Resolving a persistent locking issue hours after it occurred

A fellow DBA supported an application that had a process that compiled a number of packages every night as part of a scheduled script. Sample command:

ALTER PACKAGE schema.package_name COMPILE PACKAGE;
 
ERROR at line 1:
ORA-04021: timeout occurred while waiting to lock object
 

Over the last couple of days this script began to fail every night. I used the below SQL to identify the blocking session:

select    * 
from      gv$active_session_history
where     sample_time > sysdate -  (interval '1' day)
  and     sql_opname = 'ALTER PACKAGE'
  and not blocking_Session is null
order by  sample_time
;

This SQL returned the information identifying the blocks with the sample time exactly matching the script’s scheduled run time. I used the sample time, blocking session id, session serial and instance id and ran the below SQL:

select * 
from   gv$active_session_history
where sample_time between 
                  to_date('07-AUG-17 08.01 PM',
                          'dd-mon-yy hh.mi pm') 
                  and 
                  to_date('07-AUG-17 09.32 PM',
                          'dd-mon-yy hh.mi pm')
  and  session_id      = 
  and session_serial# = 
  and inst_id         = 
order by sample_time
;

This provided me the last sql_id that the blocking session had executed.

select  * 
from    gv$sql
where   inst_id = 
  and   sql_id in 'sql id'
;

The last SQL that the blocking session had executed happened to be a select. However, working on the assumption that it had previously performed a DML, I cancelled the session and the ALTER PACKAGES script was able to execute successfully.