InnoDB generates redundant log checkpoints

The function log_checkpoint() or log_checkpoint_low() tries to avoid redundant writes. Here is the 10.3 version of log_checkpoint():

	if (oldest_lsn

	    > log_sys.last_checkpoint_lsn + SIZE_OF_MLOG_CHECKPOINT) {

		/* Some log has been written since the previous checkpoint. */

	} else if (srv_shutdown_state > SRV_SHUTDOWN_INITIATED) {

		/* MariaDB 10.3 startup expects the redo log file to be

		logically empty (not even containing a MLOG_CHECKPOINT record)

		after a clean shutdown. Perform an extra checkpoint at

		shutdown. */

	} else {

		/* Do nothing, because nothing was logged (other than

		a MLOG_CHECKPOINT marker) since the previous checkpoint. */

		log_mutex_exit();

		return(true);

	}

...

	if (fil_names_clear(flush_lsn, do_write)) {

This logic fails to take two things into account:

1. The function fil_names_clear() may write some file names before the checkpoint record.
2. Before MDEV-14425, the checkpoint record may be interrupted by 4+12 or 8+12 bytes of log block trailer and header.

In either case, the difference of oldest_lsn and log_sys.last_checkpoint_lsn could be more than the size of the checkpoint record.

The simplest way to fix this could be to avoid writing a new checkpoint if oldest_lsn did not change since the previous checkpoint.