This is the external_lock method.
The locking methods for mysql section in
lock.cc has additional comments on this
topic that may be useful to read.
This creates a lock on the table. If you are implementing a
storage engine that can handle transactions, look at
ha_innodb.cc to see how you will want to go
about doing this. Otherwise you should consider calling
flock() here.
Called from lock.cc by
lock_external() and
unlock_external(). Also called from
sql_table.cc by
copy_data_between_tables().