justinhuynh/practical-debugging

Code for the "Practical debugging" talk

Practical debugging

Identify the type of problem you're facing by determining which question you're asking. Follow the techniques below.

Interface problems

Interface problems occur when you don’t understand the dependent structure of methods or constants.

Questions like:

• Why is this thing nil?
• Why can’t I call the method I want?
• What are the constants I can reference?
• What can this object see and do?
• What is this gem doing?

Questions to ask yourself:

• Did I handle every type of object that can be returned from this method (including nil)?
• What is the value of self in this block?
• Did I account for lexical scope?

Tools and methods to use:

• Listing methods:
  • Object#methods
  • Module#instance_methods
  • Module#private_instance_methods
• Inspecting methods:
  • Object#method
  • Module#instance_method
  • Method#source_location
  • Method#parameters
• Constants:
  • Module::nesting
  • Module#constants
  • Module#const_get

Command line utilities to use:

• bundle open (opens the source of gems in your editor)
• gem pristine (restores gems to their original state)

State problems

State problems occur when the assumptions you made about the current state of the program are incorrect.

Questions like:

• How does this value change at this point?
• What has been initialized at this point?
• How many objects are allocated in this method?

Questions to ask yourself:

• What am I assuming about the current state of the program at this point in code?

Tools and methods to use:

• Variables:
  • Kernel#local_variables
  • Object#instance_variables
  • Object#instance_variable_get
  • Object#instance_variable_set
  • Kernel#global_variables
• State:
  • Kernel#puts
  • Object#inspect
  • Kernel#p
  • Binding#irb
  • GC::stat

Flow problems

Flow problems occur when you don’t know how the ruby interpreter got to or left a location in code and its associated state.

Questions like:

• How did the interpreter get here?
• Where does the interpreter go from here?
• How did this object get here?
• Where is this object being mutated?
• When does this instance variable get set?

Questions to ask yourself:

• What am I assuming has happened or is going to happen next?

Tools and methods to use:

• Interpreter location:
  • Kernel#caller
  • TracePoint
  • Kernel#set_trace_func
  • Binding#eval
• Object lifecycle:
  • ObjectSpace
  • Object#freeze

References

• A Comprehensive Guide to Debugging Rails
• Debugging memory leaks in Ruby
• Debugging Rails Applications
• Debugging Rails Applications in Development
• I am a puts debuggerer
• Ruby Debugging Magic Cheat Sheet
• There’s More to Ruby Debugging Than puts()

Code examples

For minesweeper, run bundle install in the root of the repository to fetch the dependencies. Then run bin/minesweeper to run the program. The number of rows, columns, and mines can be specified as command line arguments in that order.