Git post-checkout hook to enable sensible foreman restart upon branch change with bonus bundle check

post-checkout

#!/usr/bin/env ruby
# post-checkout - git post checkout hook script
#
# This script will update the contents of .git/CURRENT_BRANCH with the name of
# the current branch after a checkout.
#
# Use it with rerun to restart foreman on branch changes.
# Bonus feature: sanity checks your Gemfile using $ bundle check
#
# Start foreman like this to watch CURRENT_BRANCH:
# $ rerun -d .git -p 'CURRENT_BRANCH' foreman start
#
# You'll probably need these gems:
# $ gem install rerun rb-fsevent  

BRANCH_FILENAME = "CURRENT_BRANCH"

def should_update_branchfile? previous, new
  return true if new == "HEAD" # random commit somewhere, better restart
  return previous != new
end

def current_branch
  @current_branch ||= `git rev-parse --abbrev-ref HEAD`.strip
end

def branchfile
  @branchfile ||=
    begin
      src_dir = Dir.pwd
      git_dir = ENV.fetch 'GIT_DIR'
      File.join src_dir, git_dir, BRANCH_FILENAME
    end
end

def previous_branch
  @previous_branch ||=
    begin
      IO.read(branchfile).strip
    rescue
      ""
    end
end

def update_branchfile new
  File.open(branchfile, "w") do |f|
    f.puts new
  end
end

if should_update_branchfile? previous_branch, current_branch
  puts "Branch change detected, trying to restart foreman and sanity checking deps"
  update_branchfile current_branch
  system 'bundle check'
end