row_metadata_to_tuple(), row_rec_to_index_entry_impl():
Add the parameter "pad" to determine whether
the tuple should be padded to the index fields (on ALTER TABLE it should),
or whether it should remain at its original size (on rollback).

trx_undo_page_report_modify(): Avoid out-of-bounds access to record fields.

row_quiesce_write_index_fields(): Do not crash when the table contains
an instantly dropped column.

row_rec_to_index_entry_impl<true, 2>(): Correctly convert a record
from instant ALTER TABLE format into instant ADD COLUMN format.

row_undo_search_clust_to_pcur(): Relax too tight debug assertions.

On rollback of changes to SYS_COLUMNS, we can no longer assume that
the change was the instant addition of a column at the end of the
column list. The function dict_table_t::rollback_instant(unsigned n)
was 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 locks on the table.

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.

dict_table_t::rollback_instant(): Add the parameter old_n_fields,
and simplify some logic.

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

row_undo_mod_clust_low(): FIXME: do the same

btr_index_rec_validate(): Recognize the metadata record.

For instant ADD COLUMN…LAST it was possible to remove the metadata
whenever the table becomes empty. For the generic ALTER TABLE,
we must preserve the metadata on dropped or reordered columns,
because concurrent transactions are assuming that this metadata
cannot change.

rec_is_add_metadata(): Check if a record is ADD COLUMN metadata
record (not more generic instant ALTER TABLE).

btr_discard_only_page_on_level(): Preserve the generic instant
ALTER TABLE metadata.

btr_cur_optimistic_delete_func(), btr_cur_pessimistic_delete():
When the last user record is deleted, do not delete the
generic instant ALTER TABLE metadata record. Only delete
instant ADD COLUMN metadata records.

btr_cur_optimistic_insert(): Avoid unnecessary computation of rec_size.

btr_cur_pessimistic_update(): Correctly check for generic ALTER TABLE
metadata record when update an instant ADD COLUMN record.

instant_metadata(): Invoke dict_table_t::serialise_columns()
at the earliest possibility. Previously this was only invoked
during insert in dtuple_convert_big_rec().

rec_get_converted_size_comp_prefix_low(): Assume that the metadata
field will be stored externally. In dtuple_convert_big_rec() during
the rec_get_converted_size() call, it would not be there yet.

btr_cur_instant_root_init(): Check the "infimum" and "supremum"
record strings already here, and not later in btr_cur_instant_root_init().
In this way, we can properly reject files from later versions where
instant ALTER TABLE could support further operations that change the
format of InnoDB clustered indexes.

For dropped columns, store the NOT NULL flag, the fixed length,
and for variable-length columns, whether the maximum length exceeded
255 bytes.

For partitioned tables, we must adjust each partition separately.

FIXME: For dropped columns, store the nullability and the fixed_len
in the BLOB array. Also, add a version discriminator field.

btr_cur_instant_init_low(): Read and validate the metadata BLOB page
before reconstructing the dictionary information based on it.

dict_col_t::clear_instant(): Renamed from remove_instant().
Do not assert that the column was instantly added, because we
sometimes call this unconditionally for all columns.

dict_table_t::reconstruct_columns(): Take the metadata BLOB
and length as a parameter. Invoke dict_index_t::reconstruct_fields()
upon successful completion.

For instant DROP COLUMN and ROW_FORMAT=COMPACT or ROW_FORMAT=REDUNDANT,
we must store the n_core_null_bytes in the root page, so that the
chain of node pointer records can be followed in order to reach the
leftmost leaf page where the metadata record is located.

If the PRIMARY KEY contains any variable-length column and some
nullable columns were instantly dropped, the dict_index_t::n_nullable
in the data dictionary could be smaller than it actually is in the
non-leaf pages. Because of this, the non-leaf pages could use more
bytes for the null flags than the data dictionary expects, and we
could be reading the lengths of the variable-length columns from the
wrong offset, and thus reading the child page number from wrong place.
This is the result of two design mistakes that involve unnecessary
storage of data: First, it is nonsense to store any data fields for
the leftmost node pointer records, because the comparisons would be
resolved by the MIN_REC_FLAG alone. Second, there cannot be any null
fields in the clustered index node pointer fields, but we nevertheless
reserve space for all the null flags.

btr_set_instant(): Convert a root page into "instant ALTER TABLE"
format. This replaces page_set_instant().

btr_cur_instant_init_metadata(): Do not read any lengths from the
metadata record header before reading the BLOB. At this point, we
would not actually know how many nullable fields the metadata record
contains.

btr_cur_instant_init_low(): Minimize differences from the 10.3 version.

btr_cur_instant_root_init(): Initialize n_core_null_bytes in one
of two possible ways.

FIXME: innobase_instant_try(): When needed, convert the root page
from the ADD COLUMN format to the generic instant ALTER TABLE format.

Because changes of the FIL_PAGE_TYPE or PAGE_INSTANT in the root
page are not undo-logged, it is possible that the fields suggest
that instant ADD COLUMN is in effect, even though no metadata
record exists. If the fields are set, proceed to fetch the
metadata record. If the metadata record does not exist, return
success if !index->is_instant().

Also, check that the "infimum" and "supremum" records carry the
strings in the root page. In a later format that supports instant
DROP COLUMN, we will have to store more information in the root
page, so that index->n_core_null_bytes can be determined accurately.

btr_cur_instant_init_low(): If columns were instantly added and dropped,
then index->is_instant() might not hold even though the root page type
was FIL_PAGE_TYPE_INSTANT. MariaDB 10.3 must refuse to open such files,
because instant DROP COLUMN is not supported.

Also, refuse to open the table if the metadata record has
wrong info OR status bits. Previously, we only refused to open
if both bits were wrong.

Apparently, the first fields of a clustered index may be updated
in an update_undo vector when the index is ID_IND of SYS_FOREIGN,
as part of renaming the table during ALTER TABLE.
Normally, updates of the PRIMARY KEY should be logged as
delete-mark and an insert.

The function had only one caller.

trx_undo_update_rec_get_update(): Handle the metadata BLOB correctly.

Correctly calculate the size of mblob when some last fields are missing.