de..b684140 100644
--- a/viper_test.go
+++ b/viper_test.go
@@ -487,10 +487,11 @@ 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"))
@@ -502,6 +503,17 @@ 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()