Effective Debugging Rubyconf

Debugging is an art. And to be an effective artist, an artist must be intimately familiar with their tools. In this talk, we'll start gently and finish strong to ensure that there's something for everyone. We'll cover when to use a debugger, which debugger to use, and how to use the debugger, and how to quickly configure your debugger for maximum utility. We'll touch briefly on pry and why pry is not a debugger, except when it is.

Developers are always looking for ways to boost productivity. Effective debugging allows one to more quickly discover inaccuracies between our expectations and how the software actually behaves.

Transcript

1. Effective Debugging
   

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3. Overview
   • Case Study
   
   • Recap and advanced commands
   
   • Debugging Libraries
   
   • Pry
   

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4. Case Study
   - use byebug
   - here’s the situation.
   - you’ve been handed an existing project by your boss..
   

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5. Sacculina Carcini
   - parasitic barnacle
   - takes over the host; host no longer molts; male crabs act like female crabs
   - http://www.flickr.com/photos/81858878@N00/9025250716/in/photolist-eKwLAY
   

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6. - this project has a test suite
   - previous developer completed feature and hands off a “green” test suite (or so he says)
   

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11. - turnip / rspec, explain that steps are executed in order.
    - execution stops rspec expectation is not met.
    

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12. - not enough information on line 12 to tell us why it failed
    - let’s look at the stack trace in the rspec output
    

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13. let’s examine the step definition on line 32 to see if it can tell us more
    

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14. - here we are in the step definition file
    

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15. - the internal state of the crab should have a key that points to the parasite
    - let’s take a step back and review the feature again..
    

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16. - here’s the feature
    - and we know the failure is on line 12...
    

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17. - but we don’t know when the crab’s payload should have its infection
    - it could happen on lines 3 through 11..
    

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18. - at this point, it may be tempting to investigate the internal details of the crab class and its payload and how that works but that would be premature at
    this point. instead of anticipating where the problems lies, we’re going to allow our tool, the debugger, to direct our investigations. we’re not making
    assumptions as to the root cause of the error. let’s stay “assumption free.”
    - so, let’s use the ruby gem ‘debugger’..
    

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19. - we have to do two things to use the ‘byebug’ gem
    

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20. - update our gem file to include a reference to the ‘byebug’ gem
    

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21. - call the debugger method which pauses our application.
    - but where should we put it?...
    

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22. - there’s no relationship between host, the crab, and parasite, the sacculina carcini
    

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23. - instead, let’s place our debugger statement at the first step where the parasite and the host interact.
    - note that execution is paused on the ruby expression immediately following the debugger
    method call.
    - let’s run the debugger
    

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25. - let’s take a moment to discuss what we see in the debugger
    

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26. - we see ten lines of context
    

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27. - the line numbers appear in the first column
    

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28. - here’s the current line where the application is paused
    

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29. - and that is indicated by the presence of the hash rocket.
    - great. now we know where we are in the debugger session, let’s examine the code in this step...
    

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30. - we have two lines.
    - ah, line 12 looks interesting. do you remember what the original failure? it was related to the crab’s payload.
    - the crab’s payload was nil when the test expected there to be a value
    - let’s add the crab’s payload as a display (or watched) variable
    

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34. - first thing to note is that our displayed ruby expression is still nil
    - secondly, whoa, where are we..
    

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35. - we’re actually in a new file. we’re inside the attach method of the parasite.
    - the debugger command “step” goes into a method definition. we “step” into
    

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36. - a little different than last time. we provide a number after step. this indicates how many times to run the step command. this is useful when you want to
    issue the same command multiple times. this will run “step” 3 times for us.
    

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41. - yeah, we have a value for the payload
    

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42. - uh oh, where are we?
    

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43. - we’re interested in what happens at the turnip step levels of lines 8, 9, 10 and 11.
    - how can we quickly stop execution at those lines? ...
    

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44. - well, i can think of one way, we could add ‘debugger’ statements at lines 16, 20, 24, and 28...
    

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45. - so instead of just one debugger statement, we would have five...
    

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46. - however, there’s a better way. let’s make use of the capabilities provided by the debugger tool.
    

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64. - here we are at breakpoint 4
    

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65. - the crab payload in our display list hasn’t changed from 0x007
    

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66. - the last time we stepped into a method, we took a detour that wasn’t necessary or informative.
    - let’s explore another debugger command, ‘next’...
    

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70. - hasn’t changed from 0x007.
    - but wait, what’s that?!?!
    

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71. - string vs. symbol!
    

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80. Recap and Advanced
    Commands
    - here’s the situation.
    - you’ve been handed an existing project by your boss..
    

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81. Case Study Recap
    • debugger
    
    • display
    
    • step
    
    • break
    
    • continue
    
    • next
    - debugger is a method that you may place in your application that will pause its execution allowing you to examine state, modify state, set further break
    points.
    

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82. Case Study Recap
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    - debugger is a method that you may place in your application that will pause its execution allowing you to examine state, modify state, set further break
    points.
    

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83. Case Study Recap
    • debugger byebug
    
    • disp
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    If you’re using the latest version of byebug, which is 2.3.1, then you can’t use the ‘debugger’ alias. You must use ‘byebug’
    

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84. Case Study Recap
    •!!!! s/debugger/byebug/g !!!!
    
    • disp
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    So everywhere we saw ‘debugger’ used (only on line 10 in spec/steps.rb), you would use ‘byebug.’
    

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86. My PR was accepted yesterday. A new version of the gem hasn’t been pushed to rubygems.org yet but it is coming soon.
    

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87. Case Study Recap
    • debugger
    
    • display
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    - also called watch
    

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88. Case Study Recap
    • debugger
    
    • display
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    

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89. Case Study Recap
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    

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90. Case Study Recap
    • debugger
    
    • disp[lay]
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    

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91. Case Study Recap
    • debugger
    
    • disp[lay]
    
    • step [n]
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    

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92. Case Study Recap
    • debugger
    
    • disp[lay]
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    

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93. Case Study Recap
    • debugger
    
    • disp[lay]
    
    • step
    
    • break <file name>:
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    

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94. Case Study Recap
    • debugger
    
    • disp[lay]
    
    • step
    
    • b <file name>:
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    

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95. Case Study Recap
    • debugger
    
    • disp[lay]
    
    • step
    
    • b Class.class_method
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    

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96. Case Study Recap
    • debugger
    
    • disp[lay]
    
    • step
    
    • b Class#instance_method
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    

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97. Case Study Recap
    • debugger
    
    • display
    
    • step
    
    • break
    
    • continue
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    - continue execution until the application completes or we hit another breakpoint
    

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98. Case Study Recap
    • debugger
    
    • disp[lay]
    
    • step
    
    • break
    
    • next
    • debugger
    
    • disp
    
    • step
    
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    

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99. Case Study Recap
    • debugger
    
    • disp[lay]
    
    • step
    
    • break
    
    • next [n]
    • debugger
    
    • disp
    
    • step
    
    • debugger
    
    • disp
    
    • step
    
    • break
    
    • next
    

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100. Case Study Recap
     • debugger
     
     • disp[lay]
     
     • step
     
     • break
     
     • n [n]
     • debugger
     
     • disp
     
     • step
     
     • debugger
     
     • disp
     
     • step
     
     • break
     
     • next
     

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101. Advanced Commands
     • finish
     
     • save
     
     • source
     - what did we miss?
     

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102. What did we miss?
     • finish
     
     • source
     • debugger
     
     • disp
     
     • step
     
     • break
     
     • next
     

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103. - remember in our case study when we were on line eleven and we “stepped” into the attach method on @sacculini_carcini?...
     

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104. - and at that time we stepped 3?
     - what if we had a loop, or many many lines of code?...
     

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105. - so instead of step, we use “finish” which execute code until the current stack has completed
     

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107. What did we miss?
     • finish
     
     • save
     
     • source
     • debugger
     
     • disp
     
     • step
     
     • break
     
     • next
     • debugger
     
     • disp
     
     • step
     
     • break
     
     • next
     

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115. - automatically evaluate ruby expressions
     - show full file paths or just the filename
     -
     - display context
     - auto start irb when prompt is shown
     

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116. What did we miss?
     • finish
     
     • save
     
     • source
     • debugger
     
     • disp
     
     • step
     
     • break
     
     • next
     

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122. What else did we miss?
     !
     • which versions of ruby?
     

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124. Debugger Libraries
     • 1.8 -- ruby-debug
     
     • 1.9 -- debugger, ruby-debug19, debugger2
     
     • 2.0 -- debugger, byebug, debugger2
     - lag time between new version of ruby and a fully supported debugger
     - debugger2 / byebug - use external C APIs
     

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125. DEBUGGERS STINK!
     provocative slide!
     

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126. Why?
     - why? coupling!
     - let’s take a look at the change log for the debugger gem...
     

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129. Ruby’s C API
     - in the past, most debugging tools are tightly coupled to the C internals.
     - because the debuggers were hooking into internals, every time ruby changed, you needed a new debugger
     

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130. Debugger Versions
     • 1.8 -- ruby-debug
     
     • 1.9 -- debugger, debugger2, ruby-debug19
     
     • 2.0 -- debugger, debugger2, byebug
     • debugger
     
     • disp
     
     • step
     
     • break
     
     • next
     - lag time between new version of ruby and a fully supported debugger
     - debugger2 / byebug - use external C APIs
     

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131. ~/.rdebugrc
     • set autoreload
     
     • set autoeval
     
     • set autolist

     - for ruby 1.8.7
     - if you’re using debugger, byebug or debugger2, these are the defaults
     

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132. Debugger Versions
     • 1.8 -- ruby-debug
     
     • 1.9 -- debugger, debugger2, ruby-debug19
     
     • 2.0 -- debugger, debugger2, byebug
     • debugger
     
     • disp
     
     • step
     
     • break
     
     • next
     • debugger
     
     • disp
     
     • step
     
     - lag time between new version of ruby and a fully supported debugger
     - debugger2 / byebug - use external C APIs
     

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133. Debugger Versions
     • 1.8 -- ruby-debug
     
     • 1.9 -- debugger, debugger2, ruby-debug19
     
     • 2.0 -- debugger, debugger2, byebug
     • debugger
     
     • disp
     
     • step
     
     • break
     
     • debugger
     
     • disp
     
     • step
     
     • break
     
     • next
     - lag time between new version of ruby and a fully supported debugger
     - debugger2 / byebug - use external C APIs
     

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135. Pry
     • “powerful alternative to [...] IRB [...]”
     
     • syntax highlighting
     
     • “flexible plugin architecture”
     - here’s the important thing, PRY is a REPL (read eval print loop)
     - let’s take a look; two things to do
     

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136. - add pry and pry-byebug to your Gemfile
     

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139. - similar display, column numbers on the left, hash rocket indicates the current line
     - pretty colors
     

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140. - binding.pry stops us before the binding.pry statement on
     line 10 so we don’t need “; true”
     

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142. - notice we have to use “break”, no shortcuts
     - and we must use the relative path from the project directory to when specifying the file
     

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143. - after hitting enter, we see info about the break point...
     

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144. - in addition we see the context too for the break point.
     

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146. - pry tells us how often the break point is hit
     - we have a few more things to note about pry...
     

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147. - no shortcuts for commands unless you define them
     

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148. Debugger (+ Pry)
     Versions
     • 1.9 -- pry + pry-debugger
     
     • 2.0 -- pry + pry-byebug
     

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149. byebug
     - mashup of debase, another debugger for ruby 2.0 (C ext part) and debugger (lib and test dirs)
     - i didn’t investigate debase much because there’s not a lot of documentation
     

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150. why byebug?
     

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151. - only for ruby 2.0. otherwise works as debugger in terms of commands, etc.
     

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152. Others?
     • Rubinius?
     
     • JRuby?
     

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154. - for JRuby, feel free to use any of the Java tools for debugging
     

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155. Overview
     • Case Study
     
     • Recap and advanced commands
     
     • Debugging Libraries
     
     • Pry
     - next, step, break, continue, display
     - finish, source, save
     - ruby-debug 1.8.7, debugger for 1.9, byebug for 2.0
     - can use pry w/debugger and byebug)
     

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156. Slides and Code and Image
     • https://speakerdeck.com/jwallace/effective-
     debugging-rubyconf
     
     • https://github.com/wallace/
     sacculina_carcini.git
     
     • http://www.flickr.com/photos/
     81858878@N00/9025250716/in/photolist-
     eKwLAY
     

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158. Get to know me!
     KNOW ME http://blog.jonathanrwallace.com/about
     FOLLOW ME @jonathanwallace
     WORK WITH ME http://www.bignerdranch.com
     CODE WITH ME github/wallace
     - thanks to Big Nerd Ranch, organizers and people who helped me with this talk
     

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