tie then untie on a hash that is midway through iteration mangles the hash iterator state

[nwc10]

Description

tie then untie on a hash that is midway through iteration mangles the hash iterator state. After the untie where hash iteration resumes is not consistent, and depends on whether tie chanced to happen when the iterator was midway down a hash chain.

Steps to Reproduce

$ cat collide.pl
use strict;
use warnings;
use Hash::Util qw(used_buckets);
require Tie::Hash;

sub testit {
    my @keys = @_;
    my %h;
    ++$h{$_} for @keys;

    my $k1 = each %h;

    tie %h, 'Tie::StdHash';
    untie %h;

    my @have;
    while (1) {
        my $k = each %h;
        last
            unless defined $k;
        push @have, $k;
    }

    print "For @keys, have @have\n";
    return;
}

LOOP:
for my $x ( "A" .. "Z" ) {
    for my $y ( chr( ord($x) + 1 ) .. "Z" ) {
        my %h;
        ++$h{$x};
        ++$h{$y};
        if (used_buckets %h == 1) {
            testit($x, $y);
            last LOOP;
        }
    }
}

testit('A', 'B');

__END__
$ perl collide.pl
For A F, have
For A B, have B

(Note that for this run A and F were the first pair of keys that were found to collide into the same hash chain, and (fortunately) A and B did not collide)

Expected behaviour

Expected behaviour would be "something consistent" - whether the keys collide or not the state is the same.

The bug is in this code in pp_tie:

        case SVt_PVHV:
        {
            HE *entry;
            methname = "TIEHASH";
            if (HvLAZYDEL(varsv) && (entry = HvEITER_get((HV *)varsv))) {
                HvLAZYDEL_off(varsv);
                hv_free_ent((HV *)varsv, entry);
            }
            HvEITER_set(MUTABLE_HV(varsv), 0);
            break;
        }

which (re)sets EITER but not RITER when the hash is tied, meaning that RITER is unchanged.

Fix coming once I have an issue ID.

[nwc10]

Fixed by 364906c