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Re: d1ec0231553: MDEV-26732 Assertion `0' failed in Item::val_native

 

  Hello Sergei,

On 10/25/21 11:03 PM, Sergei Golubchik wrote:
Hi, Alexander!

On Oct 25, Alexander Barkov wrote:

MDEV-26732 Assertion `0' failed in Item::val_native

Also fixes MDEV-24619 Wrong result or Assertion `0' in Item::val_native / Type_handler_inet6::Item_val_native_with_conversion

would be good to have some explanation here, what was wrong, what was
the fix.

Right, I suggest this comment:


      MDEV-26732 Assertion `0' failed in Item::val_native

      Also fixes MDEV-24619 Wrong result or Assertion `0' in
Item::val_native / Type_handler_inet6::Item_val_native_with_conversion

Type_handler::create_item_copy() created a generic Item_copy_string,
which does not implement val_native() - it has a dummy implementation
with DBUG_ASSERT(0), which made the server crash.

Why does it call val_native() if Item_copy_string doesn't implement it?

Because I forgot to implement Item_copy_inet6 - I missed the reported
scenario in my original test coverage for INET6.

The default implementation in Item::val_native(), which is called
for Item_copy_string in this scenario, intentionally consists only of DBUG_ASSERT.

If it had some working implementation, returning the expected value,
it would hide a performance problem:

Conversion from the string INET6 representation to the binary
INET6 representation would happen per row. This is not desirable.
So the DBUG_ASSERT is a trap to find such places not implementing
fast code paths for the native representation.


If the item was a string in the first place, that is, if
Item_copy_string would've been used correctly where it should've been
used, then val_native() wouldn't be called? What would it use,
val_str()?

Yes, val_str().


So why does it use val_native() here, because that
Item_copy_string uses Type_handler_inet6?

Correct.


Fix:

- Adding a new class Item_copy_inet6, which implements val_native().
- Fixing Type_handler::create_item_copy() to make Item_copy_inet6
instead of Item_copy_string.

Regards,
Sergei
VP of MariaDB Server Engineering
and security@xxxxxxxxxxx



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