Syntax:
MASTER_POS_WAIT(log_name,log_pos[,timeout][,channel])

This function is for control of source/replica synchronization. It
blocks until the replica has read and applied all updates up to the
specified position in the source's binary log. From MySQL 8.0.26,
MASTER_POS_WAIT() is deprecated and the alias SOURCE_POS_WAIT() should
be used instead. In releases before MySQL 8.0.26, use
MASTER_POS_WAIT().

The return value is the number of log events the replica had to wait
for to advance to the specified position. The function returns NULL if
the replication SQL thread is not started, the replica's source
information is not initialized, the arguments are incorrect, or an
error occurs. It returns -1 if the timeout has been exceeded. If the
replication SQL thread stops while MASTER_POS_WAIT() is waiting, the
function returns NULL. If the replica is past the specified position,
the function returns immediately.

If the binary log file position has been marked as invalid, the
function waits until a valid file position is known. The binary log
file position can be marked as invalid when the CHANGE REPLICATION
SOURCE TO option GTID_ONLY is set for the replication channel, and the
server is restarted or replication is stopped. The file position
becomes valid after a transaction is successfully applied past the
given file position. If the applier does not reach the stated position,
the function waits until the timeout. Use a SHOW REPLICA STATUS
statement to check if the binary log file position has been marked as
invalid.

On a multithreaded replica, the function waits until expiry of the
limit set by the replica_checkpoint_group, slave_checkpoint_group,
replica_checkpoint_period or slave_checkpoint_period system variable,
when the checkpoint operation is called to update the status of the
replica. Depending on the setting for the system variables, the
function might therefore return some time after the specified position
was reached.

If binary log transaction compression is in use and the transaction
payload at the specified position is compressed (as a
Transaction_payload_event), the function waits until the whole
transaction has been read and applied, and the positions have updated.

If a timeout value is specified, MASTER_POS_WAIT() stops waiting when
timeout seconds have elapsed. timeout must be greater than 0; a zero or
negative timeout means no timeout.

The optional channel value enables you to name which replication
channel the function applies to. See
https://dev.mysql.com/doc/refman/8.0/en/replication-channels.html for
more information.

URL: https://dev.mysql.com/doc/refman/8.0/en/miscellaneous-functions.html

