Debugging Effectively

Colin O’Dell

@colinodell

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Colin O’Dell

• Lead Web Developer at Unleashed Technologies• PHP League Member• league/commonmark• league/html-to-markdown

• PHP 7 Upgrade Guide e-book

@colinodell

OverviewI. Importance of debuggingII. Debugging processIII. Tools & TechniquesIV. Q&A

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Debugging is...Photo by Ewan Cross // cc by-nc-nd 2.0 // https://flic.kr/p/7MCt3m

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ (adjective)

@colinodell

Debugging is...Photo by Ewan Cross // cc by-nc-nd 2.0 // https://flic.kr/p/7MCt3m

important@colinodell

Debugging is...Photo by Ewan Cross // cc by-nc-nd 2.0 // https://flic.kr/p/7MCt3m

the single most important skill in

programming.@colinodell

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PlanningCoding

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Planning

Testing & Debugging

Coding

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Debugging is the process of finding and resolving bugs or defects that prevent correct operation of computer software or a system.

– Wikipedia

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Process is the foundation of effective debugging

Process

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Gain experience with code and tools

Experience

Process

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Develop your intuition

IntuitionExperience

Process

Junior Developers• Try the “usual” steps• bin/console cache:clear• composer install• chmod –R 777 *

• Ask somebody else• Co-worker• Google• StackOverflow post

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XY ProblemPhoto by Yoel Ben-Avraham, with changes // cc by--nd 2.0 // https://flic.kr/p/6pmtQL

•I want to solve problem X•How do I solve X?

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XY ProblemPhoto by Yoel Ben-Avraham, with changes // cc by--nd 2.0 // https://flic.kr/p/6pmtQL

•I want to solve problem X•How do I solve X?•Solution Y might work•How can I do Y?

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1.Don’t parse HTML with regex2.Solve problems the right way

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“I don’t know why”

“For some reason”

“Doesn’t make sense”

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Bugs are logical

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“The bug is not moving around in your code, trying to trick or evade you. It is just siting in one place, doing the wrong thing in the same

way every time.” – Nick Parlante, Debugging Zen

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Assume your code is the problem

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Systematic Approach

1. Gather information2. Replicate the issue3. Identify the culprit4. Fix it & re-test5. Mitigate future occurrences

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1. Gather information

• Expected behavior vs.actual behavior• Error messages• Stack traces

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• Screenshots•Browser & OS•Date & time• Log entries

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2. Replicate the Issue

Be able to replicate with 100% certainty

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3. Identify the CulpritPhoto by U.S. Navy // cc 2.0 // https://flic.kr/p/n6Wgks

• Be methodical• Make no assumptions• Understand the bug

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4. Fix it & Re-test• Attempt to replicate again• Avoid XY problem• No temporary workarounds!• Add technical debt• May introduce other issues• Never get replaced with true solutions

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5. Mitigate Future Occurrences•Add an automated test• Share your new knowledge• Project documentation• Blog post• StackOverflow

• Submit patch upstream

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Recap

1. Gather information2. Replicate the issue3. Identify the culprit4. Fix it & re-test5. Mitigate future occurrences

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Long-Term Results

• Gain experience• Learn how the system works• Build heuristics• Boost confidence

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Tools & Techniques

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Two essential tools

• Integrated development environment (IDE)• Interactive debugger

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Integrated Development Environment•Minimum features:•Syntax highlighting•Auto-completion•Fast code navigation•Debugger

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Interactive Debugger

•Pause code execution•Breakpoints•Conditional breakpoints

•Step through execution•Examine variables•Explore call stack

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Techniques1. Trace backwards from known issue2. Trace forwards from start 3. Binary search4. Use tools5. Get help6. Take a break

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1. Trace backwards•Error is thrown from known location•Use a debugger•Establish context•Work backwards

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1. Trace backwards•Error is thrown from known location•Use a debugger•Establish context•Work backwards

a()

b()

c()

d()

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1. Trace backwards•Error is thrown from known location•Use a debugger•Establish context•Work backwards

a()

b()

c()

d()

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1. Trace backwards•Error is thrown from known location•Use a debugger•Establish context•Work backwards

a()

b()

c()

d()

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1. Trace backwards•Error is thrown from known location•Use a debugger•Establish context•Work backwards

a()

b()

c()

d()

Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

1. Trace backwards•Error is thrown from known location•Use a debugger•Establish context•Work backwards

a()

b()

c()

d()

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2. Trace forwards•Opposite direction•Problematic lineisn’t known•Use debugger or logging

a()

b()

c()

d()

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2. Trace forwards•Opposite direction•Problematic lineisn’t known•Use debugger or logging

a()

b()

c()

d()

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2. Trace forwards•Opposite direction•Problematic lineisn’t known•Use debugger or logging

a()

b()

c()

d()

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2. Trace forwards•Opposite direction•Problematic lineisn’t known•Use debugger or logging

a()

b()

c()

d()

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3. Divide & Conquer• Identify different code sections• Set breakpoints at the boundaries• Isolate issue to one particular area• Focus efforts on that area

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@colinodell

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✓

X@colinodell

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4. Use tools•Variable dumps•Debug toolbars•Console utility•Profilers•git bisect

•netcat•curl• strace•etc.

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Variable dumpsPhoto by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe

•var_dump()•kint()•file_put_contents()•$logger->debug()

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Debug toolbars

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Console Utility

Symfony: bin/consoleLaravel: artisan

Drupal: drushDrupal Console

Magento: n98-magerun@colinodell

Performance Profiling

Identify slowness:• Bottlenecks• Resource hogs• Inefficient code

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Tools:• Blackfire (freemium)• New Relic (freemium)• xhprof (open-source)

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git bisect

v1.7 ? ? ? ? ? ? ? HEAD

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git bisect

v1.7 ? ? ? ? ? ? ? HEAD

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git bisect

v1.7 ? ? ? BAD ? ? ? HEAD

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git bisect

v1.7 ? ? ? BAD BAD BAD BAD HEAD

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git bisect

v1.7 ? GOOD ? BAD BAD BAD BAD HEAD

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git bisect

v1.7 GOOD GOOD ? BAD BAD BAD BAD HEAD

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git bisect

v1.7 GOOD GOOD X BAD BAD BAD BAD HEAD

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abcd123 is the first bad commit @colinodell

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netcat

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curl

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strace

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5. Get help•RTFM / RTFD•Project forums or issue queue• StackOverflow, IRC, etc.•Ask a colleague• Expert in that area• Senior developer

•Rubber ducking

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6. Take a break

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• Clear your mind; start fresh• Forget invalid assumptions• Recharge your batteries• Let your subconscious work on it

@colinodell

Four things to walk away with

1. Computers aren’t random,and neither are bugs

2. Persistence will always pay off3. Don’t be afraid to dive deep4. Don’t make assumptions or

take things for granted

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Questions?

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Learn More• https://web.duke.edu/cps001/notes/Debugging.pdf• http://www.fiveminutegeekshow.com/20• http://blog.codeunion.io/2014/09/03/teaching-novices-how-to-debug-co

de/• https://www.jetbrains.com/phpstorm/help/debugging.html• http://www.sitepoint.com/debugging-git-blame-bisect/• http://unix.stackexchange.com/a/50099/80744• http://codeception.com/docs/01-Introduction• http://chadfowler.com/blog/2014/01/26/the-magic-of-strace/ • http://c2.com/cgi/wiki?RubberDucking

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Thanks!Feedback Appreciated!• https://joind.in/talk/549fd• @colinodell / colinodell@gmail.com

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