MDEV-7871 Tests fail massively on "Assertion `status_var.memory_used == 0'"...

MDEV-7871 Tests fail massively on "Assertion `status_var.memory_used == 0'" when run with --ps --embedded.
   As the MF_THREAD_SPECIFIC was introduced to the alloc_root's and
   the prealloc added to the statement::mem_root and statement::result.alloc, we have to adjust
   the embedded server to it. The preallocation was removed for the embedded server as it
   makes no sence for it. The msyqltest should free the statement inside the proper thead to
   make the memory statistics happy.

client/mysqltest.cc

@@ -839,6 +839,7 @@ static void handle_no_active_connection(struct st_command* command,
 #define EMB_END_CONNECTION 3
 #define EMB_PREPARE_STMT 4
 #define EMB_EXECUTE_STMT 5
+#define EMB_CLOSE_STMT 6
 
 /* workaround for MySQL BUG#57491 */
 #undef MY_WME
@@ -887,6 +888,9 @@ pthread_handler_t connection_thread(void *arg)
       case EMB_EXECUTE_STMT:
         cn->result= mysql_stmt_execute(cn->stmt);
         break;
+      case EMB_CLOSE_STMT:
+        cn->result= mysql_stmt_close(cn->stmt);
+        break;
       default:
         DBUG_ASSERT(0);
     }
@@ -984,6 +988,17 @@ static int do_stmt_execute(struct st_connection *cn)
 }
 
 
+static int do_stmt_close(struct st_connection *cn)
+{
+  /* The cn->stmt is already set. */
+  if (!cn->has_thread)
+    return mysql_stmt_close(cn->stmt);
+  signal_connection_thd(cn, EMB_CLOSE_STMT);
+  wait_query_thread_done(cn);
+  return cn->result;
+}
+
+
 static void emb_close_connection(struct st_connection *cn)
 {
   if (!cn->has_thread)
@@ -1019,6 +1034,7 @@ static void init_connection_thd(struct st_connection *cn)
 #define do_read_query_result(cn) mysql_read_query_result(cn->mysql)
 #define do_stmt_prepare(cn, q, q_len) mysql_stmt_prepare(cn->stmt, q, q_len)
 #define do_stmt_execute(cn) mysql_stmt_execute(cn->stmt)
+#define do_stmt_close(cn) mysql_stmt_close(cn->stmt)
 
 #endif /*EMBEDDED_LIBRARY*/
 
@@ -1378,11 +1394,11 @@ void close_connections()
   DBUG_ENTER("close_connections");
   for (--next_con; next_con >= connections; --next_con)
   {
+    if (next_con->stmt)
+      do_stmt_close(next_con);
 #ifdef EMBEDDED_LIBRARY
     emb_close_connection(next_con);
 #endif
-    if (next_con->stmt)
-      mysql_stmt_close(next_con->stmt);
     next_con->stmt= 0;
     mysql_close(next_con->mysql);
     next_con->mysql= 0;
@@ -5635,7 +5651,11 @@ void do_close_connection(struct st_command *command)
       con->mysql->net.vio = 0;
     }
   }
-#else
+#endif /*!EMBEDDED_LIBRARY*/
+  if (con->stmt)
+    do_stmt_close(con);
+  con->stmt= 0;
+#ifdef EMBEDDED_LIBRARY
   /*
     As query could be still executed in a separate theread
     we need to check if the query's thread was finished and probably wait
@@ -5643,9 +5663,6 @@ void do_close_connection(struct st_command *command)
   */
   emb_close_connection(con);
 #endif /*EMBEDDED_LIBRARY*/
-  if (con->stmt)
-    mysql_stmt_close(con->stmt);
-  con->stmt= 0;
 
   mysql_close(con->mysql);
   con->mysql= 0;

libmysql/libmysql.c

@@ -1508,6 +1508,12 @@ my_bool cli_read_prepare_result(MYSQL *mysql, MYSQL_STMT *stmt)
     memory
 */
 
+#ifdef EMBEDDED_LIBRARY
+#define STMT_INIT_PREALLOC(S) 0
+#else
+#define STMT_INIT_PREALLOC(S) S
+#endif /*EMBEDDED_LIBRARY*/
+
 MYSQL_STMT * STDCALL
 mysql_stmt_init(MYSQL *mysql)
 {
@@ -1526,8 +1532,10 @@ mysql_stmt_init(MYSQL *mysql)
     DBUG_RETURN(NULL);
   }
 
-  init_alloc_root(&stmt->mem_root, 2048, 2048, MYF(MY_THREAD_SPECIFIC));
-  init_alloc_root(&stmt->result.alloc, 4096, 4096, MYF(MY_THREAD_SPECIFIC));
+  init_alloc_root(&stmt->mem_root, 2048, STMT_INIT_PREALLOC(2048),
+                  MYF(MY_THREAD_SPECIFIC));
+  init_alloc_root(&stmt->result.alloc, 4096, STMT_INIT_PREALLOC(4096),
+                  MYF(MY_THREAD_SPECIFIC));
   stmt->result.alloc.min_malloc= sizeof(MYSQL_ROWS);
   mysql->stmts= list_add(mysql->stmts, &stmt->list);
   stmt->list.data= stmt;
@@ -1544,6 +1552,8 @@ mysql_stmt_init(MYSQL *mysql)
   DBUG_RETURN(stmt);
 }
 
+#undef STMT_INIT_PREALLOC
+
 
 /*
   Prepare server side statement with query.