Debugging: The Science of Deduction

THE SCIENCE OF DEDUCTION
[email protected] • @DanielleSucher
DEBUGGING
Friday, November 15, 13

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"People write programs without
any expectation that they will be
right the first time."
- David Lorge Parnas

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Let’s start simple.

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When you
get an error
message

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“Oh, I think I know what
went wrong..!”

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NO.

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READ THE ERROR
MESSAGE

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/Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/factory_girl-4.2.0/
lib/factory_girl/attribute_assigner.rb:14:in `tap'
lib/factory_girl/attribute_assigner.rb:14:in òbject'
lib/factory_girl/evaluation.rb:12:in òbject'
lib/factory_girl/strategy/build.rb:9:in `result'
lib/factory_girl/factory.rb:42:in `run'
lib/factory_girl/factory_runner.rb:23:in `block in run'
/Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/activesupport-4.0.1/
lib/active_support/notifications.rb:161:in ìnstrument'
lib/factory_girl/factory_runner.rb:22:in `run'
lib/factory_girl/strategy_syntax_method_registrar.rb:19:in `block in
define_singular_strategy_method'
/Users/stan/projects/rails4/spec/models/project_spec.rb:21:in `block
(4 levels) in '
/Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/rspec-core-2.14.5/
lib/rspec/core/memoized_helpers.rb:199:in `block (2 levels) in let'
lib/rspec/core/memoized_helpers.rb:199:in `fetch'
lib/rspec/core/memoized_helpers.rb:199:in `block in let'

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How do you find
the important stuff?

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Start at the top.

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Skim for lines that refer to
your project’s codebase

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lib/factory_girl/attribute_assigner.rb:14:in `tap'
lib/factory_girl/attribute_assigner.rb:14:in òbject'
lib/factory_girl/evaluation.rb:12:in òbject'
lib/factory_girl/strategy/build.rb:9:in `result'
lib/factory_girl/factory.rb:42:in `run'
lib/factory_girl/factory_runner.rb:23:in `block in run'
/Users/stan/.rvm/gems/ruby-1.9.3-p448@rails4/gems/activesupport-4.0.1/
lib/active_support/notifications.rb:161:in ìnstrument'
lib/factory_girl/factory_runner.rb:22:in `run'
lib/factory_girl/strategy_syntax_method_registrar.rb:19:in `block in
define_singular_strategy_method'
(4 levels) in '
lib/rspec/core/memoized_helpers.rb:199:in `block (2 levels) in let'
lib/rspec/core/memoized_helpers.rb:199:in `fetch'
lib/rspec/core/memoized_helpers.rb:199:in `block in let'

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Read the line
number

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google the error
(no, really, you’re not smarter
than google)

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Unexpected Behavior

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“I don’t guess.
I observe.”

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“And once I’ve
observed...”

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“I deduce.”

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Formulate a hypothesis.
Try to disprove it.
Repeat as needed.

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Where do you start looking?

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The Streetlight Effect
searching where the keys fell
vs
searching where the light is

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Start
looking
where
the
light
is

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First test the theories
you can disprove the fastest

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To test your hypotheses,
you need to
reproduce the bug

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If you can’t reproduce the bug,
what’s different
about your environment?

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Watch somebody else
reproduce the problem.

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Sometimes it’s all in the timing

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Even partial patterns
can give you useful data

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the 500 mile bug
http://www.ibiblio.org/harris/
500milemail.html

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Try to break things down into
smaller, self-contained, pieces.

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Don’t even try
to understand everything

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Miller’s Law
The average person can only hold
7 ± 2
items in working memory

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It’s okay to think of
the rest of the system
as a black box

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Break apart methods so you can
test smaller chunks of logic
separately

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Comment out parts of your code
so you can focus on one
piece at a time

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Refactoring and adding tests
(or even just fixing typos)
keeps you focused
and improves
your understanding

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With really gnarly code
write tests to
document existing behavior

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"...what are the facts, and what is
the truth that the facts bear out?"
- Bertrand Russell

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Is the method you’re looking at
even being called at all?

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def might_be_called
puts “IN MIGHT_BE_CALLED”
puts caller
# ...
end

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“What they
don’t
confess
to is
almost
always
more
interesting.”

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What are the actual inputs
your method is receiving?

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Maybe you think you’re
getting nil, but
you’re getting ‘’ or [‘’]
instead

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Is there a rescue in the way,
swallowing useful errors?

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What is the state of the universe?

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Follow the money logs
$ tail -f log/whatevs.log

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Does doing things in a
different order
lead to different results?

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Possible Ordering Problems
Methods with side effects
Rails autoload order
Database left in a dirty state

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Clean out your environment
(if at all possible)

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Make the
problem
more concrete

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Sketch things out

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Wave your hands around
in the air
placing pieces of your system
in imaginary 3d space

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“I need to go
to my mind
palace.”

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Question your assumptions.
(Write a list or even an informal
proof, if necessary.)

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Rubber duck
debugging

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Trust no one.

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“The only truly accurate
comment I've seen was just
cursing, in Swedish.”
- Scott Vokes (@silentbicycle)

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“Without a debugger, you
basically have to go to the next
step; understand what the
program does.”
- Linus Torvalds

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print statement debugging
puts “*” * 20
puts “foo: #{foo.inspect}”
puts “*” * 20

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Where is this method defined?
m = Foo.new.method(:do_stuff)
m.source_location
=> [filename, line_number]

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“Ideas are tested by experiment.
That is the core of science.”
- Zombie Richard Feynman
(via xkcd)

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Experiment with
small pieces of the code
in IRB or Pry

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Focus on what’s
changed
since the problem started

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A few Git tricks

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test on a different branch
or a different commit

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git bisect
binary search through your
commits to figure out when the
problem started!

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Running git bisect manually
$ git bisect start
$ git bisect good SHA
$ git bisect bad HEAD

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Running git bisect
automatically
$ git bisect start HEAD HEAD~10
$ git bisect run rspec spec/

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git blame
(not actually
for blaming
people!)

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The person who wrote this code
may have been an idiot.
Or they may have been
a mad genius.

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Look at the original context
$ git blame -L(range) filename
$ git show SHA

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But what about whitespace
changes?
$ git blame -w filename
(ignores whitespace)

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But what about lines that were
just copied or moved?
$ git blame -wCCC filename
-w ignores whitespace
-C detects moved/copied lines
-CCC is like -C, but tries harder

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$ git log -p
Includes patches, not just
commit messages.
You can search with /
search_string

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$ git log -p -S “test string”
Lists only commits where that
string was added or deleted
(with patches).

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Debuggers

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byebug (Ruby 2.0.x)
debugger (Ruby 1.9.x)
ruby-debug (Ruby 1.8.x)

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Pry
https://github.com/pry/pry
https://github.com/pry/pry/
wiki/Available-plugins

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ls - list methods on an object
cd - into objects
wtf? - backtrace
show-doc - prints docs
show-method - prints source
whereami - no really
Pry

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binding.pry if @whatevs

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Pry-remote
https://github.com/mon-ouie/
pry-remote

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binding.pry_remote
$ pry-remote

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Better Errors
https://github.com/charliesome/
better_errors

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WHEN THINGS GET REALLY
WEIRD

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This shouldn’t make a difference,
but let’s try it just in case...

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It doesn’t count if
you’re right by accident

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“That’s clever. Is it clever?”
“...why is it clever?”

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If you don’t understand the cause
of the problem, it’s not fixed,
because you can’t predict when it
might come up again.

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Don’t be afraid to read the source

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$ bundle open gem_name

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occasionally
the compiler
(Rarely! But it happens.)

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“You know my methods, Watson.”

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THE SCIENCE OF DEDUCTION
[email protected] • @DanielleSucher
DEBUGGING
thank you!

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Debugging: The Science of Deduction

Debugging: The Science of Deduction

Danielle Sucher

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