Several race conditions between MDEV-15562 instant ALTER TABLE and purge
were observed.
The most obvious race condition resulted in a reported assertion failure in
dict_index_t::instant_add_field(): instant.n_core_fields == n_core_fields
would not hold if the table was emptied by purge after the time
dict_table_t::prepare_instant() was called.
During purge, it can turn out that the table is logically empty, only
containing a metadata record. If the metadata record is of the type
created by MDEV-11369 instant ADD COLUMN, it can be removed and
dict_index_t::clear_instant_add() can be called. This will convert the
table to the canonical non-instant format. (If the metadata record is
of the MDEV-15562 type, then it can only be deleted if the table becomes
empty as the result of rollback of an instant ALTER TABLE operation.)
row_purge_remove_clust_if_poss_low(): Add a debug check that ensures
that purge can never remove a MDEV-15562 metadata record.
ha_innobase::open(): Add a comment about the necessity of rolling
back any recovered instant ALTER TABLE transaction on the table.
instant_metadata_lock(): An auxiliary function to acquire a page latch
on the metadata record, to prevent race conditions.
dict_table_t::prepare_instant(), dict_index_t::instant_add_field(),
dict_table_t::rollback_instant(), innobase_instant_try():
Invoke instant_metadata_lock() in order to prevent race conditions.
dict_index_t::instant_add_field(): Correct debug assertions.
The == was guaranteed by code in dict_table_t::prepare_instant()
that was introduced in MDEV-15562. Due to the race condition,
we could occasionally have <=, but never >= like the code was
after MDEV-11369.
ha_innobase_inplace_ctx::instant_column(): Wrapper for
dict_table_t::instant_column(). Add debug assertions.
There was a race condition between ALTER TABLE and purge.
If a table turns out to be logically empty when instant ALTER TABLE
is executing, we will convert the table to the canonical format,
to avoid overhead during subsequent accesses, and to allow the
data file to be imported into older versions of MariaDB.
It could happen that at the time the table is logically empty,
there still exists an undo log record for updating the hidden
metadata record for an earlier instant ALTER TABLE operation.
If the table was converted to the canonical format before
purge processes this undo log record, the undo log record
could be referring to index fields that no longer exist,
causing a crash.
To prevent the race condition, we must delete the old undo log records.
We do this lazily by assigning a new table ID, so that the table lookup
for the old undo log records will fail.
dict_table_t::reassign_id(): Reassign the table_id to
effectively lazily delete old undo log records.
innobase_instant_try(): Invoke index->table->reassign_id() before
index->clear_instant_alter().
