From 11588a03fa7c6d33f2d4bbdcd3813001a4a92b0b Mon Sep 17 00:00:00 2001
From: Carolyn Van Slyck <me@carolynvanslyck.com>
Date: Wed, 10 Jan 2018 10:35:47 -0600
Subject: [PATCH] Prevent AutomaticEnv from clobbering an explicit mapping

When an explicit environment variable mapping is set for a key, do not
use the automatic mapping. This allows using AutomaticEnv while
specifying a special mapping for a single key.

It's important to both check for the explicit mapping first, and to not
fall back to the automatic mapping when the explicit environment variable
is not set. Otherwise it's very difficult to use AutomaticEnv when you
have a flag that uses a commonly set environment variable, such as
--path. When I map --path to SPECIAL_PATH, it shouldn't fall back to
PATH when SPECIAL_PATH isn't set.
---
 viper.go      | 17 ++++++++---------
 viper_test.go | 14 ++++++++++++++
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/viper.go b/viper.go
index ad8a037..93442b9 100644
--- a/viper.go
+++ b/viper.go
@@ -931,9 +931,14 @@ func (v *Viper) find(lcaseKey string) interface{} {
 	}
 
 	// Env override next
-	if v.automaticEnvApplied {
-		// even if it hasn't been registered, if automaticEnv is used,
-		// check any Get request
+	// Check for an explicit environment key mapping first, before applying AutomaticEnv
+	envkey, exists := v.env[lcaseKey]
+	if exists {
+		if val = v.getEnv(envkey); val != "" {
+			return val
+		}
+	} else if v.automaticEnvApplied {
+		// If it hasn't been registered, check any Get request
 		if val = v.getEnv(v.mergeWithEnvPrefix(lcaseKey)); val != "" {
 			return val
 		}
@@ -941,12 +946,6 @@ func (v *Viper) find(lcaseKey string) interface{} {
 			return nil
 		}
 	}
-	envkey, exists := v.env[lcaseKey]
-	if exists {
-		if val = v.getEnv(envkey); val != "" {
-			return val
-		}
-	}
 	if nested && v.isPathShadowedInFlatMap(path, v.env) != "" {
 		return nil
 	}
diff --git a/viper_test.go b/viper_test.go
index c93480e..44f24b8 100644
--- a/viper_test.go
+++ b/viper_test.go
@@ -409,6 +409,20 @@ func TestAutoEnv(t *testing.T) {
 	assert.Equal(t, "13", Get("foo_bar"))
 }
 
+func TestAutoEnvOverride(t *testing.T) {
+	Reset()
+
+	BindEnv("path", "SPECIAL_PATH")
+	AutomaticEnv()
+	os.Setenv("SPECIAL_PATH", "/home/me/special")
+	os.Setenv("PATH", "/usr/local/bin:/usr/bin")
+
+	assert.Equal(t, "/home/me/special", Get("path"))
+
+	os.Unsetenv("SPECIAL_PATH")
+	assert.Nil(t, Get("path"))
+}
+
 func TestAutoEnvWithPrefix(t *testing.T) {
 	Reset()