R12 E-Business Suite Output Post Processor (OPP) Fails To Pick Up Concurrent Requests With Error 'Unable to find an Output Post Processor service to post-process request nnnnn'

E-Business Suite R12 Concurrent Processing, Output Post Processing related issues
Requests fail during Post Processing with the following errors:

'Unable to find an Output Post Processor service to post-process request xxxxx.

Check that the Output Post Processor service is running'

Cause

Incorrect data is present in the OPP queue table AQ$FND_CP_GSM_OPP_AQTBL_S, which occurs when the concurrent managers are not shutdown correctly.

A quick sql query shows orphaned OPP subscribers in APPLSYS.AQ$FND_CP_GSM_OPP_AQTBL_S, causing the select_random_subscriber routine to pick a non-running process.

Solution

To resolve the issue test the following steps in a development instance and then migrate accordingly:
1. Find the active processes for OPP:

SELECT fcp.concurrent_process_id
FROM fnd_concurrent_queues fcq, fnd_concurrent_processes fcp
WHERE concurrent_queue_name = 'FNDCPOPP'
AND fcq.concurrent_queue_id = fcp.concurrent_queue_id
AND fcq.application_id = fcp.queue_application_id
AND fcp.process_status_code = 'A';

Output example:
CONCURRENT_PROCESS_ID
---------------------
93557
93558
2. Find the OPP subscribers:

select name from APPLSYS.AQ$FND_CP_GSM_OPP_AQTBL_S;

The name in the table is the concurrent_process_id prefixed with 'OPP'.
The output should normally correspond to the output of first query: have the same number of records and have as name the concurrent_process_id of the running OPP processes prefixed by OPP
Output example:
NAME
------------------------------
OPP93558
OPP93557
OPP88933
OPP92625
3. If there are extra subscribers which are left from previous runs of OPP processes, unsubscribe them via the following command:

  exec fnd_cp_opp_ipc.unsubscribe('<concurrent_process_id>');

 Example:
  exec fnd_cp_opp_ipc.unsubscribe('88933');
  exec fnd_cp_opp_ipc.unsubscribe('92625');
4. Bounce the concurrent managers.

5. Retest some concurrent requests and confirm the post processing actions now complete successfully.