Bug #14573: rb_ary_or doesn't check objects hash when the array contains less than SMALL_ARRAY_LEN

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-darwin17]
• Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN

[Willian (Willian van der Velde)]

Hello,

While debugging a build failure on a 2.5 branch of graphql-ruby (https://github.com/rmosolgo/graphql-ruby/pull/1303) we found out a difference in behavior for the Array's union operator.

On Ruby 2.5:

irb(main):001:0> class X; def initialize(h); @h = h; end; def hash; @h; end; def eql?(o); true; end; end
=> :eql?
irb(main):002:0> a = X.new(1) ; b = X.new(2)
=> #<X:0x00007fe0b00a4130 @h=2>
irb(main):003:0> a.hash
=> 1
irb(main):004:0> b.hash
=> 2
irb(main):005:0> [a] | [b]
=> [#<X:0x00007fe0b00a4158 @h=1>]

I would expect this union would result in two objects. On Ruby 2.3.6 I do get two objects:

irb(main):001:0> class X; def initialize(h); @h = h; end; def hash; @h; end; def eql?(o); true; end; end
=> :eql?
irb(main):002:0> a = X.new(1) ; b = X.new(2)
=> #<X:0x00007f816a80eec0 @h=2>
irb(main):003:0> a.hash
=> 1
irb(main):004:0> b.hash
=> 2
irb(main):005:0> [a] | [b]
=> [#<X:0x00007f816a80eee8 @h=1>, #<X:0x00007f816a80eec0 @h=2>]

I think this change is related to #13884, and it looks like when small arrays (less than SMALL_ARRAY_LEN) are given the object's hash isn't checked. If a bigger array is given we get a correct union:

irb(main):001:0> class X; def initialize(h); @h = h; end; def hash; @h; end; def eql?(o); true; end; end
=> :eql?
irb(main):002:0> a = 20.times.map { |i| X.new(i) }
=> ...
irb(main):003:0> b = 20.times.map { |i| X.new(100 + i) }
=> ...
irb(main):004:0> (a | b).size
=> 40

Thanks,

Willian

[Willian (Willian van der Velde)]

• ruby -v changed from ruby 2.6.0dev (2017-12-27 trunk 61500) [x86_64-darwin17] to ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-darwin17]

[tenderlovemaking (Aaron Patterson)]

I guess I can see how this is a behavior change, but these two objects shouldn't eql? each other. It seems like a bug in the eql? implementation of the X class. These instances won't deal with hash collisions correctly, so I'm not sure why we'd expect union operator to work too.

[lewispb (Lewis Buckley)]

Related to https://bugs.ruby-lang.org/issues/14263. We have posted a note there about our investigation and the inconsistent behaviour between small and larger arrays.

[jeremyevans0 (Jeremy Evans)]

• Related to Bug #14263: Array Intersection does not seem to use hash added

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Closed

I don't think this is a bug. If eql? returns true, hash must return the same value. Class X in this example violates that constraint, resulting in undefined behavior. If you fix class X, the behavior is correct in all ruby versions since at least 1.8:

class X; def initialize(h); @h = h; end; def hash; @h; end; def eql?(o); @h.eql?(o.instance_variable_get(:@h)); end; end