From 4f59047658836fc8e6113fe910fbf4c5a0e962be Mon Sep 17 00:00:00 2001
From: Mark Sagi-Kazar <mark.sagikazar@gmail.com>
Date: Tue, 6 Nov 2018 20:20:07 +0100
Subject: [PATCH] Add AllowEmptyEnv method to preserve backard compatibility

---
 README.md     |  7 ++++++-
 viper.go      | 14 +++++++++++++-
 viper_test.go | 16 ++++++++++++++++
 3 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 64bf474..8da6a93 100644
--- a/README.md
+++ b/README.md
@@ -179,13 +179,14 @@ viper.GetBool("verbose") // true
 ### Working with Environment Variables
 
 Viper has full support for environment variables. This enables 12 factor
-applications out of the box. There are four methods that exist to aid working
+applications out of the box. There are five methods that exist to aid working
 with ENV:
 
  * `AutomaticEnv()`
  * `BindEnv(string...) : error`
  * `SetEnvPrefix(string)`
  * `SetEnvKeyReplacer(string...) *strings.Replacer`
+  * `AllowEmptyEnvVar(bool)`
 
 _When working with ENV variables, it’s important to recognize that Viper
 treats ENV variables as case sensitive._
@@ -217,6 +218,10 @@ keys to an extent. This is useful if you want to use `-` or something in your
 `Get()` calls, but want your environmental variables to use `_` delimiters. An
 example of using it can be found in `viper_test.go`.
 
+By default empty environment variables are considered unset and will fall back to
+the next configuration source. To treat empty environment variables as set, use
+the `AllowEmptyEnv` method.
+
 #### Env example
 
 ```go
diff --git a/viper.go b/viper.go
index bd9b8a0..85742ec 100644
--- a/viper.go
+++ b/viper.go
@@ -169,6 +169,7 @@ type Viper struct {
 
 	automaticEnvApplied bool
 	envKeyReplacer      *strings.Replacer
+	allowEmptyEnv       bool
 
 	config         map[string]interface{}
 	override       map[string]interface{}
@@ -330,6 +331,14 @@ func (v *Viper) mergeWithEnvPrefix(in string) string {
 	return strings.ToUpper(in)
 }
 
+// AllowEmptyEnv tells Viper to consider set,
+// but empty environment variables as valid values instead of falling back.
+// For backward compatibility reasons this is false by default.
+func AllowEmptyEnv(allowEmptyEnv bool) { v.AllowEmptyEnv(allowEmptyEnv) }
+func (v *Viper) AllowEmptyEnv(allowEmptyEnv bool) {
+	v.allowEmptyEnv = allowEmptyEnv
+}
+
 // TODO: should getEnv logic be moved into find(). Can generalize the use of
 // rewriting keys many things, Ex: Get('someKey') -> some_key
 // (camel case to snake case for JSON keys perhaps)
@@ -341,7 +350,10 @@ func (v *Viper) getEnv(key string) (string, bool) {
 	if v.envKeyReplacer != nil {
 		key = v.envKeyReplacer.Replace(key)
 	}
-	return os.LookupEnv(key)
+
+	val, ok := os.LookupEnv(key)
+
+	return val, ok && (v.allowEmptyEnv || val != "")
 }
 
 // ConfigFileUsed returns the file used to populate the config registry.
diff --git a/viper_test.go b/viper_test.go
index 73ff024..9dc703b 100644
--- a/viper_test.go
+++ b/viper_test.go
@@ -391,6 +391,22 @@ func TestEmptyEnv(t *testing.T) {
 
 	os.Setenv("TYPE", "")
 
+	assert.Equal(t, "donut", Get("type"))
+	assert.Equal(t, "Cake", Get("name"))
+}
+
+func TestEmptyEnv_Allowed(t *testing.T) {
+	initJSON()
+
+	AllowEmptyEnv(true)
+
+	BindEnv("type") // Empty environment variable
+	BindEnv("name") // Bound, but not set environment variable
+
+	os.Clearenv()
+
+	os.Setenv("TYPE", "")
+
 	assert.Equal(t, "", Get("type"))
 	assert.Equal(t, "Cake", Get("name"))
 }