This was done to match the naming of the ALTER #sql-#-# tmp files

No need to initialize socket_accept, it always gets assigned a value.

Pass addr_len directly to accept().

Part of MDEV-19515 - Improve connect speed

Removed LOCK_connection_count.

Removed duplicate denied_connections and connection_errors_max_connection
increment from create_new_thread(). Another increment done by
CONNECT::close_with_error().

Simplified away CONNECT::thread_count_incremented. Now close_with_error()
is always called with connection_count incremented.

Part of MDEV-19515 - Improve connect speed

cached_thread_count can handle it alone.

Added "fast path": when thread is being added to the cache and there's
queued connection request, serve it immediately.

Part of MDEV-19515 - Improve connect speed

It mostly duplicates code of do_handle_one_connection(). Move it there,
so that it is easier to simplify it in future.

Moved PSI_CALL_delete_current_thread()/_db_pop_() out of mutex. Call them
consistently in all cases, unlike before:
- called for reused threads
- called for flushed threads
- called for threads that go down after 5 minutes timeout
- not called for threads attempting to cache themselves during flush
- not called for threads attempting to cache themselves when cache is full

Part of MDEV-19515 - Improve connect speed

Code properly integrated into callers instead.

do_handle_one_connection(): no need to reset thd and thd->thread_stack
as they're not updated by cache_thread().

Part of MDEV-19515 - Improve connect speed

It was meaningful only for one-thread-per-connection scheduler anyway,
so call init_new_connection_handler_thread() directly from
handle_one_connection().

It was somewhat harmful for no-threads scheduler, because it'd attempt to
detach main thread on AIX_3_2 and Siemens unix (and DEC OSF/1 3.2 too).
Also it does duplicate my_thread_init() call, which may produce a warning
under EXTRA_DEBUG_THREADS.

start_wsrep_THD() shouldn't have called it at all: it already detached
and called my_thread_init().

Part of MDEV-19515 - Improve connect speed

Gives ~10% throughput improvemet in sysbench connect benchmark.

Part of MDEV-19515 - Improve connect speed

Gives ~3% throughput improvemet in sysbench connect benchmark.

Part of MDEV-19515 - Improve connect speed

Rather than iterating global plugin collection, iterate THD local
collection. Removes two LOCK_plugin locks per connection.

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Do not reset THD ha_data in rocksdb_close_connection(), cleaner approach
is to let ha_close_connection() do it.

Removed transaction objects cleanup from rocksdb_done_func(). As we lock
plugin properly, there must be no transaction objects during RocksDB
shutdown.

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Part of MDEV-19515 - Improve connect speed

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Do not reset THD ha_data in sphinx_close_connection(), cleaner approach
is to let ha_close_connection() do it.

Part of MDEV-19515 - Improve connect speed

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Do not reset THD ha_data in mrn_close_connection(), cleaner approach
is to let ha_close_connection() do it.

Part of MDEV-19515 - Improve connect speed

Do not reset THD ha_data in spider_close_connection(), cleaner approach
is to let ha_close_connection() do it.

Part of MDEV-19515 - Improve connect speed

Use thd_get_ha_data()/thd_set_ha_data() which protect against plugin
removal until it has THD ha_data.

Do not reset THD ha_data in ha_federatedx::disconnect(), cleaner approach
is to let ha_close_connection() do it.

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

It was wrong anyway, as it may not get updated by the time created thread
reads it. And it was 0 for cached threads.

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

It may be beneficial for slow-path, where new thread is to be created.
However it is harmful for fast-path, where thread cache can fulfil
request: it caused 2 extra loads of write-contended variables.

Also shrink variables scope.

Part of MDEV-19515 - Improve connect speed

Removed FD_CLOEXEC setting: already done by mysql_socket_accept().

Moved O_NONBLOCK setting out of accepting loop.

Removed blocking acceptance attempt, which was dead code. It was supposed
to be executed before the last iteration, however it was actually executed
during the last iteration. And it is not correct to block on one socket
leaving other sockets unattended anyway.

Gives ~5% throughput improvemet in sysbench connect benchmark.

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

Part of MDEV-19515 - Improve connect speed

abort_loop is intended to break accepting connections loop in main thread,
however it is being used for other purposes.

cache_thread() is governed by kill_cached_threads, no need to check
abort_loop here.

Check in create_new_thread() is redundant, abort_loop already checked
by caller.

Part of MDEV-19515 - Improve connect speed

This commit adds a new feature to the server to add comments at the database
level. 1024 bytes is the maximum comment length allowed. If the comment length
exceeds this limit, a new error/warning code 4144 is thrown, based on whether
thd->is_strict_mode() is true/false. The database comment is also added to the
db.opt file, as well as to the information_schema.schemata table.

Make sure that SIGNALs are not overwritten before they
are received.

Use different signal names, so that subsequent WAIT_FOR is not awaken by
previous signal.