Simplified away init_new_connection_thread()

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

include/thread_pool_priv.h

@@ -82,8 +82,6 @@ bool do_command(THD *thd);
   ensure that the proper MySQL Server logic attached to these events is
   executed.
 */
-/* Initialise a new connection handler thread */
-bool init_new_connection_handler_thread();
 /* Set up connection thread before use as execution thread */
 bool setup_connection_thread_globals(THD *thd);
 /* Prepare connection as part of connection set-up */

sql/scheduler.cc

@@ -24,7 +24,6 @@
 
 #include "mariadb.h"
 #include "mysqld.h"
-#include "sql_connect.h"         // init_new_connection_handler_thread
 #include "scheduler.h"
 #include "sql_class.h"
 #include "sql_callback.h"
@@ -133,16 +132,11 @@ void one_thread_per_connection_scheduler(scheduler_functions *func,
   func->max_threads= *arg_max_connections + 1;
   func->max_connections= arg_max_connections;
   func->connection_count= arg_connection_count;
-  func->init_new_connection_thread= init_new_connection_handler_thread;
   func->add_connection= create_thread_to_handle_connection;
   func->end_thread= one_thread_per_connection_end;
   func->post_kill_notification= post_kill_notification;
 }
 #else
-bool init_new_connection_handler_thread()
-{
-  return 0;
-}
 void handle_connection_in_main_thread(CONNECT *connect)
 {
 }
@@ -158,7 +152,6 @@ void one_thread_scheduler(scheduler_functions *func)
   func->max_threads= 1;
   func->max_connections= &max_connections;
   func->connection_count= &connection_count;
-  func->init_new_connection_thread= init_new_connection_handler_thread;
   func->add_connection= handle_connection_in_main_thread;
   func->end_thread= no_threads_end;
 }

sql/scheduler.h

@@ -34,7 +34,6 @@ struct scheduler_functions
   uint max_threads, *connection_count;
   ulong *max_connections;
   bool (*init)(void);
-  bool (*init_new_connection_thread)(void);
   void (*add_connection)(CONNECT *connect);
   void (*thd_wait_begin)(THD *thd, int wait_type);
   void (*thd_wait_end)(THD *thd);

sql/sql_connect.cc

@@ -1303,6 +1303,14 @@ pthread_handler_t handle_one_connection(void *arg)
 
   mysql_thread_set_psi_id(connect->thread_id);
 
+  if (init_new_connection_handler_thread())
+  {
+    scheduler_functions *scheduler= connect->scheduler;
+    connect->close_with_error(0, 0, ER_OUT_OF_RESOURCES);
+    scheduler->end_thread(0, 0);
+    return 0;
+  }
+
   do_handle_one_connection(connect);
   return 0;
 }
@@ -1340,8 +1348,7 @@ void do_handle_one_connection(CONNECT *connect)
 {
   ulonglong thr_create_utime= microsecond_interval_timer();
   THD *thd;
-  if (connect->scheduler->init_new_connection_thread() ||
+  if (!(thd= connect->create_thd(NULL)))
-      !(thd= connect->create_thd(NULL)))
   {
     scheduler_functions *scheduler= connect->scheduler;
     connect->close_with_error(0, 0, ER_OUT_OF_RESOURCES);

sql/threadpool_common.cc

@@ -383,11 +383,6 @@ end:
 
 /* Dummy functions, do nothing */
 
-static bool tp_init_new_connection_thread()
-{
-  return 0;
-}
-
 static bool tp_end_thread(THD *, bool)
 {
   return 0;
@@ -512,7 +507,6 @@ static scheduler_functions tp_scheduler_functions=
   NULL,
   NULL,
   tp_init,                            // init
-  tp_init_new_connection_thread,      // init_new_connection_thread
   tp_add_connection,                  // add_connection
   tp_wait_begin,                      // thd_wait_begin
   tp_wait_end,                        // thd_wait_end

sql/wsrep_mysqld.cc

@@ -2619,13 +2619,6 @@ void* start_wsrep_THD(void *arg)
 
   mysql_thread_set_psi_id(thd->thread_id);
   thd->thr_create_utime=  microsecond_interval_timer();
-  if (MYSQL_CALLBACK_ELSE(thread_scheduler, init_new_connection_thread, (), 0))
-  {
-    close_connection(thd, ER_OUT_OF_RESOURCES);
-    statistic_increment(aborted_connects,&LOCK_status);
-    MYSQL_CALLBACK(thread_scheduler, end_thread, (thd, 0));
-    goto error;
-  }
 
 // </5.1.17>
   /*