ExceptionManager gem

ExceptionManager is a gem for getting extra information from your exception.

Source code: https://github.com/iliabylich/exception_manager

With this gem every time when you get an exception, it’s possible to grab subject of exception (the instance of class where raise happened), locals - local variables, subject_instance_variables and subject_class_variables

Examples:

require 'exception_manager'
ExceptionManager.enable!

class TestClassThatRaisesException
  @@class_variable = :class_value

  def test_error(*args)
    @instance_variable = :instance_value

    raise 'Test error'
  end
end

begin
  TestClassThatRaisesException.new.test_error(1, 2, 3)
rescue => e
  puts "Subject: #{e.subject}"
  puts "Locals: #{e.locals}"
  puts "Instance variables: #{e.subject_instance_variables}"
  puts "Class variables: #{e.subject_class_variables}"
  puts "Summary: #{e.summary.inspect}"
end

This code snippet prints:

Subject: #<TestClassThatRaisesException:0x00000001512268>
Locals: {:args=>[1, 2, 3]}
Instance variables: {:@instance_variable=>:instance_value}
Class variables: {:@@class_variable=>:class_value}
Summary: {
  :locals => {
    :klass => TestClassThatRaisesException,
    :args=> [1, 2, 3]
  },
  :subject => #<TestClassThatRaisesException:0x00000001512268>,
  :subject_instance_variables => {
    :@instance_variable => :instance_value
  },
  :subject_class_variables => {
    :@@class_variable => :class_value
  }
}

So, you can get local variables of you exception, instance where it happened and the whole context.

If you have pry installed, you can inject pry session into stored binding:

begin
  TestClassThatRaisesException.new.test_error(1, 2, 3)
rescue => e
  e._binding.pry
end

 Integration with NewRelic

Just use the following snippet:

class StandardError
  def original_exception
    message_for_new_relic = [message, summary.inspect].join(' ')
    self.exception(message_for_new_relic)
  end
end

NewRelic gem by default calls .original_exception.to_s on every caught exception. Yes, looks dirty, but saves a lot of time!

 How it works

In old versions of ruby there was a Kernel method called set_trace_func which allows to set trigger for every executed line (including c-calls).

Ruby 2.0.0 (and newer) has a class called TracePoint which allows to subscribe to any specific method call (raise in our case).

So, simplified version of the main code looks like:

class Exception
  attr_accessor :_binding
end

TracePoint.new(:raise) do |tp|
  tp.raised_exception._binding = tp.binding
end

And then we can get everything through this binding

 Compatibility

ExceptionManager is compatible (and tested on travis) with the following versions of ruby

(In other words, in all versions of ruby that has TracePoint class).

 Questions

If you have any questions/suggestions feel free to create issues on GitHub.
https://github.com/iliabylich/exception_manager/issues

 
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