Thanks to Zhangyuan from Alibaba for pointing out this bug.

btr_page_empty(): When a clustered index root page is emptied,
preserve PAGE_ROOT_AUTO_INC. This would occur during a page split.

page_create_empty(): Preserve PAGE_ROOT_AUTO_INC when a clustered
index root page becomes empty. Use a faster method for writing
the field.

page_zip_copy_recs(): Reset PAGE_MAX_TRX_ID when copying
clustered index pages. We must clear the field when the root page
was a leaf page and it is being split, so that PAGE_MAX_TRX_ID
will continue to be 0 in clustered index non-root pages.

page_create_zip(): Add debug assertions for validating
PAGE_MAX_TRX_ID and PAGE_ROOT_AUTO_INC.

Remove unnecessary restarts by testing multiple tables across a restart.
This change almost halves the execution time.
Some further restarts could be removed with additional effort.

This should be functionally equivalent to WL#6204 in MySQL 8.0.0, with
the notable difference that the file format changes are limited to
repurposing a previously unused data field in B-tree pages.

For persistent InnoDB tables, write the last used AUTO_INCREMENT
value to the root page of the clustered index, in the previously
unused (0) PAGE_MAX_TRX_ID field, now aliased as PAGE_ROOT_AUTO_INC.
Unlike some other previously unused InnoDB data fields, this one was
actually always zero-initialized, at least since MySQL 3.23.49.

The writes to PAGE_ROOT_AUTO_INC are protected by SX or X latch on the
root page. The SX latch will allow concurrent read access to the root
page. (The field PAGE_ROOT_AUTO_INC will only be read on the
first-time call to ha_innobase::open() from the SQL layer. The
PAGE_ROOT_AUTO_INC can only be updated when executing SQL, so
read/write races are not possible.)

During INSERT, the PAGE_ROOT_AUTO_INC is updated by the low-level
function btr_cur_search_to_nth_level(), adding no extra page
access. [Adaptive hash index lookup will be disabled during INSERT.]

If some rare UPDATE modifies an AUTO_INCREMENT column, the
PAGE_ROOT_AUTO_INC will be adjusted in a separate mini-transaction in
ha_innobase::update_row().

When a page is reorganized, we have to preserve the PAGE_ROOT_AUTO_INC
field.

During ALTER TABLE, the initial AUTO_INCREMENT value will be copied
from the table. ALGORITHM=COPY and online log apply in LOCK=NONE will
update PAGE_ROOT_AUTO_INC in real time.

innodb_col_no(): Determine the dict_table_t::cols[] element index
corresponding to a Field of a non-virtual column.
(The MySQL 5.7 implementation of virtual columns breaks the 1:1
relationship between Field::field_index and dict_table_t::cols[].
Virtual columns are omitted from dict_table_t::cols[]. Therefore,
we must translate the field_index of AUTO_INCREMENT columns into
an index of dict_table_t::cols[].)

Upgrade from old data files:

By default, the AUTO_INCREMENT sequence in old data files would appear
to be reset, because PAGE_MAX_TRX_ID or PAGE_ROOT_AUTO_INC would contain
the value 0 in each clustered index page. In new data files,
PAGE_ROOT_AUTO_INC can only be 0 if the table is empty or does not contain
any AUTO_INCREMENT column.

For backward compatibility, we use the old method of
SELECT MAX(auto_increment_column) for initializing the sequence.

btr_read_autoinc(): Read the AUTO_INCREMENT sequence from a new-format
data file.

btr_read_autoinc_with_fallback(): A variant of btr_read_autoinc()
that will resort to reading MAX(auto_increment_column) for data files
that did not use AUTO_INCREMENT yet. It was manually tested that during
the execution of innodb.autoinc_persist the compatibility logic is
not activated (for new files, PAGE_ROOT_AUTO_INC is never 0 in nonempty
clustered index root pages).

initialize_auto_increment(): Replaces
ha_innobase::innobase_initialize_autoinc(). This initializes
the AUTO_INCREMENT metadata. Only called from ha_innobase::open().

ha_innobase::info_low(): Do not try to lazily initialize
dict_table_t::autoinc. It must already have been initialized by
ha_innobase::open() or ha_innobase::create().

Note: The adjustments to class ha_innopart were not tested, because
the source code (native InnoDB partitioning) is not being compiled.

in slow shutdown mode don't stop purge threads until they've
purged everything there is

in slow shutdown mode stop all bg threads that might generate
new undo records to purge before stopping purge threads.

in slow shutdown mode purge threads really must exit only when there is
nothing to purge. Restore the trx_commit_disallowed check and
don't stop purge threads until all connection thread transactions
are gone.

The patch for bug mdev-10882 tried to fix it by providing an
implementation of the virtual method build_clone for the class
Item_cache. It's turned out that it is not easy provide a valid
implementation for Item_cache::build_clone(). At the same time
if the condition that can be pushed into a materialized view
contains a cached item this item can be substituted for a basic
constant of the same value. In such a way we can avoid building
proper clones for Item_cache objects when constructing pushdown
conditions.

* rpm upgrade fix
* update test results
* valgrind warning

        Now JSON functions return warnings if arguments are invalid.

name innodb background threads as such

MDEV-11518 Assertion `!table || (!table->read_set || bitmap_is_set(table->read_set, field_index))' failed in Field_long::val_int()

QUICK_RANGE_SELECT::init_ror_merged_scan() should
also set and restore TABLE::vcol_set

Aria supports virtual columns, but does not support indexes
on virtual columns. Let's issue an appropriate error in this case.

strangely enough, ?: variant does not link in some older gcc versions:

sql/sql_table.cc:6409: undefined reference to `Alter_inplace_info::ALTER_STORED_GCOL_EXPR'
sql/sql_table.cc:6409: undefined reference to `Alter_inplace_info::ALTER_VIRTUAL_GCOL_EXPR'

by introducing new Item::precedence() method and using it
to decide whether parentheses are required

* do setup/cleanup for charset and arena only once per table
  not for every vcol
* don't copy every vcol expression into table's memroot for parsing,
  do it in a temporary String buffer that is freed at the end
* add asserts

now, when expr_str is gone, expr_item can be unambiguously
renamed to expr.

otherwise we'd need to store sql_mode *per vcol*
(consider CREATE INDEX...) and how SHOW CREATE TABLE would
support that?

Additionally, get rid of vcol::expr_str, just to make sure
the string is always generated and never leaked in the
original form.

it must, because purge opens a table that might be being altered right now

  * remove old 5.2+ InnoDB support for virtual columns
  * enable corresponding parts of the innodb-5.7 sources
  * copy corresponding test cases from 5.7
  * copy detailed Alter_inplace_info::HA_ALTER_FLAGS flags from 5.7
     - and more detailed detection of changes in fill_alter_inplace_info()
  * more "innodb compatibility hooks" in sql_class.cc to
     - create/destroy/reset a THD (used by background purge threads)
     - find a prelocked table by name
     - open a table (from a background purge thread)

  * different from 5.7:
    - new service thread "thd_destructor_proxy" to make sure all THDs are
      destroyed at the correct point in time during the server shutdown
    - proper opening/closing of tables for vcol evaluations in
       + FK checks (use already opened prelocked tables)
       + purge threads (open the table, MDLock it, add it to tdc, close
         when not needed)
    - cache open tables in vc_templ
    - avoid unnecessary allocations, reuse table->record[0] and table->s->default_values
    - not needed in 5.7, because it overcalculates:
      + tell the server to calculate vcols for an on-going inline ADD INDEX
      + calculate vcols for correct error messages

  * update other engines (mroonga/tokudb) accordingly

remove 'table' argument, remnant of 5.6, does not exist in 5.7

* remnant of 5.6, does not exist in 5.7. bad merge?
* also remove dict_table_get_col_name_for_mysql(), it was only
  used when index_field_t::col_name was not NULL

* fix "unused pending_checkpoint_mutex_key" compiler warning
* clarify/simplify get_field_offset()
* typos, comments
* unused (forgotten) declaration of create_options_are_invalid()
* fix my_error(ER_WRONG_KEY_COLUMN) calls
* crash in row_upd_sec_index_entry()
* double if (ret != 0)
* don't duplucate PSI_INSTRUMENT_ME lines
* useless break; after return();
* remove unused xtradb-only "cursor_read_view" stuff
* code formatting
* simplify dropped column detection
* redundant assignment

and THD::inc_thread_count