Fix active-elastic-job#51 Add lazy loading of AWS credential configur…

…ation

The default configuration of AWS credentials for this gem
uses AWS instance profiles for retrieving credentials. It
seems that `Aws::InstanceProfileCredentials.new` is blocking
if not executed on an actual EC2 instance, which consequently
impacts the startup time of the application. Therefore,
we postpone the initialization of the credential configuration
to when it's actually needed in the queue adapter.

README.md

@@ -100,12 +100,19 @@ The snippet below shows the various configurable settings and their defaults.
   ```Ruby
   Rails.application.configure do
     config.active_elastic_job.process_jobs = ENV['PROCESS_ACTIVE_ELASTIC_JOBS'] == 'true'
-    config.active_elastic_job.aws_credentials = Aws::InstanceProfileCredentials.new
+    config.active_elastic_job.aws_credentials = lambda { Aws::InstanceProfileCredentials.new } # allows lambdas for lazy loading
     config.active_elastic_job.secret_key_base = Rails.application.secrets[:secret_key_base]
     cofnig.active_elastic_job.periodic_tasks_route = '/periodic_tasks'.freeze
   end
   ```
 
+If you want to provide the AWS credentials not by the EC2 instance prodfile, but via environment variables, you can do so:
+  ```Ruby
+  Rails.application.configure do
+    config.active_elastic_job.aws_credentials = Aws::Credentials.new(ENV['AWS_ACCESS_KEY_ID'], ENV['AWS_SECRET_ACCESS_KEY'])
+  end
+  ```
+
 ## FAQ
 A summary of frequently asked questions:
 ### What are the advantages in comparison to popular alternatives like Resque, Sidekiq or DelayedJob?

lib/active_elastic_job/railtie.rb

@@ -2,7 +2,7 @@ module ActiveElasticJob
   class Railtie < Rails::Railtie
     config.active_elastic_job = ActiveSupport::OrderedOptions.new
     config.active_elastic_job.process_jobs = ENV['PROCESS_ACTIVE_ELASTIC_JOBS'] == 'true'
-    config.active_elastic_job.aws_credentials = Aws::InstanceProfileCredentials.new
+    config.active_elastic_job.aws_credentials = lambda { Aws::InstanceProfileCredentials.new }
     config.active_elastic_job.periodic_tasks_route = '/periodic_tasks'.freeze
 
     initializer "active_elastic_job.insert_middleware" do |app|

lib/active_job/queue_adapters/active_elastic_job_adapter.rb

@@ -169,7 +169,11 @@ def aws_sqs_client
         end
 
         def aws_sqs_client_credentials
-          config.aws_credentials
+          @aws_credentials ||= if config.aws_credentials.kind_of?(Proc)
+                                 config.aws_credentials.call
+                               else
+                                 config.aws_credentials
+                               end
         end
 
         def aws_region