Investigate rspec-mocks incompatibility with add_method_tracer

[kaylareopelle]

When add_method_tracer is used on a method that is overridden in an RSpec test using allow_any_instance_of, the following error is raised:

Failures:

  1) Bowling#score with no strikes or spares does not like to be prepended
     Failure/Error:
       allow_any_instance_of(Bowling)
         .to receive(:hit)
         .and_return(100)

       Using `any_instance` to stub a method (hit) that has been defined on a prepended module (#<Module:0x0000000127e41240>) is not supported.
     # ./spec/bowling_spec.rb:12:in `block (3 levels) in <top (required)>'

add_method_tracer prepends onto the methods it traces to create segments and metrics.

This error occurs even when the :agent_enabled configuration is set to false.

A customer reported they would like to use add_method_tracer and allow_any_instance_of on the same method.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-277524

[kaylareopelle]

The example above was created with this class:

# lib/bowling.rb

require 'newrelic_rpm'

class Bowling
  attr_reader :score

  def initialize
    @score = 0
  end

  def hit(pin_count)
    @score += pin_count
  end

  add_method_tracer :hit
end

And this spec:

# spec/bowling_spec.rb

require 'bowling'

RSpec.describe Bowling, "#score" do
  context "with no strikes or spares" do
   # passes 
    it "sums the pin count for each roll" do
      bowling = Bowling.new
      20.times { bowling.hit(4) }
      expect(bowling.score).to eq 80
    end
   
    # fails with error
    it "does not like to be prepended" do
      allow_any_instance_of(Bowling)
        .to receive(:hit)
        .and_return(300)
    end
  end
end

With this Gemfile:

source 'https://rubygems.org'

gem "newrelic_rpm", "~> 9.10"
gem "rspec", "~> 3.13"

The example was drawn from: https://rspec.info/
It was executed using: $ NEW_RELIC_AGENT_ENABLED=false bin/rspec

[kaylareopelle]

The agent needs to be able to add method tracers in cases where the API is invoked before the agent is loaded. This means the behavior of add_method_tracer cannot be contingent upon whether the agent is enabled.

We have a feature request open that may get around this. #2701 would allow method tracers to be added by configuration rather than declaring them directly in your code.

For anyone who encounters this problem, we have a few workarounds to recommend:

1. You could refactor your tests to stub a single instance of a class rather than any instance of a class. The following code didn't raise the RSpec prepend error:

Option A:

    let(:bowling) do
      b = bowling.new

      allow(b).to receive(:hit).and_return(100)   
      
      b
    end

Option B:

      let(:bowling) { Bowling.new }

      before do
        allow(bowling)
          .to receive(:hit)
          .and_return(100)
      end

2. If you're using factory_bot, you could update the factory definition for the class to stub the return value.

3. Inside the spec/spec_helper.rb file, you could overwrite the add_method_tracer method to remove its logic before RSpec is configured. This could be done using a one-liner, like:

  class Module; def add_method_tracer(*_args); end; end