Completed fix of issue kelseyhightower#148

with respecting of env-prefix and partially saved backward compatibility by using new tag no_pfx

envconfig.go

@@ -121,14 +121,28 @@ func gatherInfo(prefix string, spec interface{}) ([]varInfo, error) {
 		if prefix != "" {
 			info.Key = fmt.Sprintf("%s_%s", prefix, info.Key)
 		}
-		// If name is overridden, switch .Key and .Alt and don't forget about prefix.
-		// Now the overridden name is for the first attempt to find value in environment.
-		// The initial name is an alternative name used if the first attempt is not successful.
+
+		// override default env-variable name with an envconfig tag
 		if info.Alt != "" {
 			if prefix != "" {
-				info.Alt = fmt.Sprintf("%s_%s", prefix, info.Alt)
+				noPfxPlace := ftype.Tag.Get("no_pfx")
+				if noPfxPlace == "key" {
+					// now the key name is overridden and prefixed
+					// alt name is not used
+					info.Key = info.Alt
+					info.Alt = ""
+				} else if noPfxPlace == "alt" {
+					// now the key name is overridden and prefixed
+					// and alt name is overridden name without prefix
+					info.Key = fmt.Sprintf("%s_%s", prefix, info.Alt)
+				} else {
+					// now key name is overridden and prefixed
+					// but alt is the prev key name (source struct field name) with prefix
+					prefixedStructFieldName := info.Key
+					info.Key = fmt.Sprintf("%s_%s", prefix, info.Alt)
+					info.Alt = prefixedStructFieldName
+				}
 			}
-			info.Key, info.Alt = info.Alt, info.Key
 		}
 		info.Key = strings.ToUpper(info.Key)
 		info.Alt = strings.ToUpper(info.Alt)
@@ -211,9 +225,6 @@ func Process(prefix string, spec interface{}) error {
 		if !ok && def == "" {
 			if isTrue(req) {
 				key := info.Key
-				if info.Alt != "" {
-					key = info.Alt
-				}
 				return fmt.Errorf("required key %s missing value", key)
 			}
 			continue

envconfig_test.go

@@ -54,14 +54,14 @@ type Specification struct {
 	SomePointerWithDefault       *string `default:"foo2baz" desc:"foorbar is the word"`
 	MultiWordVarWithAlt          string  `envconfig:"MULTI_WORD_VAR_WITH_ALT" desc:"what alt"`
 	MultiWordVarWithLowerCaseAlt string  `envconfig:"multi_word_var_with_lower_case_alt"`
-	NoPrefixWithAlt              string  `envconfig:"SERVICE_HOST"`
+	NoPrefixWithAlt              string  `envconfig:"SERVICE_HOST" no_pfx:"alt"`
 	DefaultVar                   string  `default:"foobar"`
 	RequiredVar                  string  `required:"True"`
-	NoPrefixDefault              string  `envconfig:"BROKER" default:"127.0.0.1"`
+	NoPrefixDefault              string  `envconfig:"BROKER" default:"127.0.0.1" no_pfx:"alt"`
 	RequiredDefault              string  `required:"true" default:"foo2bar"`
 	Ignored                      string  `ignored:"true"`
 	NestedSpecification          struct {
-		Property            string `envconfig:"inner"`
+		Property            string `envconfig:"inner" no_pfx:"alt"`
 		PropertyWithDefault string `default:"fuzzybydefault"`
 	} `envconfig:"outer"`
 	AfterNested  string
@@ -804,6 +804,23 @@ func TestErrorMessageForRequiredAltVar(t *testing.T) {
 		t.Error("no failure when missing required variable")
 	}
 
+	if !strings.Contains(err.Error(), " ENV_CONFIG_BAR ") {
+		t.Errorf("expected error message to contain BAR, got \"%v\"", err)
+	}
+}
+
+func TestErrorMessageForRequiredOverriddenKeyVar(t *testing.T) {
+	var s struct {
+		Foo string `envconfig:"BAR" required:"true" no_pfx:"key"`
+	}
+
+	os.Clearenv()
+	err := Process("env_config", &s)
+
+	if err == nil {
+		t.Error("no failure when missing required variable")
+	}
+
 	if !strings.Contains(err.Error(), " BAR ") {
 		t.Errorf("expected error message to contain BAR, got \"%v\"", err)
 	}