Bug #10943
Updated by ko1 (Koichi Sasada) over 9 years ago
# Abstract
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.
```ruby
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.
```ruby
class C
CONST = 1
p CONST #=> 1
def foo
p CONST #=> 1
end
end
```
Singleton class defintion should also introduce namespace for constant.
```ruby
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.
```ruby
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\n2\n". Maybe your answer is same.
However after Ruby 1.9.0, this program outputs "2\n2\n". This is a bug (not intentional behavior).
JRuby and Rubinius also output not correct answers (interestingly JRuby prints "1\n1").
```
$ 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.
```ruby
obj = Object.new
class << obj
class X
# make <singleton class::X>
end
end
```
and it also has problem with multiple definitions.
```ruby
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 $xs[0].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.
# Solution
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.
# Acknowledgement
This bug was found during investigating [Bug #10871].