Git `update` hook (server side)

update

#!/usr/bin/ruby

# Enforce custom commit message format
def check_message_format(refname, oldrev, newrev)
  message_format_regex = /\[ref: (\d+)\]/
  # You would probably have to adjust the way you're retrieving user identity.
  # You can also load user identity from each analysed commit.
  user = ENV['USER']
  # Initialize empty map to store errors
  errors = []

  puts "Enforcing Policie on #{refname} (#{oldrev[0,6]}..#{newrev[0,6]})…"
  missed_revs = `git rev-list #{oldrev}..#{newrev}`.split("\n")
  missed_revs.each do |rev|
    message = `git cat-file commit #{rev} | sed '1,/^$/d'`
    # Check if current commit message matched expected format
    if !message_format_regex.match(message)
      # If user wasn't loaded, then try to load it from first commit
      user = `git show -s --pretty=format:'%an' #{rev}` unless user
      errors << rev[0,6]
    end
  end

  # Print errors to STDOUT when something went wrong
  if errors.any?
    puts_red "[POLICY] 😡  Shame to #{user}!"
    if errors.size > 1
      puts_red "[POLICY] 😱  Some messages are not formatted correctly (commits #{errors.join(', ')})"
    else
      puts_red "[POLICY] 😨  Commit #{errors.first} message is not well formatted"
    end
    exit 1
  end
end

# We don't want any user to push on master.
# This can be done using protecting branches on GitHub, GitLab and many other Git servers.
def prevent_master_update(refname)
  if refname =~ /(refs\/)?(heads\/)?master/
    puts_red "You are not allowed to push to `master`"
    exit 1
  end
end

# Print message in red
def puts_red(message)
  puts "\e[31m#{message}\e[0m"
end

puts 'Running update hook…'

# Loads values from STDIN
refname, oldrev, newrev = *ARGV
prevent_master_update(refname)
check_message_format(refname, oldrev, newrev)

puts 'Everything went fine! 👏'

How to install on GitLab CE?

Here is a summary of GitLab custom hooks behavior.
Create a custom_hooks subdirectory in your project path with the expected file:

mkdir /var/opt/gitlab/git-data/repositories/<group>/<project>.git/custom_hooks
vi /var/opt/gitlab/git-data/repositories/<group>/<project>.git/custom_hooks/update

[copy/paste hook then save] 

chmod +x /var/opt/gitlab/git-data/repositories/<group>/<project>.git/custom_hooks/update

And you're done!

More about that example

That example has been inspired (not to say copy/pasted) from Git SCM.