MDEV-40237: Partition info nullptr dereference

[grooverdan]

thd->lex is null in InnoDB purge thd. The Item_field can take a NULL context as a pointer for resolving field names.

[gemini-code-assist]

Code Review

This pull request introduces a null check for thd->lex when initializing the name resolution context in partition_info::vers_fix_field_list. The review feedback correctly identifies that thd->lex->current_select could also be null, which would still result in a null pointer dereference. It is recommended to update the check to ensure both pointers are valid before accessing the context.

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[gemini-code-assist]

The current check only guards against thd->lex being nullptr. However, if thd->lex is non-null but thd->lex->current_select is nullptr, accessing thd->lex->current_select->context will still result in a null pointer dereference (which matches the bug title: member access within null pointer of type 'SELECT_LEX').

To prevent this, we should also check if thd->lex->current_select is non-null before accessing its context member.

  Name_resolution_context *context= (thd->lex && thd->lex->current_select) ? &thd->lex->current_select->context : nullptr;