Standby WAL Cleanup?

I'm following through the workflow from primary to standby in our
environment. However the one thing I can't seem to find is how WAL files
are cleaned up from $PGDATA/pg_xlog once they are no longer needed on the
standby.

The standby has archive_cleanup_command calling pg_archivecleanup to remove
files from the incoming archive directory. My understanding is that files
from there are restored to the pg_xlog directory via the restore_command,
which jives with what I see in pg_xlog.

However something is cleaning up files from pg_xlog and I don't see any
parameter command or cron job that does so. My documentation and list
archive searching has come up empty so I thought I'd bug you guys yet again.

TL;DR: What cleans up restored WAL files from pg_xlog when they are no
longer needed?

Don.

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Don Seiler
www.seiler.us