shared tempfile was not removed on statement_timeout (unreproducible)

I have a nagios check on ancient tempfiles, intended to catch debris left by
crashed processes.  But triggered on this file:

$ sudo find /var/lib/pgsql/12/data/base/pgsql_tmp -ls
142977    4 drwxr-x---   3 postgres postgres     4096 Dec 12 11:32 /var/lib/pgsql/12/data/base/pgsql_tmp
169868    4 drwxr-x---   2 postgres postgres     4096 Dec  7 01:35
/var/lib/pgsql/12/data/base/pgsql_tmp/pgsql_tmp11025.0.sharedfileset
169347 5492 -rw-r-----   1 postgres postgres  5619712 Dec  7 01:35
/var/lib/pgsql/12/data/base/pgsql_tmp/pgsql_tmp11025.0.sharedfileset/0.0
169346 5380 -rw-r-----   1 postgres postgres  5505024 Dec  7 01:35
/var/lib/pgsql/12/data/base/pgsql_tmp/pgsql_tmp11025.0.sharedfileset/1.0

I found:
 2019-12-07 01:35:56 | 11025 | postgres | canceling statement due to statement timeout
        | CLUSTER pg_stat_database_snap USI
 
 2019-12-07 01:35:56 | 11025 | postgres | temporary file: path "base/pgsql_tmp/pgsql_tmp11025.0.sharedfileset/2.0",
size5455872 | CLUSTER pg_stat_database_snap USI
 

I don't have a saved log entry for sharedfileset/0.0 - that may be significant,
or may be a bug in my log-archiving script.

The process has not crashed since it started:

postgres 18145     1  0 Nov18 ?        00:51:39 /usr/pgsql-12/bin/postmaster -D /var/lib/pgsql/12/data
postgres 18147 18145  0 Nov18 ?        00:00:53 postgres: logger                                      
postgres 18149 18145  0 Nov18 ?        00:38:42 postgres: checkpointer                                

version | PostgreSQL 12.1 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-39), 64-bit

Just to be sure:
$ ps -wwf 11025 || echo not running
UID        PID  PPID  C STIME TTY      STAT   TIME CMD
not running

I wasn't able to reproduce it like this.
PGOPTIONS='-c maintenance_work_mem=128MB -c client_min_messages=debug' psql postgres -c 'CREATE TABLE t (i int unique);
INSERTINTO t SELECT generate_series(1,999999)' -c 'SET statement_timeout=4999' -c 'CLUSTER t USING t_i_key'
 

Actually, I tried using pg_ls_tmpdir(), but it unconditionally masks
non-regular files and thus shared filesets.  Maybe that's worth discussion on a
new thread ?

src/backend/utils/adt/genfile.c
                /* Ignore anything but regular files */
                if (!S_ISREG(attrib.st_mode))
                        continue;

BTW there's no other tablespaces.

Justin