From 9be8744bb96ab00cbf25afb60ab54db5cbad95dc Mon Sep 17 00:00:00 2001
From: renadeen <renadeen@yandex-team.com>
Date: Thu, 28 Aug 2025 16:41:01 +0300
Subject: YT-26037: Fix empty polymorphic yson struct
 commit_hash:918f0da0ab743372aebb95a69ed93cc6dad32df2

---
 yt/yt/core/ytree/polymorphic_yson_struct-inl.h | 15 ++++++----
 yt/yt/core/ytree/polymorphic_yson_struct.h     |  2 +-
 yt/yt/core/ytree/unittests/yson_struct_ut.cpp  | 39 ++++++++++++++++++++++++--
 3 files changed, 47 insertions(+), 9 deletions(-)

diff --git a/yt/yt/core/ytree/polymorphic_yson_struct-inl.h b/yt/yt/core/ytree/polymorphic_yson_struct-inl.h
index ed0352c2438..382742609bf 100644
--- a/yt/yt/core/ytree/polymorphic_yson_struct-inl.h
+++ b/yt/yt/core/ytree/polymorphic_yson_struct-inl.h
@@ -45,6 +45,16 @@ CreateInstance(TEnum value)
 
 ////////////////////////////////////////////////////////////////////////////////
 
+template <CPolymorphicEnumMapping TMapping>
+TPolymorphicYsonStruct<TMapping>::TPolymorphicYsonStruct()
+{
+    if constexpr (DefaultType_) {
+        HeldType_ = *DefaultType_;
+    } else {
+        HeldType_ = {};
+    }
+}
+
 template <CPolymorphicEnumMapping TMapping>
 TPolymorphicYsonStruct<TMapping>::TPolymorphicYsonStruct(TKey key)
     : TPolymorphicYsonStruct(key, TMapping::CreateInstance(key))
@@ -77,11 +87,6 @@ void TPolymorphicYsonStruct<TMapping>::Load(
     SerializedStorage_ = TTraits::AsNode(source);
     IMapNodePtr map = SerializedStorage_->AsMap();
 
-    if (!map || map->GetChildCount() == 0) {
-        // Empty struct.
-        return;
-    }
-
     auto key = map->FindChildValue<TKey>("type");
     THROW_ERROR_EXCEPTION_UNLESS(
         key.has_value() || DefaultType_,
diff --git a/yt/yt/core/ytree/polymorphic_yson_struct.h b/yt/yt/core/ytree/polymorphic_yson_struct.h
index 50c4b206380..effc27ae034 100644
--- a/yt/yt/core/ytree/polymorphic_yson_struct.h
+++ b/yt/yt/core/ytree/polymorphic_yson_struct.h
@@ -160,7 +160,7 @@ class TPolymorphicYsonStruct
 public:
     using TImplementsYsonStructField = void;
 
-    TPolymorphicYsonStruct() = default;
+    TPolymorphicYsonStruct();
 
     explicit TPolymorphicYsonStruct(TKey key);
     TPolymorphicYsonStruct(TKey key, TIntrusivePtr<TBase> ptr) noexcept;
diff --git a/yt/yt/core/ytree/unittests/yson_struct_ut.cpp b/yt/yt/core/ytree/unittests/yson_struct_ut.cpp
index 31217b26b2f..792c5636c76 100644
--- a/yt/yt/core/ytree/unittests/yson_struct_ut.cpp
+++ b/yt/yt/core/ytree/unittests/yson_struct_ut.cpp
@@ -3478,6 +3478,18 @@ TEST(TYsonStructTest, TestPolymorphicYsonStructDefault)
 {
     TMyPolyDefault poly;
 
+    auto empty = BuildYsonNodeFluently()
+        .BeginMap()
+        .EndMap();
+
+    Deserialize(poly, empty->AsMap());
+    EXPECT_EQ(poly.GetCurrentType(), EMyPolyDefaultType::Drv1);
+
+    auto drv1Ptr = poly.TryGetConcrete<TPolyDerived1>();
+    EXPECT_TRUE(drv1Ptr.operator bool());
+
+    ///////////////
+
     auto node = BuildYsonNodeFluently()
         .BeginMap()
             .Item("base_field").Value(11)
@@ -3487,11 +3499,13 @@ TEST(TYsonStructTest, TestPolymorphicYsonStructDefault)
     Deserialize(poly, node->AsMap());
     EXPECT_EQ(poly.GetCurrentType(), EMyPolyDefaultType::Drv1);
 
-    auto drv1Ptr = poly.TryGetConcrete<TPolyDerived1>();
+    drv1Ptr = poly.TryGetConcrete<TPolyDerived1>();
     EXPECT_TRUE(drv1Ptr.operator bool());
     EXPECT_EQ(drv1Ptr->BaseField, 11);
     EXPECT_EQ(drv1Ptr->Field1, 123);
 
+    ///////////////
+
     node = BuildYsonNodeFluently()
         .BeginMap()
             .Item("type").Value("drv2")
@@ -3502,6 +3516,8 @@ TEST(TYsonStructTest, TestPolymorphicYsonStructDefault)
     Deserialize(poly, node->AsMap());
     EXPECT_EQ(poly.GetCurrentType(), EMyPolyDefaultType::Drv2);
 
+    ///////////////
+
     node = BuildYsonNodeFluently()
     .BeginMap()
         .Item("base_field").Value(11)
@@ -3528,6 +3544,18 @@ TEST(TYsonStructTest, TestPolymorphicYsonStructDefaultEnum)
 {
     TMyPolyDefaultEnum poly;
 
+    auto empty = BuildYsonNodeFluently()
+        .BeginMap()
+        .EndMap();
+
+    Deserialize(poly, empty->AsMap());
+
+    EXPECT_EQ(poly.GetCurrentType(), EMyPolyDefaultEnum::Base);
+    auto basePtr = poly.TryGetConcrete<TPolyBase>();
+    EXPECT_TRUE(basePtr.operator bool());
+
+    ///////////////
+
     auto node = BuildYsonNodeFluently()
         .BeginMap()
             .Item("base_field").Value(11)
@@ -3537,10 +3565,12 @@ TEST(TYsonStructTest, TestPolymorphicYsonStructDefaultEnum)
     Deserialize(poly, node->AsMap());
     EXPECT_EQ(poly.GetCurrentType(), EMyPolyDefaultEnum::Base);
 
-    auto basePtr = poly.TryGetConcrete<TPolyBase>();
+    basePtr = poly.TryGetConcrete<TPolyBase>();
     EXPECT_TRUE(basePtr.operator bool());
     EXPECT_EQ(basePtr->BaseField, 11);
 
+    ///////////////
+
     node = BuildYsonNodeFluently()
         .BeginMap()
             .Item("type").Value("drv1")
@@ -3551,6 +3581,8 @@ TEST(TYsonStructTest, TestPolymorphicYsonStructDefaultEnum)
     Deserialize(poly, node->AsMap());
     EXPECT_EQ(poly.GetCurrentType(), EMyPolyDefaultEnum::Drv1);
 
+    ///////////////
+
     node = BuildYsonNodeFluently()
     .BeginMap()
         .Item("base_field").Value(11)
@@ -3585,7 +3617,8 @@ TEST(TYsonStructTest, TestPolymorphicYsonStructSerializeEmpty)
 
     auto node = ConvertToNode(holder);
 
-    Deserialize(holder, node->AsMap());
+    // Field `type` is mandatory for structs without configured default type.
+    EXPECT_THROW(Deserialize(holder, node->AsMap()), std::exception);
 }
 
 TEST(TYsonStructTest, TestPolymorphicYsonStructAsField)
-- 
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