Code Archaeology Notes

presentation.sh

#!/usr/bin/env bash

# Useful git commands using the rails codebase as an example:
# git clone git@github.com:rails/rails.git

# and my own example later
# git clone git@github.com:mowat27/maths.git

# You'll also need bat (brew install bat)

# ------------------------------------------------------------------------------

# 1. What happened?

cd /Users/adrian/desktop/rails

# What's changed recently
git log

# First ever commit
git log --reverse

# Abbreviated history
git log --oneline

# For a range of commits
git log cdb51080b1..b4029ba4a9 --oneline

# Since a commit
git log cdb51080b1.. --oneline

# What changed?
# 5b0680437c is interesting, commit message was :golf:
git show 5b0680437c

# Why does a file exist?
# .yardopts is new to me, why is it there?
# see all the commits against a file
git log .yardopts

# now we can dig into one
git show e555388e1603f9915051b873b18bf60642bc3e85
# or we can abbreviate
git show e555388e16


# ------------------------------------------------------------------------------

# 2. Who Changed What?

# It might be useful to see who was the most recent person to change
# each line in a file
git blame .yardopts

# or for a bigger file we can zoom in
# e.g to a method in
# activerecord/lib/active_record/tasks/database_tasks.rb
git blame -L 63,77 activerecord/lib/active_record/tasks/database_tasks.rb

# Looks like something changed in 2018:
#
# a2827ec9811 (eileencodes  2018-01-10 10:25:13 -0500 65)           current = ActiveRecord::Base.connection.migration_context.current_environment
# a2827ec9811 (eileencodes  2018-01-10 10:25:13 -0500 66)           stored  = ActiveRecord::Base.connection.migration_context.last_stored_environment
#
# so let's have a look
git log a2827ec9811


# ------------------------------------------------------------------------------

# 3. When did it break?

cd /Users/adrian/desktop/example

# The script does simple maths
./maths.rb + 1 2 3
./maths.rb - 1 2 3
./maths.rb / 4 2

# The problem is that multiply is always returning 0
./maths.rb x 2 4

# Looking at the code - it's a bit obscure!
bat --plain maths.rb

# What happened?
git log --oneline

# Who did it?
git blame maths.rb

# When did it break?
git bisect start
git bisect bad
git bisect good 52a9b0c

# repeatedly
./maths.rb x 2 4
git bisect # [good|bad]

# until
# afa85672c8f2b1a41a920e9be162646018b775f0 is the first bad commit
# Lands you at the last good commit so we can see what changed
git show afa85672c8f2b1a41a920e9be162646018b775f0

# Get out of bisect mode
git bisect reset

# Alternatively
# We can just run a script

git bisect start
git bisect bad
git bisect good 52a9b0c
git bisect run tmp/try_multiply.sh

# 3.1 Looking for the first working commit in rails

# Get the first commit on inflector
git log --oneline --reverse activesupport/lib/active_support/inflector.rb

# Write a script that checks for good and bad
bat --plain tmp/try_inflector.rb

# Confusingly we need to say invert good and bad
# the working version is bad and a broken version is good
git bisect start
git bisect bad
git bisect good dc3d6eb9b4

# Run the script
tmp/try_inflector.rb
# and git bisect good|bad
# ad nauseum

# Automate
git bisect run tmp/try_inflector.rb