MDEV-38993 Assertion `trx->undo_no == 1' fails upon ALTER IGNORE

[Thirunarayanan]

Problem:
========
During ALTER TABLE ... IGNORE, a partial rollback on duplicate key
error resets trx->undo_no to 0. The subsequent insert then enters
the undo rewrite block with undo_no == 0, hitting the assertion
that expected undo_no == 1.

Solution:
=========
Partial rollback which truncates the last insert undo record
via trx_undo_truncate_end(), which rewrites TRX_UNDO_PAGE_FREE
on the page. By checking trx->undo_no as part of the rewrite
predicate, InnoDB correctly skips the rewrite logic after partial
rollback.

trx_undo_report_row_operation(): Pre-compute the full predicate
(clear_ignore) before trx_undo_assign_low(), since old_offset
and top_offset are not modified by that call.

trx_undo_rewrite_ignore(): Extract the rewrite body into a
separate ATTRIBUTE_COLD ATTRIBUTE_NOINLINE static function.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[dr-m]

Please check the generated AMD64 code for CMAKE_BUILD_TYPE=RelWithDebInfo.

[dr-m]

The indentation is a bit weird here. I would format it as follows to conform to the original InnoDB formatting style that is being used in this function:

		const bool clear_ignore = *pundo && trx->undo_no
			&& (*pundo)->old_offset <= (*pundo)->top_offset
			&& index->table->skip_alter_undo
			== dict_table_t::IGNORE_UNDO;

[dr-m]

The parameters will not fit in the 6 registers that are available in the commonly used AMD64 calling conventions. I assume that this is the limiting factor on our common target platforms. On ARMv8 there seem to be 8 registers for passing parameters.

We can freely choose the order of the function body. I would make mtr, undo_block the first parameters so that no registers have to be shuffled for the first call, like this:

static ATTRIBUTE_COLD ATTRIBUTE_NOINLINE
std::pair<trx_mod_tables_t::iterator, bool>
trx_undo_rewrite_ignore(mtr_t *mtr, buf_block_t *undo_block,
                        dict_table_t *table, trx_t *trx, trx_undo_t *undo)
  noexcept
{
  mtr->write<2>(*undo_block, undo_block->page.frame + TRX_UNDO_PAGE_HDR +
                TRX_UNDO_PAGE_FREE, undo->old_offset);
  ut_ad(trx->undo_no == 1);
  undo->top_offset= undo->old_offset;
  undo->top_undo_no= 0;
  trx->undo_no= 0;
  trx->mod_tables.clear();
  return trx->mod_tables.emplace(table, 0);
}

This will also enable a nice tail call optimization for the return value, which the caller would assign to its local variable m.