Keep only one build up for each PR in travis

.travis.yml

env:
  global:
    # TRAVIS_ACCESS_TOKEN used to access travis API from within builds
    - secure: "XXXXXXXXXXXXXXXXXXXXXXXXXXXXX"

cache:
  bundler: true

before_install:
  - bundle install
  - bundle exec travis_build_killer.rb

travis_build_killer.rb

#!/usr/bin/env ruby

# Highly inspirated by https://github.com/ManageIQ/miq_bot/blob/master/app/workers/travis_build_killer.rb

require 'rubygems'
require 'bundler/setup'

require 'travis'

logger = Logger.new(STDOUT)

Travis::Pro.access_token = ENV['TRAVIS_ACCESS_TOKEN']
travis_repo = Travis::Pro::Repository.find(ENV['TRAVIS_REPO_SLUG'])

pending_builds = travis_repo.builds.take_while { |b| b.pending? || b.canceled? }.reject(&:canceled?)

logger.info "Found #{pending_builds.count} builds in queue"

out_dated_builds = pending_builds.group_by { |b| b.pull_request_number || b.branch_info }.flat_map { |_key, builds| builds[1..-1] }
out_dated_builds.each do |b|
    if b.pull_request?
      logger.info "Canceling Travis build ##{b.number} for PR ##{b.pull_request_number}"
      b.cancel
    elsif b.running?
      logger.info "Skipping currently running Travis build ##{b.number} for merge commit"
    else
      logger.info "Canceling Travis build ##{b.number} for merge commit"
      b.cancel
    end
end

long_running_jobs = pending_builds.select(&:running?).flat_map { |b| b.jobs.select { |j| j.running? && (Time.now.getlocal - j.started_at) >= 3600 } }

logger.info "Found #{long_running_jobs.count} long running jobs"

long_running_jobs.each do |b|
    logger.info "  - canceling long running Travis build ##{b.number}, something must be wrong."
    b.cancel
end