* varobj.h (enum varobj_update_error): New enum.
* varobj.c (struct varobj_root): Add is_valid member.
(varobj_get_type): Check for invalid varobj.
(varobj_get_attributes): Likewise.
(variable_editable):Likewise.
(varobj_update): Likewise. Use varobj_update_error.
(new_root_variable): Set root varobj as valid by default.
(varobj_invalidate): New function.
* symfile.c (clear_symtab_users): Use varobj_invalidate.
* mi/mi-cmd-var.c (varobj_update_one): Change return type to void.
Use varobj_update_error.
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