Launch postmortem debug on test failure

debug_helper.rb

require 'debug'

module DebugHelper
  class << self
    def debug_on_error
      yield
    rescue => e
      enter_postmortem_session(e)
      raise e
    end

    def enter_postmortem_session(error)
      error = error.cause while error.cause

      puts "Enter postmortem mode with #{error.inspect}"
      puts error.backtrace.map { |b| "\t#{b}" }
      puts "\n"

      DEBUGGER__::SESSION.enter_postmortem_session(error)
    end

    def enable!
      DEBUGGER__::SESSION.postmortem = true
    end
  end
end

if ENV['DEBUG_ON_ERROR']
  DebugHelper.enable!

  RSpec.configure do |c|
    c.around(:each) do |example|
      DebugHelper.debug_on_error do
        example.run
      end
    end

    c.after(:each) do |example|
      DebugHelper.enter_postmortem_session(example.exception) if example.exception
    end
  end
end

rspec_inline.rb

# USAGE: DEBUG_ON_ERROR=t ruby rspec_inline.rb

require 'bundler/inline'

gemfile do
  gem 'rspec'
  gem 'debug'
end

require_relative 'debug_helper'

RSpec.describe 'rspec-debug-on-error' do
  it 'raises exception' do
    1.undefined_method
  end

  it 'fails' do
    expect(true).to be false
  end
end

RSpec::Core::Runner.invoke