3 files changed, 128 insertions, 6 deletions
diff --git a/yt/yt/core/ytree/unittests/yson_struct_update_ut.cpp b/yt/yt/core/ytree/unittests/yson_struct_update_ut.cpp
index 0fb160fdcd..fd82265dd7 100644
--- a/yt/yt/core/ytree/unittests/yson_struct_update_ut.cpp
+++ b/yt/yt/core/ytree/unittests/yson_struct_update_ut.cpp
@@ -176,5 +176,36 @@ TEST(TUpdateYsonStructTest, Nested)
     EXPECT_EQ(updatedCommand, "sort");
 }
 
+TEST(TUpdateYsonStructTest, Validate)
+{
+    auto oldSpec = ConvertTo<TSpecWithPoolPtr>(TYsonString(TString("{pool=pool;}")));
+    auto longPoolSpec = ConvertTo<TSpecWithPoolPtr>(TYsonString(TString("{pool=new_pool;}")));
+    auto shortPoolSpec = ConvertTo<TSpecWithPoolPtr>(TYsonString(TString("{pool=p;}")));
+
+    std::string updatedPool;
+
+    auto configurator = TConfigurator<TSpecWithPool>();
+    configurator.Field("pool", &TSpecWithPool::Pool)
+        .Validator(BIND([&] (const std::string& newPool) {
+            THROW_ERROR_EXCEPTION_IF(
+                newPool.size() > 4,
+                "Pool name too long");
+        }))
+        .Updater(BIND([&] (const std::string& newPool) {
+            updatedPool = newPool;
+        }));
+
+    auto sealed = std::move(configurator).Seal();
+
+    EXPECT_THROW_WITH_SUBSTRING(
+        sealed.Validate(oldSpec, longPoolSpec),
+        "Pool name too long");
+
+    sealed.Validate(oldSpec, shortPoolSpec);
+    sealed.Update(oldSpec, shortPoolSpec);
+
+    EXPECT_EQ(updatedPool, "p");
+}
+
 } // namespace
 } // namespace NYT::NYTree
diff --git a/yt/yt/core/ytree/yson_struct_update-inl.h b/yt/yt/core/ytree/yson_struct_update-inl.h
index 3ba88ec853..c6ae5f1ead 100644
--- a/yt/yt/core/ytree/yson_struct_update-inl.h
+++ b/yt/yt/core/ytree/yson_struct_update-inl.h
@@ -49,9 +49,27 @@ struct TUnwrapYsonStructIntrusivePtr<TIntrusivePtr<T>>
 ////////////////////////////////////////////////////////////////////////////////
 
 template <class TValue>
+TFieldRegistrar<TValue>& TFieldRegistrar<TValue>::Validator(TCallback<void(const TValue&, const TValue&)> validator)
+{
+    VerifyEmptyValidator();
+    Validator_ = validator;
+    return *this;
+}
+
+template <class TValue>
+TFieldRegistrar<TValue>& TFieldRegistrar<TValue>::Validator(TCallback<void(const TValue&)> validator)
+{
+    VerifyEmptyValidator();
+    Validator_ = BIND_NO_PROPAGATE([validator = std::move(validator)] (const TValue& /*oldValue*/, const TValue& newValue) {
+        validator(std::move(newValue));
+    });
+    return *this;
+}
+
+template <class TValue>
 TFieldRegistrar<TValue>& TFieldRegistrar<TValue>::Updater(TCallback<void(const TValue&, const TValue&)> updater)
 {
-    VerifyEmpty();
+    VerifyEmptyUpdater();
     Updater_ = updater;
     return *this;
 }
@@ -59,7 +77,7 @@ TFieldRegistrar<TValue>& TFieldRegistrar<TValue>::Updater(TCallback<void(const T
 template <class TValue>
 TFieldRegistrar<TValue>& TFieldRegistrar<TValue>::Updater(TCallback<void(const TValue&)> updater)
 {
-    VerifyEmpty();
+    VerifyEmptyUpdater();
     Updater_ = BIND_NO_PROPAGATE([updater = std::move(updater)] (const TValue& /*oldValue*/, const TValue& newValue) {
         updater(std::move(newValue));
     });
@@ -90,7 +108,7 @@ TFieldRegistrar<TValue>& TFieldRegistrar<TValue>::NestedUpdater(
             TUnwrappedValue,
             typename NDetail::TUnwrapYsonStructIntrusivePtr<TValue>::TStruct>);
 
-    VerifyEmpty();
+    VerifyEmptyUpdater();
     auto configurator = configureCallback();
     Updater_ = BIND_NO_PROPAGATE([configurator = std::move(configurator)] (const TValue& oldValue, const TValue& newValue) {
         configurator.Update(oldValue, newValue);
@@ -99,6 +117,23 @@ TFieldRegistrar<TValue>& TFieldRegistrar<TValue>::NestedUpdater(
 }
 
 template <class TValue>
+void TFieldRegistrar<TValue>::DoValidate(
+    IYsonStructParameterPtr parameter,
+    TYsonStructBase* oldStruct,
+    TYsonStructBase* newStruct) const
+{
+    if (!Validator_) {
+        return;
+    }
+
+    auto typedParameter = DynamicPointerCast<TYsonStructParameter<TValue>>(parameter);
+    YT_VERIFY(typedParameter);
+    Validator_(
+        typedParameter->GetValue(oldStruct),
+        typedParameter->GetValue(newStruct));
+}
+
+template <class TValue>
 void TFieldRegistrar<TValue>::DoUpdate(
     IYsonStructParameterPtr parameter,
     TYsonStructBase* oldStruct,
@@ -116,7 +151,13 @@ void TFieldRegistrar<TValue>::DoUpdate(
 }
 
 template <class TValue>
-void TFieldRegistrar<TValue>::VerifyEmpty() const
+void TFieldRegistrar<TValue>::VerifyEmptyValidator() const
+{
+    YT_VERIFY(!Validator_);
+}
+
+template <class TValue>
+void TFieldRegistrar<TValue>::VerifyEmptyUpdater() const
 {
     YT_VERIFY(!Updater_);
 }
@@ -169,6 +210,7 @@ TSealedConfigurator<TStruct> TConfigurator<TStruct>::Seal() &&
 {
     return std::move(*this);
 }
+
 ////////////////////////////////////////////////////////////////////////////////
 
 template <CYsonStructDerived TStruct>
@@ -177,10 +219,27 @@ TSealedConfigurator<TStruct>::TSealedConfigurator(TConfigurator<TStruct> configu
 { }
 
 template <CYsonStructDerived TStruct>
+void TSealedConfigurator<TStruct>::Validate(
+    TIntrusivePtr<TStruct> oldStruct,
+    TIntrusivePtr<TStruct> newStruct) const
+{
+    Do(oldStruct, newStruct, &NDetail::IFieldRegistrar::DoValidate);
+}
+
+template <CYsonStructDerived TStruct>
 void TSealedConfigurator<TStruct>::Update(
     TIntrusivePtr<TStruct> oldStruct,
     TIntrusivePtr<TStruct> newStruct) const
 {
+    Do(oldStruct, newStruct, &NDetail::IFieldRegistrar::DoUpdate);
+}
+
+template <CYsonStructDerived TStruct>
+void TSealedConfigurator<TStruct>::Do(
+    TIntrusivePtr<TStruct> oldStruct,
+    TIntrusivePtr<TStruct> newStruct,
+    TFieldRegistrarMethod fieldMethod) const
+{
     const auto* meta = oldStruct->GetMeta();
     YT_VERIFY(meta == newStruct->GetMeta());
     const auto& parameterToFieldRegistrar = RegisteredFields_->ParameterToFieldRegistrar;
@@ -194,7 +253,7 @@ void TSealedConfigurator<TStruct>::Update(
         if (fieldDescIter == parameterToFieldRegistrar.end()) {
             THROW_ERROR_EXCEPTION("Field %Qv is not marked as updatable, but was changed", name);
         } else {
-            fieldDescIter->second->DoUpdate(parameter, oldStruct.Get(), newStruct.Get());
+            (*(fieldDescIter->second).*fieldMethod)(parameter, oldStruct.Get(), newStruct.Get());
         }
     }
 }
diff --git a/yt/yt/core/ytree/yson_struct_update.h b/yt/yt/core/ytree/yson_struct_update.h
index 4b472021d1..7a517d6f57 100644
--- a/yt/yt/core/ytree/yson_struct_update.h
+++ b/yt/yt/core/ytree/yson_struct_update.h
@@ -26,6 +26,11 @@ DECLARE_REFCOUNTED_STRUCT(TRegisteredFieldDirectory);
 struct IFieldRegistrar
     : public TRefCounted
 {
+    virtual void DoValidate(
+        IYsonStructParameterPtr parameter,
+        TYsonStructBase* oldStruct,
+        TYsonStructBase* newStruct) const = 0;
+
     virtual void DoUpdate(
         IYsonStructParameterPtr parameter,
         TYsonStructBase* oldStruct,
@@ -42,6 +47,12 @@ class TFieldRegistrar
     : public IFieldRegistrar
 {
 public:
+    // Registers validator that accepts old and new values as arguments.
+    TFieldRegistrar& Validator(TCallback<void(const TValue&, const TValue&)> validator);
+
+    // Registers validator that accepts only new value as an argument.
+    TFieldRegistrar& Validator(TCallback<void(const TValue&)> validator);
+
     // Registers updater that accepts old and new values as arguments.
     TFieldRegistrar& Updater(TCallback<void(const TValue&, const TValue&)> updater);
 
@@ -58,10 +69,17 @@ public:
         TYsonStructBase* oldStruct,
         TYsonStructBase* newStruct) const override;
 
+    void DoValidate(
+        IYsonStructParameterPtr parameter,
+        TYsonStructBase* oldStruct,
+        TYsonStructBase* newStruct) const override;
+
 private:
-    void VerifyEmpty() const;
+    void VerifyEmptyUpdater() const;
+    void VerifyEmptyValidator() const;
 
     TCallback<void(const TValue&, const TValue&)> Updater_;
+    TCallback<void(const TValue&, const TValue&)> Validator_;
 };
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -114,11 +132,25 @@ class TSealedConfigurator
 public:
     TSealedConfigurator(TConfigurator<TStruct> configurator);
 
+    void Validate(
+        TIntrusivePtr<TStruct> oldStruct,
+        TIntrusivePtr<TStruct> newStruct) const;
+
     void Update(
         TIntrusivePtr<TStruct> oldStruct,
         TIntrusivePtr<TStruct> newStruct) const;
 
 private:
+    using TFieldRegistrarMethod = void(NDetail::IFieldRegistrar::*)(
+        IYsonStructParameterPtr parameter,
+        TYsonStructBase* oldStruct,
+        TYsonStructBase* newStruct) const;
+
+    void Do(
+        TIntrusivePtr<TStruct> oldStruct,
+        TIntrusivePtr<TStruct> newStruct,
+        TFieldRegistrarMethod fieldMethod) const;
+
     NDetail::TRegisteredFieldDirectoryPtr RegisteredFields_;
 };