Correct aws_lb_listener_rule priorities to avoid Terraform recreating them unnecessarily

fix-aws-lb-listener-rule-priorities.rb

#!/usr/bin/ruby

# Terraform can't adjust priorities of `aws_lb_listener_rule`s without recreating them.
# If you have external dependencies on the ARNs this can be quite annoying.
# This script will fetch the ARNs out of terraform state and produce JSON to apply the defined priorities using `aws elbv2 set-rule-priorities`.

# Usage:
#   $ terraform workspace select <environment-or-whatever>
#   $ aws --region eu-west-1 elbv2 set-rule-priorities --rule-priorities $(./fix-aws-lb-listener-rule-priorities.rb)

require 'json'

Expecting = {
	'aws_lb_listener_rule.my_rule' => 101,
	'aws_lb_listener_rule.my_other_rule' => 301,
}

State = JSON.parse(ARGV[0] ? File.read(ARGV[0]) : `terraform state pull`)
Ids = {}

State['modules'].each do |m|
	next unless m['path'] == ['root']
	m['resources'].each do |name, details|
		Ids[name] = details['primary']['id'] if Expecting.keys.include?(name)
	end
end

results = []
Ids.each do |name, id|
	results << {RuleArn: id, Priority: Expecting[name] || raise}
end
puts JSON.dump(results)