/usr/share/dirsrv/updates/90subtreerename.pl is in 389-ds-base 1.3.2.16-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57  | use Mozilla::LDAP::Conn;
use Mozilla::LDAP::Utils qw(normalizeDN);
use Mozilla::LDAP::API qw(:constant ldap_url_parse ldap_explode_dn);
use DSUpdate qw(isOffline);
sub runinst {
    my ($inf, $inst, $dseldif, $conn) = @_;
    my $rc, @errs;
    ($rc, @errs) = isOffline($inf, $inst, $conn);
    if (!$rc) {
        return @errs;
    }
    my $ent0 = $conn->search("cn=config", "base", "(objectclass=*)");
    if (!$ent0) {
        return ('error_finding_config_entry', 'cn=config',
                $conn->getErrorString());
    }
    my $ent1 = $conn->search("cn=config,cn=ldbm database,cn=plugins,cn=config",
                            "base", "(objectclass=*)");
    if (!$ent1) {
        return ('error_finding_config_entry',
                'cn=config,cn=ldbm database,cn=plugins,cn=config',
                $conn->getErrorString());
    }
    # Get the value of nsslapd-subtree-rename-switch.
    my $need_update = 0;
    my $switch = $ent1->getValues('nsslapd-subtree-rename-switch');
    if ("" eq $switch) {
        $ent1->addValue('nsslapd-subtree-rename-switch', "on");
        $need_update = 1;
    } elsif ("off" eq $switch || "OFF" eq $switch) {
        $ent1->setValues('nsslapd-subtree-rename-switch', "on");
        $need_update = 1;
        $conn->update($ent1);
    }
    if (1 == $need_update) {
        $conn->update($ent1);
        # Convert the database format from entrydn to entryrdn
        my $instdir = $ent0->getValue('nsslapd-instancedir');
        my $prog = $instdir . "/dn2rdn";
        my $output = `$prog 2>&1`;
        my $stat = $?;
        if (0 != $stat) {
            $ent1->setValues('nsslapd-subtree-rename-switch', "off");
            $conn->update($ent1);
        }
    }
    return ();
}
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