MDEV-3945 - do not hold LOCK_thread_count when freeing THD.

  
The patch decreases the duration of LOCK_thread_count, so it is not hold during THD destructor and freeing memory.
This mutex  now only protects the integrity of threads list, when removing THD from it,  and thread_count variable.
  
The add_to_status() function that updates global status during client disconnect,  is now correctly protected by the LOCK_status mutex.

Benchmark : in a  "non-persistent" sysbench test (oltp_ro with reconnect after each query),  ~ 25% more connects/disconnects were measured

sql/mysqld.cc

@@ -2440,21 +2440,6 @@ void dec_connection_count(THD *thd)
 }
 
 
-/*
-  Delete the THD object and decrease number of threads
-
-  SYNOPSIS
-    delete_thd()
-    thd		 Thread handler
-*/
-
-void delete_thd(THD *thd)
-{
-  thread_count--;
-  delete thd;
-}
-
-
 /*
   Unlink thd from global list of available connections and free thd
 
@@ -2473,14 +2458,23 @@ void unlink_thd(THD *thd)
 
   thd_cleanup(thd);
   dec_connection_count(thd);
+
+  mysql_mutex_lock(&LOCK_status);
+  add_to_status(&global_status_var, &thd->status_var);
+  mysql_mutex_unlock(&LOCK_status);
+
   mysql_mutex_lock(&LOCK_thread_count);
+  thread_count--;
+  thd->unlink();
   /*
     Used by binlog_reset_master.  It would be cleaner to use
     DEBUG_SYNC here, but that's not possible because the THD's debug
     sync feature has been shut down at this point.
   */
   DBUG_EXECUTE_IF("sleep_after_lock_thread_count_before_delete_thd", sleep(5););
-  delete_thd(thd);
+  mysql_mutex_unlock(&LOCK_thread_count);
+
+  delete thd;
   DBUG_VOID_RETURN;
 }
 
@@ -2589,10 +2583,13 @@ bool one_thread_per_connection_end(THD *thd, bool put_in_cache)
   /* Mark that current_thd is not valid anymore */
   my_pthread_setspecific_ptr(THR_THD,  0);
   if (put_in_cache)
+  {
+    mysql_mutex_lock(&LOCK_thread_count);
     put_in_cache= cache_thread();
-  mysql_mutex_unlock(&LOCK_thread_count);
-  if (put_in_cache)
-    DBUG_RETURN(0);                             // Thread is reused
+    mysql_mutex_unlock(&LOCK_thread_count);
+    if (put_in_cache)
+      DBUG_RETURN(0);                             // Thread is reused
+  }
 
   /* It's safe to broadcast outside a lock (COND... is not deleted here) */
   DBUG_PRINT("signal", ("Broadcasting COND_thread_count"));

sql/scheduler.cc

@@ -35,7 +35,6 @@
 static bool no_threads_end(THD *thd, bool put_in_cache)
 {
   unlink_thd(thd);
-  mysql_mutex_unlock(&LOCK_thread_count);
   return 1;                                     // Abort handle_one_connection
 }
 

sql/sql_class.cc

@@ -1444,7 +1444,6 @@ THD::~THD()
   mysql_mutex_lock(&LOCK_thd_data);
   mysys_var=0;					// Safety (shouldn't be needed)
   mysql_mutex_unlock(&LOCK_thd_data);
-  add_to_status(&global_status_var, &status_var);
 
   /* Close connection */
 #ifndef EMBEDDED_LIBRARY

sql/threadpool_common.cc

@@ -173,7 +173,6 @@ void threadpool_remove_connection(THD *thd)
   close_connection(thd, 0);
 
   unlink_thd(thd);
-  mysql_mutex_unlock(&LOCK_thread_count);
   mysql_cond_broadcast(&COND_thread_count);
 
   /*