Effective Debugging - RubyconfIndia

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Effective Debugging 1 Saturday, June 22, 13

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

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

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

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

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

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

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

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

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

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

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

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

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35 Saturday, June 22, 13

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

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

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

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

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

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

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

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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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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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Case Study Recap • debugger • display • step • break • next 84 Saturday, June 22, 13 - also called watch

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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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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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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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~/.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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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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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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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 Saturday, June 22, 13 - add pry and pry-debugger to your Gemﬁle

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

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

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

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

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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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why byebug? 134 Saturday, June 22, 13

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

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

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

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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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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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Get to know me! • http://blog.jonathanrwallace.com/about • [email protected] • @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