debian-mirror-gitlab/doc/development/utilities.md

GitLab utilities

We developed a number of utilities to ease development.

MergeHash

• Deep merges an array of hashes:

  Gitlab::Utils::MergeHash.merge(
    [{ hello: ["world"] },
     { hello: "Everyone" },
     { hello: { greetings: ['Bonjour', 'Hello', 'Hallo', 'Dzien dobry'] } },
      "Goodbye", "Hallo"]
  )
  

  Gives:

  [
    {
      hello:
        [
          "world",
          "Everyone",
          { greetings: ['Bonjour', 'Hello', 'Hallo', 'Dzien dobry'] }
        ]
    },
    "Goodbye"
  ]
  

• Extracts all keys and values from a hash into an array:

  Gitlab::Utils::MergeHash.crush(
    { hello: "world", this: { crushes: ["an entire", "hash"] } }
  )
  

  Gives:

  [:hello, "world", :this, :crushes, "an entire", "hash"]
  

Override

• This utility could help us check if a particular method would override another method or not. It has the same idea of Java's @Override annotation or Scala's override keyword. However we only do this check when ENV['STATIC_VERIFICATION'] is set to avoid production runtime overhead. This is useful to check:

  • If we have typos in overriding methods.

  • If we renamed the overridden methods, making original overriding methods overrides nothing.

    Here's a simple example:

    class Base
      def execute
      end
    end
    
    class Derived < Base
      extend ::Gitlab::Utils::Override
    
      override :execute # Override check happens here
      def execute
      end
    end
    

    This also works on modules:

    module Extension
      extend ::Gitlab::Utils::Override
    
      override :execute # Modules do not check this immediately
      def execute
      end
    end
    
    class Derived < Base
      prepend Extension # Override check happens here, not in the module
    end
    

StrongMemoize

• Memoize the value even if it is nil or false.

  We often do @value ||= compute, however this doesn't work well if compute might eventually give nil and we don't want to compute again. Instead we could use defined? to check if the value is set or not. However it's tedious to write such pattern, and StrongMemoize would help us use such pattern.

  Instead of writing patterns like this:

  class Find
    def result
      return @result if defined?(@result)
  
      @result = search
    end
  end
  

  We could write it like:

  class Find
    include Gitlab::Utils::StrongMemoize
  
    def result
      strong_memoize(:result) do
        search
      end
    end
  end
  

• Clear memoization

  class Find
    include Gitlab::Utils::StrongMemoize
  end
  
  Find.new.clear_memoization(:result)
  

RequestCache

This module provides a simple way to cache values in RequestStore, and the cache key would be based on the class name, method name, optionally customized instance level values, optionally customized method level values, and optional method arguments.

A simple example that only uses the instance level customised values:

class UserAccess
  extend Gitlab::Cache::RequestCache

  request_cache_key do
    [user&.id, project&.id]
  end

  request_cache def can_push_to_branch?(ref)
    # ...
  end
end

This way, the result of can_push_to_branch? would be cached in RequestStore.store based on the cache key. If RequestStore is not currently active, then it would be stored in a hash saved in an instance variable, so the cache logic would be the same.

We can also set different strategies for different methods:

class Commit
  extend Gitlab::Cache::RequestCache

  def author
    User.find_by_any_email(author_email)
  end
  request_cache(:author) { author_email }
end