[ruby-core:104257] [Ruby master Bug#17951] Collistions in Proc#hash values for blocks defined at the same line

Issue #17951 has been updated by xtkoba (Tee KOBAYASHI).


A possible fix:

```diff
--- a/proc.c
+++ b/proc.c
@@ -1451,7 +1451,7 @@ rb_hash_proc(st_index_t hash, VALUE prc)
     GetProcPtr(prc, proc);
     hash = rb_hash_uint(hash, (st_index_t)proc->block.as.captured.code.val);
     hash = rb_hash_uint(hash, (st_index_t)proc->block.as.captured.self);
-    return rb_hash_uint(hash, (st_index_t)proc->block.as.captured.ep >> 16);
+    return rb_hash_uint(hash, (st_index_t)proc->block.as.captured.ep);
 }
 
 MJIT_FUNC_EXPORTED VALUE
```

I do not understand the meaning of the 16-bit right shift in the original code.

----------------------------------------
Bug #17951: Collistions in Proc#hash values for blocks defined at the same line
https://bugs.ruby-lang.org/issues/17951#change-92436

* Author: decuplet (Nikita Shilnikov)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-darwin20]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
```ruby
require 'set'

def capture(&block)
  block
end

# it creates 1k of same blocks
blocks = Array.new(1000) { capture { :foo } }

hashes = blocks.map(&:hash).uniq
ids = blocks.map(&:object_id).uniq
equality = blocks.map { blocks[0].eql?(_1) }.tally
hash = blocks.to_h { [_1, nil] }
set = blocks.to_set

puts(hashes.size)      # => 11
puts(ids.size)         # => 1000
puts(equality.inspect) # => {true=>1, false=>999}
puts(hash.size)        # => 1000
puts(set.size)         # => 1000
```

The script builds one thousand blocks and then compares them in various ways. I would expect proc objects to be completely opaque and thus be treated as separate objects. As in, they are not equal. All tests but first confirm this expectation. However, `Proc#hash` doesn't return 1000 different results rather it varies between 3 and 20 on my machine.

As I understand, current behavior doesn't violate ruby's guarantees. But I would expect `Proc#hash` results to be as unique as `Proc#object_id`, at least a lot more unique than they currently are.

The problem is likely to occur only for blocks defined at the same line.

ref to similar/related issue https://bugs.ruby-lang.org/issues/6048



-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:[e-mail]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>