While the record type MLOG_UNDO_INIT feels redundant, it does save
redo log volume, and subtle changes to redo log volume appear to
make a big difference for the log_sys.mutex contention.

trx_undo_parse_page_init(): Allow type==0, which is what we write
since MDEV-12288. Parse type in a simpler way; it always was written
as a single byte.

trx_undo_page_init(): Write a MLOG_UNDO_INIT record.

Check index capabilities before executing HANDLER READ command.

Thanks to Sergey Vojtovich for feedback and many ideas.

purge_state_t: Remove. The states are replaced with
purge_sys_t::enabled() and purge_sys_t::paused() as follows:
PURGE_STATE_INIT, PURGE_STATE_EXIT, PURGE_STATE_DISABLED: !enabled().
PURGE_STATE_RUN, PURGE_STATE_STOP: paused() distinguishes these.

purge_sys_t::m_paused: Renamed from purge_sys_t::n_stop.
Protected by atomic memory access only, not purge_sys_t::latch.

purge_sys_t::m_enabled: An atomically updated Boolean that
replaces purge_sys_t::state.

purge_sys_t::running: Remove, because it duplicates
srv_sys.n_threads_active[SRV_PURGE].

purge_sys_t::running(): Accessor for srv_sys.n_threads_active[SRV_PURGE].

purge_sys_t::stop(): Renamed from trx_purge_stop().

purge_sys_t::resume(): Renamed from trx_purge_run().
Do not acquire latch; solely rely on atomics.

purge_sys_t::is_initialised(), purge_sys_t::m_initialised: Remove.

purge_sys_t::create(), purge_sys_t::close(): Instead of invoking
is_initialised(), check whether event is NULL.

purge_sys_t::event: Move before latch, so that fields that are
protected by latch can reside on the same cache line with latch.

srv_start_wait_for_purge_to_start(): Merge to the only caller srv_start().

Because the InnoDB implementation in MariaDB has diverged from MySQL,
it is not meaningful to report a MySQL version number for InnoDB
any more. Some examples include:

MariaDB 10.1 (which is based on MySQL 5.6) included encryption and
variable-size page compression before MySQL 5.7 introduced them.
MariaDB 10.2 (based on MySQL 5.7) introduced persistent AUTO_INCREMENT
(MDEV-6076) in a GA release before MySQL 8.0.
MariaDB 10.3 (based on MySQL 5.7) introduced instant ADD COLUMN
(MDEV-11369) before MySQL.

All of these features use a different implementation and file format.
Also, some features were never merged from MySQL 5.7, and thus MariaDB
is not affected by related bugs. Examples include CREATE TABLESPACE
and the reimplementation of the partitioning engine.

row_trx_id_offset(): Add the missing rec_offs_init() call.
This was broken by commit 97e51d24.

Before invoking fil_space_t::acquire(), check space->is_stopping()
to ensure that the tablespace actually is accessible. This fixes
a regression introduced by MDEV-15983.

fil_space_next(): Remove some duplicated code for prev_space==NULL,
and check is_stopping() also on the first tablespace.

i_s_tablespaces_encryption_fill_table(),
i_s_tablespaces_scrubbing_fill_table(): Check is_stopping().

These directives did not have any affect.

All these affected lines:

1.  | INTERVAL_SYM expr interval '+' expr

2.  | INTERVAL_SYM '(' expr ',' expr ')'

3.  | INTERVAL_SYM '(' expr ',' expr ',' expr_list ')'

4.  | DATE_ADD_INTERVAL '(' expr ',' INTERVAL_SYM expr interval ')'

5.  | DATE_SUB_INTERVAL '(' expr ',' INTERVAL_SYM expr interval ')'

define the grammar unambiguously.

"bison -v sql_yacc.yy" produces exactly the same sql_yacc.output
before and after the change.

It caused compile failures:

commit 6620fbd6
Author: Teodor Mircea Ionita <teodor@mariadb.org>
Date:   Thu May 10 12:23:35 2018 +0300

    MDEV-15778: On macOS pthread_t is opaque, requires explicit cast

    On macOS pthread id is a pointer to struct _opaque_pthread_t type,
    requires explicit cast to ulint which in turn is size_t;
    Was failing with Clang 9.1.0 Debug build on macOS 10.13.4:

    sync0policy.h:53:4: error: cannot initialize a member subobject of type 'ulint'
    (aka 'unsigned long') with an rvalue of type 'os_thread_id_t' (aka '_opaque_pthread_t *')
    m_thread_id(os_thread_id_t(ULINT_UNDEFINED))
                            ^           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    sync0policy.h:79:4: error: cannot initialize a parameter of type 'int64' (aka 'long long') with
    an rvalue of type 'os_thread_id_t' (aka '_opaque_pthread_t *')
    my_atomic_storelint(&m_thread_id, os_thread_get_curr_id());

MDEV-15576: Server crashed in Cached_item_str::cmp / sortcmp or Assertion `item->null_value' failed in Type_handler_temporal_result::make_sort_key upon SELECT with NULLIF and ROLLUP

Fixed null_value processing and is_null() usage.

Problem was that I in a previous patch enabled ifdef:ed code intended for
10.3 but that never worked.

Reverted to original code

MDEV-654 Assertion `share->now_transactional' failed in flush_log_for_bitmap on concurrent workload with Aria tables

I was able to repeat the problem with old version of randgen

Reason for crash:

- It's not safe to change share->now_transactional if there are changed
bitmaps in the pagecache as flushing these can cause redo-entries and
the bitmap flush code checks that share->now_transactional is set.

Fixed by flushing bitmaps in _ma_tmp_disable_logging_for_table() before
we set share->now_transactional to 0

MDEV-15465 Server crash or ASAN heap-use-after-free in Item_func_match::cleanup upon using FT search with partitioning.

Partition engine FT keys are implemented in such a way that
the FT function's cleanup() methods use table's internals.
So calling them after close_thread_tables is unsafe.

MDEV-15778: 10.3 macOS build fixes

Fix mysql_load iterator to skip invisible fields.

MDEV-16100 FOR SYSTEM_TIME erroneously resolves string user variables as transaction IDs

Problem:

Vers_history_point::resolve_unit() tested item->result_type() before
item->fix_fields() was called.

- Item_func_get_user_var::result_type() returned REAL_RESULT by default.
  This caused MDEV-16100.
- Item_func_sp::result_type() crashed on assert.
  This caused MDEV-16094

Changes:
1. Adding item->fix_fields() into Vers_history_point::resolve_unit()
   before using data type specific properties of the history point
   expression.

2. Adding a new virtual method Type_handler::Vers_history_point_resolve_unit()

3. Implementing type-specific
   Type_handler_xxx::Type_handler::Vers_history_point_resolve_unit()
    in the way to:
    a. resolve temporal and general purpose string types to TIMESTAMP
    b. resolve BIT and general purpose INT types to TRANSACTION
    c. disallow use of non-relevant data type expressions in FOR SYSTEM_TIME

    Note, DOUBLE and DECIMAL data types are disallowed intentionally.
    - DOUBLE does not have enough precision to hold huge BIGINT UNSIGNED values
    - DECIMAL rounds on conversion to INT
    Both lack of precision and rounding might potentionally lead to
    very unpredictable results when a wrong transaction ID would be chosen.
    If one really wants dangerous use of DOUBLE and DECIMAL, explicit CAST
    can be used:

      FOR SYSTEM_TIME AS OF CAST(double_or_decimal AS UNSIGNED)

    QQ: perhaps DECIMAL(N,0) could still be allowed.

4. Adding a new virtual method Item::type_handler_for_system_time(),
   to make HEX hybrids and bit literals work as TRANSACTION rather
   than TIMESTAMP.

5. sql_yacc.yy: replacing the rule temporal_literal to "TIMESTAMP TEXT_STRING".
   Other temporal literals now resolve to TIMESTAMP through the new
   Type_handler methods. No special grammar needed. This removed
   a few shift/resolve conflicts.
   (TIMESTAMP related conflicts in "history_point:" will be removed separately)

6. Removing the "timestamp_only" parameter from
   vers_select_conds_t::resolve_units() and Vers_history_point::resolve_unit().
   It was a hint telling that a table did not have any TRANSACTION-aware
   system time columns, so it's OK to resolve to TIMESTAMP in case of uncertainty.
   In the new reduction it works as follows:
   - the decision between TIMESTAMP and TRANSACTION is first made
     based only on the expression data type only
   - then, in case if the expression resolved to TRANSACTION, the table
     is checked if TRANSACTION-aware columns really exist.
   This way is safer against possible ALTER TABLE statements changing
   ROW START and ROW END columns from "BIGINT UNSIGNED" to "TIMESTAMP(x)"
   or the other way around.

This problem occured because the reorganization of the list of values when the
number of elements exceeds 32 was not handled correctly.  I have fixed the
problem by fixing the way that the list values are reorganized when the number
of list values exceeds 32.

Author:
  Jacob Mathew.

Reviewer:
  Alexey Botchkov.

Merged From:
  Branch bb-10.3-MDEV-16101

Forced columns of recursive CTEs to be nullable. SQL standard
requires this only from recursive columns, but in our code
so far we do not differentiate between recursive and non-recursive
columns when aggregating types of the union that specifies a
recursive CTE.

This problem occured because the reorganization of the list of values when the
number of elements exceeds 32 was not handled correctly.  I have fixed the
problem by fixing the way that the list values are reorganized when the number
of list values exceeds 32.

Author:
  Jacob Mathew.

Reviewer:
  Alexey Botchkov.

Problem was that we used table->s->db_type() for accessing
handlerton of opened file instead of table->file->ht

Other bug fixed:
- Ensure that we set error if reopen_tables() fails
  (This was the cause of assert)

The predicate dict_table_is_discarded() checks whether
ALTER TABLE…DISCARD TABLESPACE has been executed.

Replace most occurrences of dict_table_is_discarded() with
checks of dict_table_t::space. A few checks for the flag
DICT_TF2_DISCARDED are necessary; write them inline.

Because !is_readable() implies !space, some checks for
dict_table_is_discarded() were redundant.

Fixed by using exactly the same filtering conditions as specified by standard
in 7.6 <table reference> General Rules

MDEV-14823 Wrong error message upon selecting from a system_time partition
MDEV-15956 Strange ER_UNSUPPORTED_ACTION_ON_GENERATED_COLUMN upon ALTER on versioning column

MDEV-16043 Assertion thd->Item_change_list::is_empty() failed in mysql_parse upon SELECT from a view reading from a versioned table

Lost restore_active_arena().
Using of Query_arena_stmt is suggested instead.

Store transaction start time in thd->transaction.start_time.
THD::transaction_time() wraps over transaction.start_time taking into
account current status of BEGIN.