About

Trip is a concurrent tracer that can pause and resume the code it is tracing. Trip yields control between two Fibers - typically the root Fiber and a Fiber that Trip creates. The process of yielding control back and forth between the two Fibers can be repeated until the code being traced has finished and exits normally. Trip is currently implemented using TracePoint.

Examples

Concurrency

A concurrent tracer

In the context of Trip - a concurrent tracer can be explained as a tracer that spawns a new Fiber to run, and trace a block of Ruby code. Trip then pauses the new Fiber when a condition is met, and yields control back to the root Fiber.

The root Fiber can then resume the tracer, and repeat this process until the new Fiber exits. While the new Fiber is paused, the root Fiber can examine event information and evaluate code in the Binding (context) of where an event occurred. The following example hopes to paint a clearer picture of what that means in practice:

require "trip"

module Stdout
  def self.write(message)
    puts(message)
  end
end

##
# Create a new Trip.
# Pause for events coming from "Stdout.write".
trip = Trip.new { Stdout.write("Ruby is") }
trip.pause_when { |event| event.self == Stdout && event.method_id == :write }

##
# Enter "Stdout.write" - then mutate a local
# variable while the tracer is paused.
event = trip.start
event.binding.eval("message << ' cool.'")

##
# Execute the "puts(message)" line, and pause
# for the return of "Stdout.write".
event = trip.resume

##
# Exit the "Stdout.write" method, and the
# tracer.
event = trip.resume

##
# Ruby is cool.

Filter

Events

Trip will listen for method call and return events from methods implemented in either C or Ruby by default. The first argument given to Trip.new can specify a list of event names to listen for other than the defaults. All events can be included by using Trip.new('*') { ... }. A full list of event names can be found in the Trip::Event docs. The following example listens for call and return events from Ruby methods, and excludes methods implemented in C:

require "trip"

def add(x, y)
  puts(x + y)
end

trip = Trip.new(%i[call return]) { add(20, 50) }
while event = trip.resume
  print event.name, " ", event.method_id, "\n"
end

##
# call add
# 70
# return add

Trip#pause_when

In the previous example we saw how to specify what events to listen for. The events specified by the first argument given to Trip.new decide what events will be made available to Trip#pause_when. By default Trip#pause_when will pause the tracer on call and return events from methods implemented in either C or Ruby.

The following example demonstrates how to pause the tracer when a new module / class is defined with the module Name or class Name syntax:

require "trip"

trip = Trip.new(%i[class]) do
  class Foo
  end

  class Bar
  end

  class Baz
  end
end

trip.pause_when(&:module_opened?)
while event = trip.resume
  print event.self, " class opened", "\n"
end

##
# Foo class opened
# Bar class opened
# Baz class opened

Analysis

Require count

The Trip#to_a method can perform a trace from start to finish, and then return an array of Trip::Event objects. The following example returns the number of files that Pry v0.14.1 requires, including duplicate calls to require, and without any plugins being used.

When we exclude require "pry" from the count, the number is 168 rather than 169:

require "trip"

trip = Trip.new(%i[c_call]) { require "pry" }
trip.pause_when { _1.method_id == :require }
events = trip.to_a

##
# The number of calls to require
p events.size

##
# The paths that were required
p events.map { _1.binding.eval('path') }

##
# 169
# ["pry", "pry/version", "pry/last_exception",
#  "pry/forwardable", "forwardable",  "forwardable/impl",
#  ...]

Rescue

IRB

Trip can listen for the raise event, and then pause the tracer when it is encountered. Afterwards, an IRB session can be started in the Binding (context) of where an exception was raised. The following example demonstrates how that works in practice:

require "trip"

module Stdout
  def self.write(message)
    putzzz(message)
  end
end

trip = Trip.new(%i[raise]) { Stdout.write("hello") }
trip.pause_when(&:raise?)
event = trip.start
event.binding.irb

Sources

• Source code (GitHub)
• Source code (GitLab)

Install

trip.rb is distributed as a RubyGem through its git repositories.
GitHub, and GitLab are available as sources.

License

BSD Zero Clause.
See LICENSE.