btr_pcur_store_position(): Replace a too strict debug assertion.
It is possible to have a clustered index B-tree for a logically
empty table, which will consist of a node pointer from the root
page to a leaf page that contains the metadata record.

The too strict debug assertion was added in
commit 0e5a4ac2 (MDEV-15562).

MDEV-21546 main.backup_stages occasionally fails with lock wait timeout

commit f74023b9 (MDEV-15090)
inadvertently removed a mtr_t::commit() call from
trx_undo_report_rename(), causing an InnoDB hang if
we failed to log a RENAME operation.

It is unclear whether this condition is possible in practice.
The test case involved SET GLOBAL innodb_trx_rseg_n_slots_debug=1
and a failed CREATE TABLE...SELECT, whose error handling would
internally invoke RENAME in InnoDB.

Problem:
=======
  While evicting the uncompressed page from buffer pool, InnoDB writes
the checksum for the compressed page in buf_LRU_free_page().
So while flushing the compressed page, checksum validation fails
when innodb_checksum_algorithm variable changed to strict_none.

Solution:
========
- Calculate the checksum only during flushing of page. Removed the
checksum write in buf_LRU_free_page().

Compare to trx_roll_crash_recv_trx directly where needed.

innobase_init(): On every path to refused startup, log the reason
to refuse startup as an error, instead of a note.

Part of MDEV-19061 - table_share used for reading statistical tables is
                     not protected

Previously multiple threads were allowed to load histograms concurrently.
There were no known problems caused by this. But given amount of data
races in this code, it'd happen sooner or later.

To avoid scalability bottleneck, histograms loading is protected by
per-TABLE_SHARE atomic variable.

Whenever histograms were loaded by preceding statement (hot-path), a
scalable load-acquire check is performed.

Whenever histograms have to be loaded anew, mutual exclusion for loaders
is established by atomic variable. If histograms are being loaded
concurrently, statement waits until load is completed.

- Table_statistics::total_hist_size moved to TABLE_STATISTICS_CB: only
  meaningful within TABLE_SHARE (not used for collected stats).
- TABLE_STATISTICS_CB::histograms_can_be_read and
  TABLE_STATISTICS_CB::histograms_are_read are replaced with a tri state
  atomic variable.
- Simplified away alloc_histograms_for_table_share().

Note: there's still likely a data race if a thread attempts accessing
histograms data after it failed to load it (because of concurrent load).
It was there previously and goes out of the scope of this effort. One way
of fixing it could be reviving TABLE::histograms_are_read and adding
appropriate checks whenever it is needed.

Part of MDEV-19061 - table_share used for reading statistical tables is
                     not protected

Previously multiple threads were allowed to load statistics concurrently.
There were no known problems caused by this. But given amount of data
races in this code, it'd happen sooner or later.

To avoid scalability bottleneck, statistics loading is protected by
per-TABLE_SHARE atomic variable.

Whenever statistics were loaded by preceding statement (hot-path), a
scalable load-acquire check is performed.

Whenever statistics have to be loaded anew, mutual exclusion for loaders
is established by atomic variable. If statistics are being loaded
concurrently, statement waits until load is completed.

TABLE_STATISTICS_CB::stats_can_be_read and
TABLE_STATISTICS_CB::stats_is_read are replaced with a tri state atomic
variable.

Part of MDEV-19061 - table_share used for reading statistical tables is
                     not protected

Removed redundant loops, integrated logics into the caller instead.
Unified condition in read_statistics_for_tables(), less
"table_share != NULL" checks, no more potential "table_share == NULL"
dereferencing.

Part of MDEV-19061 - table_share used for reading statistical tables is
                     not protected

(The table already existed in the storage engine)

Wrong algorithm of closing partitions on error doesn't close last
partition.

MDEV-22744 *SAN: sql/item_xmlfunc.cc:791:43: runtime error: downcast of address ... which does not point to an object of type 'Item_func' note: object is of type 'Item_bool' (on optimized builds)

In Item_nodeset_func_predicate::val_nodeset, args[1] is not necessarily
an Item_func descendant. It can be Item_bool.

Removing a wrong cast. It was not really needed anyway.

when checking for free port, use the same logic (IPv6 socket address
/ dual socket), like the server would.

Previous solution for testing whether port is free was trying to bind
IPv4 socket on INADDR_ANY.

This not work now on some reason, that attempt succeeds, even if there is
an existing IPv6-dual socket listening on 0.0.0.0:3306

It works, but irritates people who look into the log and see
traces of 32bit custom action server.

In the merge 9e6e4355 we replaced
direct use of std::atomic with a wrapper class, so that
dict_index_t::lock will support the default assignment operator.

As part of that change, one occurrence of std::memory_order_release
was accidentally replaced with std::memory_order_relaxed.

Thanks to Sergey Vojtovich for noticing this.

Respect system fields in NO_ZERO_DATE mode.

This is the subject for refactoring in MDEV-19597

Conflict resolution from 7d5223310789f967106d86ce193ef31b315ecff0

Respect system fields in NO_ZERO_DATE mode.

This is the subject for refactoring in MDEV-19597

UPDATE gets access to history records because versioning conditions
are not set for VIEW. This leads to endless loop of inserting history
records when clustered index is rebuilt and ha_rnd_next() returns
newly inserted history record.

Return back original behavior of failing on write-locked table in
historical query.

35b679b9 assumed that SELECT_LEX::lock_type influences anything, but
actually at this point table is already locked. Original bug report
was tempesta-tech/mariadb#102

Pre-definitions are allowed for non-embedded.
Failur catched with:
```
cmake ../../10.1 -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_COMPILER=g++-9
-DCMAKE_C_COMPILER=gcc-9 -DWITH_EMBEDDED_SERVER=ON -DCMAKE_BUILD_TYPE=Debug
-DPLUGIN_{ARCHIVE,TOKUDB,MROONGA,OQGRAPH,ROCKSDB,PERFSCHEMA,SPIDER,SPHINX}=N
-DMYSQL_MAINTAINER_MODE=ON -DNOT_FOR_DISTRIBUTION=ON
```
Alternative fix would be
```
--- a/sql/sql_acl.cc
+++ b/sql/sql_acl.cc
@@ -201,8 +201,10 @@ LEX_STRING current_user= { C_STRING_WITH_LEN("*current_user") };
 LEX_STRING current_role= { C_STRING_WITH_LEN("*current_role") };
 LEX_STRING current_user_and_current_role= { C_STRING_WITH_LEN("*current_user_and_current_role") };

+#ifndef EMBEDDED_LIBRARY
 class ACL_USER;
 static ACL_USER *find_user_or_anon(const char *host, const char *user, const char *ip);
+#endif
```

Bogus if() logic inside the func.

System versioning assertion fix. Since DROP SYSTEM VERSIONING does not
change list of dropped keys we should handle a special case.

Caused by MDEV-19751. This fix deprecates MDEV-17091.

- `SET DEFAULT ROLE xxx [FOR yyy]` should say:
  "User yyy has not been granted a role xxx" if:
    - The current user (not the user `yyy` in the FOR clause) can see the
    role xxx. It can see the role if:
      * role exists in `mysql.roles_mappings` (traverse the graph),
      * If the current user has read access on `mysql.user` table - in
    that case, it can see all roles, granted or not.
    - Otherwise it should be "Invalid role specification".

In other words, it should not be possible to use `SET DEFAULT ROLE` to discover whether a specific role exist or not.