Description

Singleton class scopes should make their own namespace for constants.

However, Ruby versions from Ruby 1.9.0 do not respect this specification.

Background and Problem¶

Singleton class is useful feature to define object specific methods, especially for Class objects.

obj = Object . new # Define something obj's singleton class. class << obj def foo end end # Idiom to make class methods class C class << self def foo # Define C.foo method end end end

A class syntax has another job: making new namespace, especially for constants.

class C CONST = 1 p CONST #=> 1 def foo p CONST #=> 1 end end

Singleton class defintion should also introduce namespace for constant.

obj = Object . new class << obj CONST = 1 def foo CONST end end p obj . foo #=> 1

No problem.

Problem is sharing a singleton class definition with multiple objects.

objs = [] $i = 0 2 . times { objs << obj = Object . new class << obj CONST = ( $i += 1 ) def foo CONST end end } p objs [ 0 ]. foo p objs [ 1 ]. foo

Please think about the answers (outputs).

The above code makes two singleton classes independently.

So that constant namespace should be made for each singleton classes.

In fact, before Ruby 1.9.0, this program outputs "1

2

". Maybe your answer is same.

However after Ruby 1.9.0, this program outputs "2

2

". This is a bug (not intentional behavior).

JRuby and Rubinius also output not correct answers (interestingly JRuby prints "1

1").

$ ruby -v t.rb ruby 2.3.0dev (2015-01-27 trunk 49421) [x86_64-linux] 2 2 $ jruby-1.7.19/bin/jruby -v t.rb jruby 1.7.19 (1.9.3p551) 2015-01-29 20786bd on OpenJDK 64-Bit Server VM 1.7.0_75-b13 +jit [linux-amd64] 1 1 $ jruby-9.0.0.0.pre1/bin/jruby -v t.rb jruby 9.0.0.0.pre1 (2.2.0p0) 2015-01-20 d537cab OpenJDK 64-Bit Server VM 24.75-b04 on 1.7.0_75-b13 +jit [linux-amd64] 1 1 $ rbx-2.5.2/bin/rbx -v t.rb rubinius 2.5.2 (2.1.0 7a5b05b1 2015-01-30 3.4 JI) [x86_64-linux-gnu] 2 2

I asked Matz and his answer is "This is a bug behavior".

Moreover, on the independent namespace can make new classes/modules.

obj = Object . new class << obj class X # make <singleton class::X> end end

and it also has problem with multiple definitions.

objs = [] $xs = [] $i = 0 2 . times { objs << obj = Object . new class << obj CONST = ( $i += 1 ) class X $xs << self CONST = ( $i += 1 ) def foo CONST end end def x X end end } p $xs #=> [#<Class:0x25d56cc>::X, #<Class:0x25d55f0>::X] p objs [ 0 ]. x #=> #<Class:0x25d55f0>::X <- should be #<Class:0x25d56cc>::X p objs [ 1 ]. x #=> #<Class:0x25d55f0>::X p $xs [ 0 ]. new . foo #=> 4 <- should be 2 p $xs [ 1 ]. new . foo #=> 4

This is a bug.

(BTW, mruby works correctly!)

Reason of this behavior¶

On MRI, the reason of this bug is wrong sharing a namespace with

multiple namespaces. On MRI, the term "CREF" is a name of namespace data

structure.

Simply saying, I couldn't recognize such case, sharing a namespace by

multiple namespaces. I was surprising that each singleton class

expression make their own namespace. So that I store CREF data into each

local ISeq (method bytecode). It means that each bytecode knows their

own CREF. I had believed that one bytecode only has one namespace (CREF).

But this assumption is not correct, as I described above.

I decided to renew this feature. ISeq data should not have their own

CREF, but method only should have. Push CREF onto each method frame

(value stack, same location of SVAR). There is a (not small) patch and I

will commit soon for Ruby 2.3.

Previous versions¶

This is bug fix, but it changes Ruby semantics largely. I'm not sure how

to treat it.

Matz said that "we can not expect how affect this change for existing applications, so that no

need for older versions".

Please discuss about it.

This bug was found during investigating [Bug #10871].