I know what you are thinking!
Why make this more complicate than it needs to be and is an ounce of prevention worth a pound of cure?
Oracle support started patching Oracle Exadata Cloud@Customer (ExaCC) environment.
After patching, the listener did not start because there were entries in the listener referencing database and oracle home that have been removed.
There are multiple database homes for the same database versions and this is how it was implemented.
Primary database (DB_ASHBURN) was used to create a second standby (DB_PHOENIX).
The primary database (DB_ASHBURN) was switchover to second standby (DB_PHOENIX)
DB_PHOENIX is now the new primary and DB_ASHBURN is the standby.
DB_ASHBURN (standby) was decommissioned and ORACLE_HOME was removed.
Unfortunately, listener.ora was not modified and failed to start after patching was completed.
Here is an example for LISTENER failed to start.
[oracle@ol7-112-dg1 admin]$ lsnrctl start
LSNRCTL for Linux: Version 11.2.0.4.0 - Production on 18-APR-2021 16:19:49
Copyright (c) 1991, 2013, Oracle. All rights reserved.
Starting /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/tnslsnr: please wait...
TNSLSNR for Linux: Version 11.2.0.4.0 - Production
System parameter file is /u01/app/oracle/product/11.2.0.4/dbhome_1/network/admin/listener.ora
Log messages written to /u01/app/oracle/diag/tnslsnr/ol7-112-dg1/listener/alert/log.xml
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=ol7-112-dg1.local)(PORT=1521)))
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC1521)))
TNS-01201: Listener cannot find executable /u01/app/oracle/product/11.2.0.4/dbhome_2/bin/oracle for SID DB_PHOENIX
[oracle@ol7-112-dg1 admin]$
---------------------------------------------------------
--- ORACLE_HOME may be in /etc/oratab but does not exist.
---------------------------------------------------------
[oracle@ol7-112-dg1 admin]$ sort -u -t : -k 2,2 /etc/oratab | grep -v "^#" | awk -F ":" '{print $2}'
/u01/app/oracle/product/11.2.0.4/dbhome_1
/u01/app/oracle/product/11.2.0.4/dbhome_2
[oracle@ol7-112-dg1 admin]$ ls -ld /u01/app/oracle/product/11.2.0.4/
drwxr-xr-x. 3 oracle oinstall 22 Apr 14 18:29 /u01/app/oracle/product/11.2.0.4/
[oracle@ol7-112-dg1 admin]$
Having separate listeners, LISTENER started without issues
[oracle@ol7-112-dg1 admin]$ lsnrctl start LISTENER
LSNRCTL for Linux: Version 11.2.0.4.0 - Production on 18-APR-2021 16:32:24
Copyright (c) 1991, 2013, Oracle. All rights reserved.
Starting /u01/app/oracle/product/11.2.0.4/dbhome_1/bin/tnslsnr: please wait...
TNSLSNR for Linux: Version 11.2.0.4.0 - Production
System parameter file is /u01/app/oracle/product/11.2.0.4/dbhome_1/network/admin/listener.ora
Log messages written to /u01/app/oracle/diag/tnslsnr/ol7-112-dg1/listener/alert/log.xml
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=ol7-112-dg1.local)(PORT=1521)))
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC1521)))
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=ol7-112-dg1.local)(PORT=1521)))
STATUS of the LISTENER
------------------------
Alias LISTENER
Version TNSLSNR for Linux: Version 11.2.0.4.0 - Production
Start Date 18-APR-2021 16:32:25
Uptime 0 days 0 hr. 0 min. 0 sec
Trace Level off
Security ON: Local OS Authentication
SNMP OFF
Listener Parameter File /u01/app/oracle/product/11.2.0.4/dbhome_1/network/admin/listener.ora
Listener Log File /u01/app/oracle/diag/tnslsnr/ol7-112-dg1/listener/alert/log.xml
Listening Endpoints Summary...
(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=ol7-112-dg1.local)(PORT=1521)))
(DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC1521)))
Services Summary...
Service "hawk" has 1 instance(s).
Instance "hawk", status UNKNOWN, has 1 handler(s) for this service...
Service "hawk_DGMGRL" has 1 instance(s).
Instance "hawk", status UNKNOWN, has 1 handler(s) for this service...
The command completed successfully
[oracle@ol7-112-dg1 admin]$
What’s your preference, having separate listeners to play it safe or decommissioning the environment properly and completely?