A while ago a fellow DBA asked me if the listener cached TLS certificates. My immediate answer was “Sure, not caching would hurt performance severely.”
But, I couldn’t be certain so I ran a trace on it.

As the listener.log shows I did connect three times using TLS enabled endpoint:

...
07-SEP-2018 11:05:30 * (CONNECT_DATA=(SERVICE_NAME=DEV1.localdomain)(CID=(PROGRAM=C:\app\oracle\product\client1830\bin\sqlplus.exe)(HOST=WIN2012CLI1)(USER=user1))) * (ADDRESS=(PROTOCOL=tcps)(HOST=192.168.56.29)(PORT=49211)) * establish * DEV1.localdomain * 0
07-SEP-2018 11:05:46 * (CONNECT_DATA=(SERVICE_NAME=DEV1.localdomain)(CID=(PROGRAM=C:\app\oracle\product\client1830\bin\sqlplus.exe)(HOST=WIN2012CLI1)(USER=user1))) * (ADDRESS=(PROTOCOL=tcps)(HOST=192.168.56.29)(PORT=49212)) * establish * DEV1.localdomain * 0
07-SEP-2018 11:05:48 * (CONNECT_DATA=(SERVICE_NAME=DEV1.localdomain)(CID=(PROGRAM=C:\app\oracle\product\client1830\bin\sqlplus.exe)(HOST=WIN2012CLI1)(USER=user1))) * (ADDRESS=(PROTOCOL=tcps)(HOST=192.168.56.29)(PORT=49213)) * establish * DEV1.localdomain * 0
...

During the entire time I had a listener trace and a “strace” on the “tnslsnr” process.
Going through the strace output I found the open calls for both wallet files (ewallet.p12 and cwallet.sso).

Line 419: open("/u01/app/oracle/etc/wallet/auth/ewallet.p12", O_RDONLY) = 19
Line 506: open("/u01/app/oracle/etc/wallet/auth/cwallet.sso", O_RDONLY) = 20

Then the listener maps anonymous memory and reads data from cwallet.sso (file descriptor 20).

Line 514: mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f0520168000
Line 517: read(20, "\272\333\241\211\10\to\264\306\247/w\217#\0n+[\t\371\v\266\244\230d\214e3\246ZV\22"..., 1149) = 1149
...
Line 531: read(20, "\241\370N8\0\0\0\6\0\0\0!\6\303\20]{\207\16_\246\247\3579'\234h\35I\301m="..., 4096) = 4096
...
Line 542: read(20, "\272\333\241\211\10\to\264\306\247/w\217#\0n+[\t\371\v\266\244\230d\214e3\246ZV\22"..., 4096) = 1149

Shortly thereafter, the listener closes the file handles and unmaps the anonymous memory.

Line 551: close(19)
Line 561: close(20)
Line 562: munmap(0x7f0520168000, 4096)            = 0

All this happens on the first incoming TLS connection request. After that it never touches any of the wallet files again.

The same can be observed in the listener.og: it opens/reads/closes the wallet file on the first incoming TLS connection request only.

Line 4473: CONNECTION REQUEST
Line 4627: snzdfo_open_file:Opening file /u01/app/oracle/etc/wallet/auth/ewallet.p12 with READ ONLY permissions
Line 4631: snzdfo_open_file:Opening file /u01/app/oracle/etc/wallet/auth/cwallet.sso with READ ONLY permissions
Line 4667: nztwOpenWallet:exit

I didn’t do any long running tests but I this proves that the listener does cache the certificate from the wallet (at least temporarily).