MDEV-37262 XMLISVALID() schema validation function.

[holyfoot]

XMLVALID function added to the XMLTYPE plugin.

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[gemini-code-assist]

Code Review

This pull request introduces the new native function XMLISVALID to the type_xmltype plugin, adding comprehensive XML schema validation tests and registering the function descriptor. However, two critical issues must be addressed: the newly referenced source file item_func_xml_isvalid.cc is missing from the pull request, which will cause immediate build failures, and the raw pointer member m_data in Item_func_xml_isvalid is left uninitialized, risking undefined behavior or segmentation faults.

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[gemini-code-assist]

The file item_func_xml_isvalid.cc has been added to the MYSQL_ADD_PLUGIN source list, but the file itself is missing from this pull request. This will cause the build to fail immediately. Please ensure that item_func_xml_isvalid.cc is committed and included in the pull request.

[gemini-code-assist]

The member pointer m_data is not initialized in the constructors of Item_func_xml_isvalid. Since it is a raw pointer, it will contain garbage values upon object creation. If any method (such as val_bool(), cleanup(), or a destructor) checks if (m_data) before allocation or deallocation, it can lead to undefined behavior or segmentation faults. Please initialize m_data to nullptr using in-class member initialization.

Suggested change

	MY_XML_VALIDATION_DATA *m_data;
	MY_XML_VALIDATION_DATA *m_data= nullptr;