pp_tie should completely reset the underlying hash's iterator state.

Previously it would mangle it, resetting EITER but not RITER, meaning that
after untie continuing iteration would be inconsistent - normally it would
carry on exactly where it left off, but if iteration had been in the middle
of a chain of HEs, it would skip the rest of the chain.

Fixes GH #19077

pp_sys.c

@@ -881,6 +881,7 @@ PP(pp_tie)
                 hv_free_ent((HV *)varsv, entry);
             }
             HvEITER_set(MUTABLE_HV(varsv), 0);
+            HvRITER_set(MUTABLE_HV(varsv), -1);
             break;
         }
         case SVt_PVAV:

t/op/tiehash.t

@@ -72,4 +72,82 @@ package TestIterators {
     is(each %h, undef, "third iterator is undef");
 }
 
+{
+    require Tie::Hash;
+
+    my %h = (
+        lolcat => "OH HAI!",
+        lolrus => "I HAS A BUCKET",
+    );
+
+    my @want = sort keys %h;
+
+    my @have;
+    while (1) {
+        my $k = each %h;
+        last
+            unless defined $k;
+        push @have, $k;
+    }
+    @have = sort @have;
+
+    # This is a sanity test:
+    is("@have", "@want", "get all keys from a loop");
+
+    @have = ();
+    keys %h;
+
+    my $k1 = each %h;
+
+    ok(defined $k1, "Got a key");
+
+    # no tie/untie here
+
+    while(1) {
+        my $k = each %h;
+        last
+            unless defined $k;
+        push @have, $k;
+    }
+
+    # As are these:
+    is(scalar @have, 1, "just 1 key from the loop this time");
+    isnt($k1, $have[0], "two different keys");
+
+    @have = sort @have, $k1;
+    is("@have", "@want", "get all keys just once");
+
+    # And this is the real test.
+    #
+    # Previously pp_tie would mangle the hash iterator state - it would reset
+    # EITER but not RITER, meaning that if the iterator happened to be partway
+    # down a chain of entries, the rest of that chain would be skipped, but if
+    # the iterator's next position was the start of a (new) chain, nothing would
+    # be skipped.
+    # We don't have space to store the complete older iterator state (and really
+    # nothing should be relying on it), so it seems better to correctly reset
+    # the iterator (every time), than leave it in a mess just occasionally.
+
+    @have = ();
+    keys %h;
+
+    my $k1 = each %h;
+
+    ok(defined $k1, "Got a key");
+
+    tie %h, 'Tie::StdHash';
+    untie %h;
+
+    while(1) {
+        my $k = each %h;
+        last
+            unless defined $k;
+        push @have, $k;
+    }
+
+    @have = sort @have;
+    is(scalar @have, 2, "2 keys from the loop this time");
+    is("@have", "@want", "tie/untie resets the hash iterator");
+}
+
 done_testing();