binding_of_caller, pure FFI version (no compiler needed)

binding_of_caller-ffi.rb

#!/usr/bin/env ruby

# =============================================================================
# Quick proof-of-concept of an FFI-based binding_of_caller (no compiling!)
#
# Author: Steve Shreeve <steve.shreeve@gmail.com>
#  Basis: binding_of_caller and ffi gems
#   Date: 28 May 2015
#
#  Legal: MIT License
# =============================================================================

# NOTE: FFI requires a dynamic library, it cannot be a static library
#
# To install a shared libruby using ruby-install, use this:
#
#   ruby-install ruby 2.2.2 -- --enable-shared --disable-install-doc
#
# To nuke everything if you're using ruby-install, use this:
#
#   rm -rf ~/.gem/ruby/2.2.2 ~/.rubies/ruby-2.2.2 ~/src/ruby-2.2.2

require 'ffi'

module FFI
  module Binding

    # Not sure the best way to refer to the dynamic library for ruby
    LIB_PATH ||= "#{ENV['HOME']}/.rubies/ruby-2.2.2/lib/libruby.2.2.0.dylib"

    extend FFI::Library
    ffi_lib LIB_PATH

    class Value
      class << self
        extend FFI::Library
        ffi_lib LIB_PATH

        attach_function :rb_obj_id  , [:pointer], :pointer
        attach_function :rb_num2long, [:pointer], :long

        def native_type
          FFI::Type::POINTER
        end

        def from_native(value, ctx=nil)
          id =
            if value.address & 0x0f == 0xe
              value.address >> 8
            else
              rb_num2long rb_obj_id value
            end
          ObjectSpace._id2ref id
        end

        def to_native(obj, ctx=nil)
          id = obj.__id__
          case obj
            when Symbol                                  then FFI::Pointer.new id << 8 | 0xe
            when Fixnum, FalseClass, TrueClass, NilClass then FFI::Pointer.new id
          else                                                FFI::Pointer.new id << 1
          end
        end
      end
    end

    typedef FFI::Type::Mapped.new(Value), :value

    attach_function :rb_debug_inspector_open, [callback([:pointer, :pointer], :pointer), :pointer], :value

    class Context < FFI::Struct
      layout :thread        , :pointer,
             :frame         , :pointer,
             :backtrace     , :pointer,
             :contexts      , :pointer,
             :backtrace_size, :long
    end

    Contexts = -> (ptr, _) { Context.new(ptr)[:contexts] }

    class << self
      def callers
        rb_debug_inspector_open(Contexts, nil).drop(3).map {|ary| ary[2]}
      end
    end
  end
end

class ::Binding
  def of_caller(n)
    c = ::FFI::Binding.callers
    n < c.size or raise "No such frame, gone beyond end of stack!"
    c[n]
  end
end

# ==[ Show that it works... ]==

outer = 10

class Z
  def z
    u = 10
    A.new.a
  end
end

class A
  def a
    y = 10
    B.new.b
  end
end

class B
  def b
    x = 10
    puts binding.of_caller(0).eval('local_variables')
    puts binding.of_caller(1).eval('local_variables')
    puts binding.of_caller(2).eval('local_variables')
    puts binding.of_caller(3).eval('local_variables')
    # puts binding.of_caller(400).eval('local_variables') # should throw an Exception
  end
end

Z.new.z

__END__

# output:
# => x
# => y
# => u
# => outer
# Exception