lambda.rb

# MIT No Attribution

# Permission is hereby granted, free of charge, to any person obtaining a copy of this
# software and associated documentation files (the "Software"), to deal in the Software
# without restriction, including without limitation the rights to use, copy, modify,
# merge, publish, distribute, sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so.

# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
# INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
# PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
# HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
# SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

require 'json'
require 'rack'
require 'base64'

# Global object that responds to the call method. Stay outside of the handler
# to take advantage of container reuse
$app ||= Rack::Builder.parse_file("#{__dir__}/app/config.ru").first
ENV['RACK_ENV'] ||= 'production'


def handler(event:, context:)
  # Check if the body is base64 encoded. If it is, try to decode it
  body = if event['isBase64Encoded']
    Base64.decode64 event['body']
  else
    event['body']
  end || ''

  # Rack expects the querystring in plain text, not a hash
  headers = event.fetch 'headers', {}

  # Environment required by Rack (http://www.rubydoc.info/github/rack/rack/file/SPEC)
  env = {
    'REQUEST_METHOD' => event.fetch('httpMethod'),
    'SCRIPT_NAME' => '',
    'PATH_INFO' => event.fetch('path', ''),
    'QUERY_STRING' => (event['queryStringParameters'] || {}).map { |k,v| "#{k}=#{v}" }.join('&'),
    'SERVER_NAME' => headers.fetch('Host', 'localhost'),
    'SERVER_PORT' => headers.fetch('X-Forwarded-Port', 443).to_s,

    'rack.version' => Rack::VERSION,
    'rack.url_scheme' => headers.fetch('CloudFront-Forwarded-Proto') { headers.fetch('X-Forwarded-Proto', 'https') },
    'rack.input' => StringIO.new(body),
    'rack.errors' => $stderr,
  }

  # Pass request headers to Rack if they are available
  headers.each_pair do |key, value|
    # 'CloudFront-Forwarded-Proto' => 'CLOUDFRONT_FORWARDED_PROTO'
    # Content-Type and Content-Length are handled specially per the Rack SPEC linked above.
    name = key.upcase.gsub '-', '_'
    header = case name
      when 'CONTENT_TYPE', 'CONTENT_LENGTH'
        name
      else
        "HTTP_#{name}"
    end
    env[header] = value.to_s
  end

  begin
    # Response from Rack must have status, headers and body
    status, headers, body = $app.call env

    # body is an array. We combine all the items to a single string
    body_content = ""
    body.each do |item|
      body_content += item.to_s
    end

    # We return the structure required by AWS API Gateway since we integrate with it
    # https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html
    response = {
      'statusCode' => status,
      'headers' => headers,
      'body' => body_content
    }
    if event['requestContext'].has_key?('elb')
      # Required if we use Application Load Balancer instead of API Gateway
      response['isBase64Encoded'] = false
    end
  rescue Exception => exception
    # If there is _any_ exception, we return a 500 error with an error message
    response = {
      'statusCode' => 500,
      'body' => exception.message
    }
  end

  # By default, the response serializer will call #to_json for us
  response
end