Use-after-free in DBUG_PRINT in ha_innobase::commit_inplace_alter_table()

In ha_innobase::commit_inplace_alter_table(), DBUG_PRINT("to_be_dropped", ("table: %s", ctx->old_table->name.m_name)) runs after trx->commit(deleted). If the table was rebuilt, commit_try_rebuild() has called trx->drop_table(*ctx->old_table), and trx_t::commit() has already freed ctx->old_table (dict_sys.remove() -> dict_mem_table_free()).

The DBUG_PRINT is only evaluated when the keyword to_be_dropped is enabled, e.g. with debug_dbug='d', or after SET debug_dbug='+d,x' followed by SET debug_dbug='-d,x', which leaves "d" with an empty keyword list, i.e. all keywords enabled (MDEV-21361). On an ASAN debug build (of our MDEV-10267 branch; handler0alter.cc differs from main only in inplace_alter_table()):

CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=InnoDB;

SET debug_dbug='d,to_be_dropped';

ALTER TABLE t1 FORCE;

ERROR: AddressSanitizer: heap-use-after-free ... READ of size 8

    #0 ha_innobase::commit_inplace_alter_table storage/innobase/handler/handler0alter.cc

freed by:

    dict_mem_table_free storage/innobase/dict/dict0mem.cc:223

    dict_sys_t::remove storage/innobase/dict/dict0dict.cc:1776

    trx_t::commit storage/innobase/dict/drop.cc:275

    ha_innobase::commit_inplace_alter_table storage/innobase/handler/handler0alter.cc

Any ALTER TABLE that rebuilds the table in place triggers it (FORCE, ENGINE=InnoDB, OPTIMIZE TABLE, the first FULLTEXT index on a table without an FTS_DOC_ID column), also on an empty table; ALGORITHM=COPY, INSTANT and in-place changes without a rebuild do not. A debug build without ASAN printed the stale name (test/#sql-ib19) and did not crash. The code is the same from 10.6 (MDEV-25506 moved the drop of the old table into trx->commit()) to main.

Fix: print ctx->tmp_name, which is the same name (asserted before the commit) and stays valid until the ctx is destroyed, or drop the DBUG_PRINT.