Fix JRuby compatibility

[c-lliope]

See the comments on #27

• Ran into prolems identifying the line and file of instantiation
  for traceable null objects because of slight differences in
  MRI's and JRuby's backtraces.
  • Create a RubyEngine module to handle slight differences like
    the above between different ruby engines.
• JRuby's backtrace doesn't give a clear indication of when methods
  are called from inside blocks.
  • It's not vital that this information be tracked by
    pebble objects, so we'll drop the specs that try to verify that.
• JRuby's backtrace also prints out a different trace for methods:
  "from calling_method" instead of "in calling_method".
  • Drop the preposition and just verify against the method name

[coveralls]

Coverage increased (+0.0%) when pulling c2da4d4 on graysonwright:jruby into 5641a7c on avdi:master.

[coveralls]

Coverage remained the same when pulling 9a86eae on graysonwright:jruby into 5641a7c on avdi:master.

[c-lliope]

I had to delete two specs that just weren't compatible with JRuby:

  context "when is called from a block" do
    it "prints the indication of a block" do
      expect(test_output).to receive(:puts).twice.
        with(/from block/)
      Caller.new.call_method_inside_block(null)
    end
  end

  context "when is called from many levels blocks" do
    it "prints the indication of blocks and its levels" do
      expect(test_output).to receive(:puts).exactly(4).times.
        with(/from block \(2 levels\)/)
      Caller.new.call_method_inside_nested_block(null)
    end
  end

They were failing because JRuby (and RBX) don't give a clear indication of blocks in their stacktrace.

As an example, the following code:

def foo
  2.times { raise 'ohno' }
end

foo

when run in the three different environments, give these stack traces:

MRI:

RuntimeError: ohno
    from (irb):2:in `block in foo'
    from (irb):2:in `times'
    from (irb):2:in `foo'
    from (irb):4

JRuby:

RuntimeError: ohno
    from (irb):2:in `foo'
    from org/jruby/RubyFixnum.java:280:in `times'
    from (irb):2:in `foo'
    from (irb):4:in `evaluate'
    ...

RBX:

RuntimeError: ohno
    from (irb):2:in `foo'
    from kernel/common/integer.rb:83:in `times'
    from (irb):2:in `foo'
    from (irb):4
    ...

They all give similar information, but the specs are looking for a specific indication that the call is coming from inside a block.

I don't think it's really necessary for Pebble to collect detailed information on the blocks, so I decided to scrap those specs. Pebbles will collect slightly less information on JRuby/RBX, but still capture the method name that the pebble was called from, etc.

Let me know if you think those specs were necessary.

[avdi]

Looks like @sferik has included JRuby support in his 1.8 compatibility PR. Thanks for getting the ball rolling!

[sferik]

I came to the same conclusion but with a somewhat different style implementation. If @avdi prefers your version, I’m happy to merge in this part of the pull request.