YT-25312: Fix TWithExtraSpace::GetExtraSpace to handle zero returned by malloc_usable_size

commit_hash:d4310bf35afeb00d408d28e96cf82ebafd82886b

Author: babenko

library/cpp/yt/memory/new-inl.h

@@ -340,12 +340,17 @@ void* TWithExtraSpace<T>::GetExtraSpacePtr()
 }
 
 template <class T>
-size_t TWithExtraSpace<T>::GetUsableSpaceSize() const
+std::optional<size_t> TWithExtraSpace<T>::GetUsableSpaceSize() const
 {
 #ifdef _win_
     return 0;
 #else
-    return malloc_usable_size(const_cast<T*>(static_cast<const T*>(this))) - sizeof(T);
+    size_t usableSize = malloc_usable_size(const_cast<T*>(static_cast<const T*>(this)));
+    if (usableSize == 0) {
+        return std::nullopt;
+    }
+    YT_ASSERT(usableSize >= sizeof(T));
+    return usableSize - sizeof(T);
 #endif
 }
 

library/cpp/yt/memory/new.h

@@ -130,9 +130,14 @@ template <class T>
 class TWithExtraSpace
 {
 protected:
+    //! Returns the pointer to the extra space associated with this instance.
     const void* GetExtraSpacePtr() const;
     void* GetExtraSpacePtr();
-    size_t GetUsableSpaceSize() const;
+
+    //! Returns the size of the extra space associated with this instance.
+    //! This is determined via the call to |malloc_usable_size| and may be
+    //! null if the allocator support is unavailable.
+    std::optional<size_t> GetUsableSpaceSize() const;
 };
 
 ////////////////////////////////////////////////////////////////////////////////

library/cpp/yt/memory/ref.cpp

@@ -150,8 +150,8 @@ class TDefaultAllocationHolder
         TRefCountedTypeCookie cookie)
     {
         if (options.ExtendToUsableSize) {
-            if (auto usableSize = GetUsableSpaceSize(); usableSize != 0) {
-                size = usableSize;
+            if (auto usableSize = GetUsableSpaceSize()) {
+                size = *usableSize;
             }
         }
         Initialize(size, options, cookie);