fix: merge environment values by ovewriting with empty values (#1162)

Fixes #1154

pkg/environment/environment.go

@@ -58,7 +58,7 @@ func (e *Environment) Merge(other *Environment) (*Environment, error) {
 	}
 	copy := e.DeepCopy()
 	if other != nil {
-		if err := mergo.Merge(&copy, other, mergo.WithOverride); err != nil {
+		if err := mergo.Merge(&copy, other, mergo.WithOverride, mergo.WithOverwriteWithEmptyValue); err != nil {
 			return nil, err
 		}
 	}

pkg/environment/environment_test.go

@@ -0,0 +1,98 @@
+package environment
+
+import (
+	"github.com/google/go-cmp/cmp"
+	"testing"
+)
+
+// See https://github.com/roboll/helmfile/issues/1150
+func TestMerge_OverwriteNilValue_Issue1150(t *testing.T) {
+	dst := &Environment{
+		Name: "dst",
+		Values: map[string]interface{}{
+			"components": map[string]interface{}{
+				"etcd-operator": nil,
+			},
+		},
+		Defaults: nil,
+	}
+
+	src := &Environment{
+		Name: "src",
+		Values: map[string]interface{}{
+			"components": map[string]interface{}{
+				"etcd-operator": map[string]interface{}{
+					"version": "0.10.3",
+				},
+			},
+		},
+		Defaults: nil,
+	}
+
+	merged, err := dst.Merge(src)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	actual := merged.Values
+
+	expected := map[string]interface{}{
+		"components": map[string]interface{}{
+			"etcd-operator": map[string]interface{}{
+				"version": "0.10.3",
+			},
+		},
+	}
+
+	if diff := cmp.Diff(expected, actual); diff != "" {
+		t.Errorf(diff)
+	}
+}
+
+// See https://github.com/roboll/helmfile/issues/1154
+func TestMerge_OverwriteWithNilValue_Issue1154(t *testing.T) {
+	dst := &Environment{
+		Name: "dst",
+		Values: map[string]interface{}{
+			"components": map[string]interface{}{
+				"etcd-operator": map[string]interface{}{
+					"version": "0.10.0",
+				},
+			},
+		},
+		Defaults: nil,
+	}
+
+	src := &Environment{
+		Name: "src",
+		Values: map[string]interface{}{
+			"components": map[string]interface{}{
+				"etcd-operator": map[string]interface{}{
+					"version": "0.10.3",
+				},
+				"prometheus": nil,
+			},
+		},
+		Defaults: nil,
+	}
+
+	merged, err := dst.Merge(src)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	actual := merged.Values
+
+	expected := map[string]interface{}{
+		"components": map[string]interface{}{
+			"etcd-operator": map[string]interface{}{
+				"version": "0.10.3",
+			},
+			"prometheus": nil,
+		},
+	}
+
+	if diff := cmp.Diff(expected, actual); diff != "" {
+		t.Errorf(diff)
+	}
+}