MDEV-29502 Clean up spider_db_seek_next() a bit

Also moved spider_conn_before_query() and spider_conn_after_query() to
be used by more places

And clean up other functions in the same file (spd_db_conn.cc).

storage/spider/spd_conn.cc

@@ -245,6 +245,24 @@ void spider_conn_done(
   DBUG_VOID_RETURN;
 }
 
+void spider_conn_before_query(ha_spider *spider, SPIDER_CONN *conn, int link_idx)
+{
+  pthread_mutex_assert_not_owner(&conn->mta_conn_mutex);
+  spider_conn_mutex_lock(conn);
+  conn->need_mon = &spider->need_mons[link_idx];
+  spider_conn_mutex_locked(conn);
+  conn->disable_connect_retry= TRUE;
+}
+
+int spider_conn_after_query(SPIDER_CONN *conn, int error_num, bool unlock)
+{
+  conn->disable_connect_retry= FALSE;
+  spider_conn_mutex_unlocking(conn);
+  if (unlock)
+    spider_conn_mutex_unlock(conn);
+  return error_num;
+}
+
 int spider_reset_conn_setted_parameter(
   SPIDER_CONN *conn,
   THD *thd

storage/spider/spd_conn.h

@@ -66,6 +66,39 @@ void spider_conn_done(
   SPIDER_CONN *conn
 );
 
+inline void spider_conn_mutex_lock(SPIDER_CONN *conn)
+{
+  pthread_mutex_lock(&conn->mta_conn_mutex);
+  SPIDER_SET_FILE_POS(&conn->mta_conn_mutex_file_pos);
+}
+
+inline void spider_conn_mutex_unlock(SPIDER_CONN *conn)
+{
+  SPIDER_CLEAR_FILE_POS(&conn->mta_conn_mutex_file_pos);
+  pthread_mutex_unlock(&conn->mta_conn_mutex);
+}
+
+inline void spider_conn_mutex_locked(SPIDER_CONN *conn)
+{
+  DBUG_ASSERT(!conn->mta_conn_mutex_lock_already);
+  DBUG_ASSERT(!conn->mta_conn_mutex_unlock_later);
+  conn->mta_conn_mutex_lock_already = TRUE;
+  conn->mta_conn_mutex_unlock_later = TRUE;
+}
+
+inline void spider_conn_mutex_unlocking(SPIDER_CONN *conn)
+{
+  DBUG_ASSERT(conn->mta_conn_mutex_lock_already);
+  DBUG_ASSERT(conn->mta_conn_mutex_unlock_later);
+  conn->mta_conn_mutex_lock_already = FALSE;
+  conn->mta_conn_mutex_unlock_later = FALSE;
+}
+
+void spider_conn_before_query(ha_spider *spider, SPIDER_CONN *conn,
+                              int link_idx);
+
+int spider_conn_after_query(SPIDER_CONN *conn, int error_num, bool clear);
+
 int spider_reset_conn_setted_parameter(
   SPIDER_CONN *conn,
   THD *thd