Git hook to disallow non-fastforward updates (also disallows merges into feature branches in order to keep them cleanly rebaseable)

gistfile1.rb

#!/usr/bin/env ruby

# This update hook enforces the following policies
# 1) release and master branches are fast-forward only
# 2) feature branches cannot contain merges

$refname = ARGV[0] 
$oldrev = ARGV[1] 
$newrev = ARGV[2]

puts "Enforcing Policies... \n(#{$refname}) (#{$oldrev[0,6]}) (#{$newrev[0,6]})"

# Enforce fast-forward-only branches.
NO_NON_FASTFORWARD_BLACKLIST = [/\brelease\b/, /\bmaster\b/]

# Disallow merges in feature branches.
NO_MERGE_BLACKLIST    = [/\bfeature\b/]

def abort_on_non_fastforward(blacklist)
  if blacklist.any?{ |pattern| $refname =~ pattern}
    missed_refs = `git rev-list #{$newrev}..#{$oldrev}`
    missed_ref_count = missed_refs.split("\n").size
    if missed_ref_count > 0
      puts "[POLICY] Cannot push a non fast-forward reference"
      exit 1
    end
  end
end 

def abort_on_merge(blacklist)
  if blacklist.any?{|pattern|  $refname =~ pattern} 
    feature_commits = `git rev-list #{$refname} ^master ^$(git merge-base master #{$refname})`.lines.map do |l|
      l.strip
    end
    if feature_commits.any?{|sha| is_merge_commit?(sha.strip) }
      STDERR.puts <<-MSG
[ POLICY ] There are merge commits on #{$refname}. Feature branches should not
contain merge commits. In order to keep a feature branch up to date with
master, rebase it. Once you have rebased your feature branch to remove the
merge commits, you will be able to push it here.
  MSG
      exit 1
    end
  end
end

def is_merge_commit?(sha)
  `git cat-file -p #{sha}`.lines.select{|l| l =~ /^parent/}.count > 1
end

abort_on_non_fastforward(NO_NON_FASTFORWARD_BLACKLIST)
abort_on_merge(NO_MERGE_BLACKLIST)