NoMethodError for ObjectSpace::WeakMap#values

[yahonda]

Environment Information

• JRuby version (jruby -v) and command line (flags, JRUBY_OPTS, etc)

% jruby -v
jruby 9.2.11.1 (2.5.7) 2020-03-25 b1f55b1a40 OpenJDK 64-Bit Server VM 11.0.2+9 on 11.0.2+9 +jit [darwin-x86_64]

• Operating system and platform (e.g. uname -a)

% uname -a
Darwin mymacbookpro 19.6.0 Darwin Kernel Version 19.6.0: Sun May 17 22:15:23 PDT 2020; root:xnu-6153.140.21~10/RELEASE_X86_64 x86_64

Expected Behavior

• Steps to reproduce

weakmap = ObjectSpace::WeakMap.new
weakmap.values

weakmap.values should return empty Array as CRuby does.

• Result with CRuby 2.7.1

% ruby -v
ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-darwin19]
% irb
irb(main):001:0> weakmap = ObjectSpace::WeakMap.new
irb(main):002:0> weakmap.values
=> []
irb(main):003:0>

Actual Behavior

It raises NoMethodError

% ruby -v
jruby 9.2.11.1 (2.5.7) 2020-03-25 b1f55b1a40 OpenJDK 64-Bit Server VM 11.0.2+9 on 11.0.2+9 +jit [darwin-x86_64]
% irb
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by com.headius.backport9.modules.Modules to method sun.nio.ch.NativeThread.signal(long)
WARNING: Please consider reporting this to the maintainers of com.headius.backport9.modules.Modules
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
irb(main):001:0> weakmap = ObjectSpace::WeakMap.new
=> #<ObjectSpace::WeakMap:0x189b5fb1>
irb(main):002:0> weakmap.values
Traceback (most recent call last):
        6: from /Users/yahonda/.rbenv/versions/jruby-9.2.11.1/bin/irb:13:in `<main>'
        5: from org/jruby/RubyKernel.java:1189:in `catch'
        4: from org/jruby/RubyKernel.java:1189:in `catch'
        3: from org/jruby/RubyKernel.java:1442:in `loop'
        2: from org/jruby/RubyKernel.java:1048:in `eval'
        1: from (irb):2:in `evaluate'
NoMethodError (undefined method `values' for #<ObjectSpace::WeakMap:0x189b5fb1>)
irb(main):003:0>

[yahonda]

This is required to support rsim/oracle-enhanced#2027

[yahonda]

Let me update that his issue needs to be addressed to support rails/rails#39121 .

[headius]

Should be easy enough to support.

[headius]

Ok, so I don't have a problem with adding the missing features from Weakmap, but officially it is not a public API. It has never been discussed (naming, behavior, etc) and I believe that needs to happen before it sees usage in third-party libraries. Unfortunately that would mean this should not be used until after Ruby 2.8/3.0, though JRuby would probably implement everything that CRuby provides in our 9.3 release.

https://bugs.ruby-lang.org/issues/16959

I'd like to get some feedback on this before we proceed to implement the rest of Weakmap just because CRuby does it that way.

[headius]

FWIW I don't fault you for using the feature... it's sorely needed in Ruby, as I've pointed out in my ReferenceQueue issue (linked from the Ruby issue above), and now that there's specs it's hard to tell that it's not a public API.

[yahonda]

Thanks for the update. I with ObjectSpace::WeakMap#values will be implemented for JRuby.

[headius]

FWIW this can be implemented in Ruby using some JRuby internal APIs (non-public, but useful for cases like this):

require 'jruby'

class org::jruby::RubyObjectSpace::WeakMap
  field_reader :map
end

class ObjectSpace::WeakMap
  def values
    JRuby.ref(self).map.values.reject(&:nil?)
  end
end

# Example usage follows...

wm = ObjectSpace::WeakMap.new

x = nil

# This is inside a block to avoid references leaking to the main scope
tap {
  wm["foo"] = Object.new
  wm["bar"] = x = Object.new

  p wm.values # [#<Object:0x62e136d3>, #<Object:0x51b7e5df>]
}

5.times { JRuby.gc }

p wm.values # [#<Object:0x62e136d3>]

The reject is necessary because the data structure in map does not omit vacated references (I assume the CRuby implementation does this filtering interally too).