TL;DR Under certain circumstances, dynamically defined symbols may break keyword arguments in Ruby 2.2. This was fixed in Ruby 2.2.3 and 2.3.

Specifically, when …
- there is a method with several keyword arguments and a double-splat argument (e.g. def m(foo: 'bar, option: 'will be lost', **further_options))
- there is a dynamically created Symbol (e.g. 'culprit'.to_sym) that is created before the method is parsed
- the method gets called with both the option and a culprit keyword argument

… then the option keyword argument will be nil inside of #m.

Affected Ruby Versions

• Affected version: 2.2.0, 2.2.1, 2.2.2
• Unaffected versions: 1.x, 2.0, 2.1, 2.2.3+, 2.3+

How to Expose it

The following code exposes the bug. Save it to a file, make sure you have Ruby 2.2 and run it.

Copy
test_symbol = '__test'

# The bug only occurs when a symbol used in a keyword argument is dynamically
# added to the Ruby symbols table *before* Ruby first sees the keyword argument.
existing = Symbol.all_symbols.map(&:to_s).grep('__test')
raise "Symbol #{test_symbol} already exists in symbol table!" if existing.any?

'__test'.to_sym # breaks it
# :__test # does not break it

# GC.start # fixes it

# Why #eval?
# Without, Ruby would parse the symbols in this code into its symbol table
# before running the file, which prevents the bug.
eval <<-RUBY
  $hash = { __test: '__test', lost: 'lost', q: 'q' }

  def _report(name, value)
    puts name.to_s << ': ' << (value ? 'ok' : 'broken')
  end

  # Confirmed broken when:
  # - `lost` is the second keyword argument Oo
  # - there is a double-splat argument
  def vulnerable_method_1(p: 'p', lost: 'lost', **options)
    _report(__method__, lost)
  end

  def vulnerable_method_2(p: 'p', lost: 'lost', q: 'q', **options)
    _report(__method__, lost)
  end

  def immune_method_1(lost: 'lost', p: 'p', **options)
    _report(__method__, lost)
  end

  def immune_method_2(q: 'q', lost: 'lost', __test: '__test')
    _report(__method__, lost)
  end

  def immune_method_3(lost: 'lost', **options)
    _report(__method__, lost)
  end
RUBY

# Exposure #####################################################################

puts '', 'Broken when calling with a hash'
vulnerable_method_1($hash)
vulnerable_method_2($hash)
immune_method_1($hash)
immune_method_2($hash)
immune_method_3($hash)

puts '', 'Double splat (**) has no influence:'
vulnerable_method_1(**$hash)
vulnerable_method_2(**$hash)
immune_method_1(**$hash)
immune_method_2(**$hash)
immune_method_3(**$hash)

puts '', 'Hash order does not matter:'
inversed_hash = Hash[$hash.to_a.reverse]
vulnerable_method_1(inversed_hash)
vulnerable_method_2(inversed_hash)
immune_method_1(inversed_hash)
immune_method_2(inversed_hash)
immune_method_3(inversed_hash)

References

• Related: https://bugs.ruby-lang.org/issues/11027