From 092b4888d855727b724e7aacb02ef54b14a4fd4e Mon Sep 17 00:00:00 2001
From: Yonas Habteab <yonas.habteab@icinga.com>
Date: Fri, 29 Mar 2024 17:57:27 +0100
Subject: [PATCH] Preserve defaults for zero initialised structs as well

---
 decode.go      | 12 ++++++++----
 decode_test.go | 25 +++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/decode.go b/decode.go
index 72af5e22..04b94bca 100644
--- a/decode.go
+++ b/decode.go
@@ -989,18 +989,22 @@ func (d *Decoder) createDecodedNewValue(
 			return newValue, nil
 		}
 	}
+	var newValue reflect.Value
 	if node.Type() == ast.NullType {
-		return reflect.Zero(typ), nil
+		newValue = reflect.New(typ).Elem()
+	} else {
+		newValue = d.createDecodableValue(typ)
 	}
-	newValue := d.createDecodableValue(typ)
 	for defaultVal.Kind() == reflect.Ptr {
 		defaultVal = defaultVal.Elem()
 	}
 	if defaultVal.IsValid() && defaultVal.Type().AssignableTo(newValue.Type()) {
 		newValue.Set(defaultVal)
 	}
-	if err := d.decodeValue(ctx, newValue, node); err != nil {
-		return newValue, errors.Wrapf(err, "failed to decode value")
+	if node.Type() != ast.NullType {
+		if err := d.decodeValue(ctx, newValue, node); err != nil {
+			return newValue, errors.Wrapf(err, "failed to decode value")
+		}
 	}
 	return newValue, nil
 }
diff --git a/decode_test.go b/decode_test.go
index f0b0c080..0d47a29c 100644
--- a/decode_test.go
+++ b/decode_test.go
@@ -3027,3 +3027,28 @@ func TestMapKeyCustomUnmarshaler(t *testing.T) {
 		t.Fatalf("expected to have value \"value\", but got %q", val)
 	}
 }
+
+func TestDecoderPreservesDefaultValues(t *testing.T) {
+	type nested struct {
+		Val string `yaml:"val"`
+	}
+
+	type test struct {
+		First   string `yaml:"first"`
+		Default nested `yaml:"nested"`
+	}
+
+	yml := `
+first: "Test"
+nested:
+  # Just some comment here
+#  val: "default"
+`
+	v := test{Default: nested{Val: "default"}}
+	if err := yaml.Unmarshal([]byte(yml), &v); err != nil {
+		t.Fatal(err)
+	}
+	if v.Default.Val != "default" {
+		t.Fatal("decoder doesn't preserve struct defaults")
+	}
+}