Bug #16503: "ret: 2, hash modified during iteration" error in Ruby 2.7.0

• Status: Closed
• Priority: Normal
• Assignee: -
• Target version: -
• ruby -v: ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux] (heroku-18 stack)
• Backport: 2.5: DONTNEED, 2.6: DONTNEED, 2.7: DONE

[jnchito (Junichi Ito)]

I am running a Rails app on Heroku. After upgrading to Ruby 2.7, I sometimes get ret: 2, hash modified during iteration error.

The same issue is reported here (I'm using meta-tags gem too):
https://github.com/kpumuk/meta-tags/issues/209

This gem deletes elements while iterating hash (HashWithIndifferentAccess actually), so I guess it might involve this error:
https://github.com/kpumuk/meta-tags/blob/v2.13.0/lib/meta_tags/renderer.rb#L168
https://github.com/kpumuk/meta-tags/blob/v2.13.0/lib/meta_tags/renderer.rb#L181
https://github.com/kpumuk/meta-tags/blob/v2.13.0/lib/meta_tags/meta_tags_collection.rb#L85

However, this error does not occur every time (sometimes works, sometimes fails), and it didn't occur in Ruby 2.6. So I'm wondering why. Do you have any idea on this?

[ko1 (Koichi Sasada)]

Could you make reproducible script?

[ko1 (Koichi Sasada)]

All right. I got a script:

loop{
  h = {a:1, b:2, c:3, d:4, e:5, f:6, g:7, h:8}
  h.each{|k,v| GC.start; h.delete(k)}
}

The patch is:

diff --git a/hash.c b/hash.c
index 878f2b1790..644f85d6da 100644
--- a/hash.c
+++ b/hash.c
@@ -974,6 +974,7 @@ ar_foreach_check(VALUE hash, st_foreach_check_callback_func *func, st_data_t arg
 
             switch (retval) {
               case ST_CHECK: {
+                  pair = RHASH_AR_TABLE_REF(hash, i);
                   if (pair->key == never) break;
                   ret = ar_find_entry_hint(hash, hint, key);
                   if (ret == RHASH_AR_TABLE_MAX_BOUND) {

This patch should be back-ported for Ruby 2.6.

Thank you for your report.

[ko1 (Koichi Sasada)]

This patch should be back-ported for Ruby 2.6.

sorry I mean 2.7. 2.6 doesn't have this problem.

[ko1 (Koichi Sasada)]

• Status changed from Open to Closed

350dafd56a9cff58d36303aeb7515ab41c5dbbb3

[jnchito (Junichi Ito)]

Thank you very much for your quick fix. I left a comment in the meta-tags issue:

https://github.com/kpumuk/meta-tags/issues/209

[nobu (Nobuyoshi Nakada)]

• Backport changed from 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN to 2.5: DONTNEED, 2.6: DONTNEED, 2.7: REQUIRED

[naruse (Yui NARUSE)]

• Backport changed from 2.5: DONTNEED, 2.6: DONTNEED, 2.7: REQUIRED to 2.5: DONTNEED, 2.6: DONTNEED, 2.7: DONE

ruby_2_7 8e8841f6bf58031a1fe5b0dbacb5a1fb442102df.