`pg_ls_dir` can query some directories, but not others

[Brennan Vincent <brennan@xxxxxxxxxxxxxx>]

Copying here a question I asked on StackOverflow:
https://stackoverflow.com/questions/58846076

=======================================

On my system, `/home` and `/etc` have exactly the same permissions:

```
$ ls -ld /home /etc
drwxr-xr-x 67 root root 4096 Nov 13 15:59 /etc
drwxr-xr-x  3 root root 4096 Oct 18 13:45 /home
```

However, Postgres can read one, but not the other:

```
test=# select count(*) from (select pg_ls_dir('/etc')) a;
count 
-------
  149
(1 row)

test=# select count(*) from (select pg_ls_dir('/home')) a;
ERROR:  could not open directory "/home": Permission denied
```

Even though the user the DB is running as can, in fact, run `ls /home`:
```
$ sudo -u postgres ls /home > /dev/null && echo "ls succeeded"
ls succeeded
```

What is going on?

My postgres version is 11.5, running on Arch Linux.