row_build_index_entry_low(): Assert that when the value of a
virtual column is not available, this can only happen when
the index creation was completed but not committed yet.

This change is not fixing any bug, making a debug assertion
stricter, so that bugs can be caught in the future.

Ultimately, we should change the InnoDB undo log format so that
all actual secondary index keys are stored there, also for
virtual or spatial indexes. In that way, purge and rollback would
be more straightforward.

FIXME: Why the condition !recovery?

btr_cur_pessimistic_update(): Invoke dtuple_convert_big_rec()
for metadata records if the record is too large, or if the
metadata field is not yet marked as externally stored.

row_upd_index_replace_metadata(): Correctly copy the ALTER TABLE
metadata field.

btr_cur_trim(): Handle the generic ALTER TABLE metadata record.

btr_cur_pessimistic_update(): Do not invoke dtuple_convert_big_rec()
for ALTER TABLE metadata records on rollback.

row_undo_mod_clust(): Relax an assertion. The delete-mark flag was
repurposed for ALTER TABLE metadata records.

row_upd_index_replace_metadata(): Remove a bogus assertion
which would not hold on rollback during recovery.

row_undo_mod_parse_undo_rec(): Handle both types of metadata records.

row_upd_index_replace_metadata(): Correctly process DB_TRX_ID,DB_ROLL_PTR,
and assert that a new value for the metadata BLOB is present.

row_build_index_entry_low(): Clean up the code, removing
redundant variables and conditions. For instantly dropped columns,
generate a dummy value that is NULL, the empty string, or a
fixed length of NUL bytes, depending on the type of the dropped column.

FIXME: For some reason, during the test gcol.innodb_virtual_debug_purge
we are returning NULL for indexes on virtual columns, meaning that
purge on the indexes is being skipped. Does this lead to missed purge,
or is it merely causing unnecessary memory allocation? Why are we not
evaluating the virtual column values before calling this function?
(And why not write all index keys (virtual columns or spatial index MBR)
directly to the undo log records?)

row_upd_clust_rec_by_insert_inherit_func(): On the update of PRIMARY KEY
of a record that contained a dropped column whose value was stored
externally, we will be inserting a dummy NULL or empty string value
to the field of the dropped column. The externally stored column would
eventually be dropped when purge removes the delete-marked record for
the old PRIMARY KEY value.

dict_index_t::clear_instant_alter(): Move the fields for the
dropped columns to the end, and sort the surviving index fields
in ascending order of column position.

ib_counter_t: make all reads/writes to m_counter relaxed atomical

Reduce the test diff from 10.4.

This reverts commit 26897301
and adds some changes.

Only a table on which the 10.3 ADD COLUMN...LAST was performed
can "lose its instantness" when it becomes empty. After
instant DROP COLUMN or reordering columns, we cannot simply
convert the table to the canonical format, because the data
dictionary cache and all possibly existing references to it
from other client connection threads would have to be adjusted.

rec_init_offsets_comp_ordinary(): Remove a FIXME comment.

row_log_allocate(): Replace dict_index_t::is_instant()
with a more appropriate condition that ignores dict_table_t::instant.

On the rollback of changes to SYS_COLUMNS, MDEV-15562 will
break the assumption that the only instantaneous changes to columns
are the addition to the end of the column list.

The function dict_table_t::rollback_instant(unsigned n)
is inherently incompatible with instantly dropping or reordering
columns.

When a change to SYS_COLUMNS is rolled back, we must simply evict
the affected table definition, at the end of the rollback. We cannot
free the table object immediately, because the current transaction
that is being rolled back may be holding a lock on the table and
its metadata record.

dict_table_remove_from_cache_low(): Replaced
by dict_table_remove_from_cache().

dict_table_remove_from_cache(): Add a third parameter keep=false,
so that the table can be freed by the caller.

trx_lock_t::evicted_tables: List of tables on which trx_t::evict_table()
was invoked.

trx_t::evict_table(): Evict a table definition during rollback.

trx_commit_in_memory(): Empty the trx->lock.evicted_tables list
after the locks were released, by freeing the table objects.

row_undo_ins_remove_clust_rec(), row_undo_mod_clust_low():
Invoke trx_t::evict_table() on the affected table if a change to
SYS_COLUMNS is being rolled back.

The error handling for ALTER TABLE…ALGORITHM=COPY as well as
CREATE TABLE used to commit the CREATE TABLE transaction and then
issue DROP TABLE in a separate transaction. This is unnecessarily
breaking atomicity during DDL operations. Let us revise it so
that the DROP TABLE will be executed within the same transaction,
which will finally be rolled back.

FIXME: Introduce an undo log record so that the data file would be
deleted on rollback and no DROP TABLE would be needed at all.

FIXME: Avoid unnecessary access to per-table tablespace during DROP TABLE.
If the .ibd file is going to be deleted anyway, we should not bother
to mark the pages free.

dict_create_add_foreigns_to_dictionary(): Do not commit the transaction.
We want simple rollback in case dict_load_foreigns() would fail.

create_table_info_t::create_table(), row_create_index_for_mysql(),
row_table_add_foreign_constraints(): Before invoking rollback, drop
the table. Rollback would invoke trx_t::evict_table(), and after that
dropping the table would be a no-op.

ha_innobase::create(): Before rollback, drop the table. If the SQL
layer invoked ha_innobase::delete_table() later, it would be a no-op
because the rollback would have invoked trx_t::evict_table().

dict_index_t::first_user_field(): Return the first data field in
a clustered index, that is, the field after the PRIMARY KEY and
the two system columns DB_TRX_ID, DB_ROLL_PTR.

dtuple_convert_big_rec(): Remove some local variables.

Ensure that no redo log checkpoint occurs in a critical section
of a recovery test.