Effective Debugging - RubyconfIndia

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
   1
   Saturday, June 22, 13
   

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2. Overview
   • Case Study
   • Recap and advanced commands
   • Debugging Libraries
   2
   Saturday, June 22, 13
   

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3. Case Study
   3
   Saturday, June 22, 13
   - here’s the situation.
   - you’ve been handed an existing project by your boss..
   

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4. Sacculina Carcini
   4
   Saturday, June 22, 13
   - 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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5. 5
   Saturday, June 22, 13
   - 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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6. 6
   Saturday, June 22, 13
   

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7. 7
   Saturday, June 22, 13
   

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8. 8
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9. 9
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10. 10
    Saturday, June 22, 13
    - turnip / rspec, explain that steps are executed in order.
    - execution stops rspec expectation is not met.
    

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11. 11
    Saturday, June 22, 13
    - 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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12. 12
    Saturday, June 22, 13
    let’s examine the step definition on line 32 to see if it can tell us more
    

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13. 13
    Saturday, June 22, 13
    - here we are in the step definition file
    

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14. 14
    Saturday, June 22, 13
    - 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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15. 15
    Saturday, June 22, 13
    - here’s the feature
    - and we know the failure is on line 12...
    

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16. 16
    Saturday, June 22, 13
    - 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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17. 17
    Saturday, June 22, 13
    - 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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18. 18
    Saturday, June 22, 13
    - we have to do two things to use the ‘debugger’ gem
    

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19. 19
    Saturday, June 22, 13
    - update our gem file to include a reference to the ‘debugger’ gem
    

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20. 20
    Saturday, June 22, 13
    - call the debugger method which pauses our application.
    - but where should we put it?...
    

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21. 21
    Saturday, June 22, 13
    - there’s no relationship between host, the crab, and parasite, the sacculina carcini
    

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22. 22
    Saturday, June 22, 13
    - instead, let’s place our debugger statement at the first step where the parasite and the
    host interact.
    - note that we have the ruby statement “true” after the debugger method call.
    - the reason for this is that execution is paused on the ruby expression immediately
    following the debugger
    method call.
    - let’s run the debugger
    

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23. 23
    Saturday, June 22, 13
    

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24. 24
    Saturday, June 22, 13
    - let’s take a moment to discuss what we see in the debugger
    

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25. 25
    Saturday, June 22, 13
    - we see ten lines of context
    

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26. 26
    Saturday, June 22, 13
    - the line numbers appear in the first column
    

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27. 27
    Saturday, June 22, 13
    - here’s the current line where the application is paused
    

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28. 28
    Saturday, June 22, 13
    - 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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29. 29
    Saturday, June 22, 13
    - 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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30. 30
    Saturday, June 22, 13
    

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31. 31
    Saturday, June 22, 13
    

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32. 32
    Saturday, June 22, 13
    

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33. 33
    Saturday, June 22, 13
    - we’ve now advanced the debugger session to line 11. we have yet to execute the ruby
    expression on line 11 (we have executed the “true” statement on line 10.
    - let’s “step” again and see what happens
    

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34. 34
    Saturday, June 22, 13
    - note that executing ‘true’ did not change our displayed ruby expression
    

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35. 35
    Saturday, June 22, 13
    

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36. 36
    Saturday, June 22, 13
    - first thing to note is that our displayed ruby expression is still nil
    - secondly, whoa, where are we..
    

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37. 37
    Saturday, June 22, 13
    - 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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38. 38
    Saturday, June 22, 13
    - 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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39. 39
    Saturday, June 22, 13
    

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40. 40
    Saturday, June 22, 13
    

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41. 41
    Saturday, June 22, 13
    

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42. 42
    Saturday, June 22, 13
    

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43. 43
    Saturday, June 22, 13
    - yeah, we have a value for the payload
    

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44. 44
    Saturday, June 22, 13
    - uh oh, where are we?
    

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45. 45
    Saturday, June 22, 13
    - 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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46. 46
    Saturday, June 22, 13
    - well, i can think of one way, we could add ‘debugger; true’ statements at lines 16, 20, 24,
    and 28...
    

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47. 47
    Saturday, June 22, 13
    - so instead of just one debugger statement, we would have five...
    

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48. 48
    Saturday, June 22, 13
    - however, there’s a better way. let’s make use of the capabilities provided by the debugger
    tool.
    

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49. 49
    Saturday, June 22, 13
    

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50. 50
    Saturday, June 22, 13
    

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51. 51
    Saturday, June 22, 13
    

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52. 52
    Saturday, June 22, 13
    

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53. 53
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54. 54
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55. 55
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56. 56
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57. 57
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58. 58
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59. 59
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60. 60
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61. 61
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62. 62
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63. 63
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64. 64
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65. 65
    Saturday, June 22, 13
    

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66. 66
    Saturday, June 22, 13
    - here we are at breakpoint 4
    

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67. 67
    Saturday, June 22, 13
    - the crab payload in our display list hasn’t changed from 0x007
    

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68. 68
    Saturday, June 22, 13
    - 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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69. 69
    Saturday, June 22, 13
    

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70. 70
    Saturday, June 22, 13
    

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71. 71
    Saturday, June 22, 13
    

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72. 72
    Saturday, June 22, 13
    - hasn’t changed from 0x007.
    - but wait, what’s that?!?!
    

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73. 73
    Saturday, June 22, 13
    - string vs. symbol!
    

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74. 74
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75. 75
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76. 76
    Saturday, June 22, 13
    

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77. 77
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78. 78
    Saturday, June 22, 13
    

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79. 79
    Saturday, June 22, 13
    

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80. 80
    Saturday, June 22, 13
    

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81. 81
    Saturday, June 22, 13
    

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82. Case Study Recap
    • debugger
    • display
    • step
    • break
    • continue
    • next
    82
    Saturday, June 22, 13
    - 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
    • disp
    • step
    • break
    • next
    83
    Saturday, June 22, 13
    - 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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84. Case Study Recap
    • debugger
    • display
    • step
    • break
    • next
    84
    Saturday, June 22, 13
    - also called watch
    

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85. Case Study Recap
    • debugger
    • display
    • step
    • break
    • next
    85
    Saturday, June 22, 13
    

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86. Case Study Recap
    • debugger
    • disp
    • step
    • break
    • next
    86
    Saturday, June 22, 13
    

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87. Case Study Recap
    • debugger
    • disp[lay]
    • step
    • break
    • next
    87
    Saturday, June 22, 13
    

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88. Case Study Recap
    • debugger
    • disp[lay]
    • step [n]
    • break
    • next
    88
    Saturday, June 22, 13
    

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89. Case Study Recap
    • debugger
    • disp[lay]
    • step
    • break
    • next
    89
    Saturday, June 22, 13
    

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90. Case Study Recap
    • debugger
    • disp[lay]
    • step
    • break <file name>:
    • next
    90
    Saturday, June 22, 13
    

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91. Case Study Recap
    • debugger
    • disp[lay]
    • step
    • b <file name>:
    • next
    91
    Saturday, June 22, 13
    

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92. Case Study Recap
    • debugger
    • disp[lay]
    • step
    • b Class.class_method
    • next
    92
    Saturday, June 22, 13
    

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93. Case Study Recap
    • debugger
    • disp[lay]
    • step
    • b Class#instance_method
    • next
    93
    Saturday, June 22, 13
    

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94. Case Study Recap
    • debugger
    • display
    • step
    • break
    • continue
    • next
    94
    Saturday, June 22, 13
    - continue execution until the application completes or we hit another breakpoint
    

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95. Case Study Recap
    • debugger
    • disp[lay]
    • step
    • break
    • next
    95
    Saturday, June 22, 13
    

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96. Case Study Recap
    • debugger
    • disp[lay]
    • step
    • break
    • next [n]
    96
    Saturday, June 22, 13
    

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97. Case Study Recap
    • debugger
    • disp[lay]
    • step
    • break
    • n [n]
    97
    Saturday, June 22, 13
    

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98. What did we miss?
    • finish
    • source
    98
    Saturday, June 22, 13
    

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99. What did we miss?
    • finish
    • source
    99
    Saturday, June 22, 13
    

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100. 100
     Saturday, June 22, 13
     - 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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101. 101
     Saturday, June 22, 13
     - and at that time we stepped 3?
     - what if we had a loop, or many many lines of code?...
     

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102. 102
     Saturday, June 22, 13
     - so instead of step, we use “finish” which execute code until the current stack has
     completed
     

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103. 103
     Saturday, June 22, 13
     

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104. What did we miss?
     • finish
     • source
     104
     Saturday, June 22, 13
     - source has an analog called save, but in my explorations it does not save displayed ruby
     expressions in debugger
     

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105. 105
     Saturday, June 22, 13
     

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106. What else did we miss?
     • which versions of ruby?
     106
     Saturday, June 22, 13
     

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107. 107
     Saturday, June 22, 13
     

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108. Debugger Versions
     • 1.8 -- ruby-debug
     • 1.9 -- debugger, ruby-debug19, debugger2
     • 2.0 -- debugger, byebug, debugger2
     108
     Saturday, June 22, 13
     - lag time between new version of ruby and a fully supported debugger
     - debugger2 / byebug - use external C APIs
     

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109. DEBUGGERS STINK!
     109
     Saturday, June 22, 13
     provocative slide!
     

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110. Why?
     110
     Saturday, June 22, 13
     - why? coupling!
     - let’s take a look at the change log for the debugger gem...
     

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111. 111
     Saturday, June 22, 13
     

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112. 112
     Saturday, June 22, 13
     

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113. Ruby’s C API
     113
     Saturday, June 22, 13
     - 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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114. Debugger Versions
     • 1.8 -- ruby-debug
     • 1.9 -- debugger, debugger2, ruby-debug19
     • 2.0 -- debugger, debugger2, byebug
     114
     Saturday, June 22, 13
     - lag time between new version of ruby and a fully supported debugger
     - debugger2 / byebug - use external C APIs
     

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115. ~/.rdebugrc
     • set autoreload
     • set autoeval
     • set autolist
     115
     Saturday, June 22, 13
     - for ruby 1.8.7
     - if you’re using debugger, byebug or debugger2, these are the defaults
     

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116. Debugger Versions
     • 1.8 -- ruby-debug
     • 1.9 -- debugger, debugger2, ruby-debug19
     • 2.0 -- debugger, debugger2, byebug
     116
     Saturday, June 22, 13
     - lag time between new version of ruby and a fully supported debugger
     - debugger2 / byebug - use external C APIs
     

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117. Debugger Versions
     • 1.8 -- ruby-debug
     • 1.9 -- debugger, debugger2, ruby-debug19
     • 2.0 -- debugger, debugger2, byebug
     117
     Saturday, June 22, 13
     - lag time between new version of ruby and a fully supported debugger
     - debugger2 / byebug - use external C APIs
     

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118. 118
     Saturday, June 22, 13
     

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119. Pry
     • “powerful alternative to [...] IRB [...]”
     • syntax highlighting
     • “flexible plugin architecture”
     119
     Saturday, June 22, 13
     - 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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120. 120
     Saturday, June 22, 13
     - add pry and pry-debugger to your Gemfile
     

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121. 121
     Saturday, June 22, 13
     

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122. 122
     Saturday, June 22, 13
     

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123. 123
     Saturday, June 22, 13
     - similar display, column numbers on the left, hash rocket indicates the current line
     - pretty colors
     

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124. 124
     Saturday, June 22, 13
     - binding.pry stops us before the binding.pry statement on
     line 10 so we don’t need “; true”
     

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125. 125
     Saturday, June 22, 13
     

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126. 126
     Saturday, June 22, 13
     - 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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127. 127
     Saturday, June 22, 13
     - after hitting enter, we see info about the break point...
     

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128. 128
     Saturday, June 22, 13
     - in addition we see the context too for the break point.
     

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129. 129
     Saturday, June 22, 13
     

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130. 130
     Saturday, June 22, 13
     - pry tells us how often the break point is hit
     - we have a few more things to note about pry...
     

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131. 131
     Saturday, June 22, 13
     - no shortcuts for commands unless you define them
     

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132. Debugger (+ Pry)
     Versions
     • 1.9 -- pry + pry-debugger
     132
     Saturday, June 22, 13
     

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133. byebug
     133
     Saturday, June 22, 13
     - 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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134. why byebug?
     134
     Saturday, June 22, 13
     

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135. 135
     Saturday, June 22, 13
     - only for ruby 2.0. otherwise works as debugger in terms of commands, etc.
     

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136. Others?
     • Rubinius?
     • JRuby?
     136
     Saturday, June 22, 13
     

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137. 137
     Saturday, June 22, 13
     

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138. 138
     Saturday, June 22, 13
     - for JRuby, feel free to use any of the Java tools for debugging
     

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139. Overview
     • Case Study
     • Recap and advanced commands
     • Debugging Libraries
     139
     Saturday, June 22, 13
     - next, step, break, continue, display
     - finish, source
     - ruby-debug 1.8.7, debugger for 1.9, byebug for 2.0
     - can use pry w/debugger and byebug)
     

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140. Slides and Code and Image
     • https://speakerdeck.com/jwallace/effective-
     debugging
     • https://github.com/wallace/
     sacculina_carcini.git
     • http://www.flickr.com/photos/
     81858878@N00/9025250716/in/photolist-
     eKwLAY
     140
     Saturday, June 22, 13
     

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141. Get to know me!
     • http://blog.jonathanrwallace.com/about
     • jonathan.wallace@gmail.com
     • @jonathanwallace
     • http://www.bignerdranch.com/about_us/
     nerds/jonathan_wallace
     141
     Saturday, June 22, 13
     - thanks to Big Nerd Ranch, organizers and people who
     helped me with this talk
     

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