GitLab utilities

We have developed a number of utilities to help ease development:

MergeHash

Refer to merge_hash.rb:

• 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

Refer to override.rb:

• This utility can help you check if one method would override another or not. It is the same concept as Java's @Override annotation or Scala's override keyword. However, we only run this check when ENV['STATIC_VERIFICATION'] is set to avoid production runtime overhead. This is useful for checking:

  • If you have typos in overriding methods.

  • If you renamed the overridden methods, which make the original override methods irrelevant.

    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

    Note that the check only happens when either:

    • The overriding method is defined in a class, or:
    • The overriding method is defined in a module, and it's prepended to a class or a module.

    Because only a class or prepended module can actually override a method. Including or extending a module into another cannot override anything.

Interactions with ActiveSupport::Concern, prepend, and class_methods

When you use ActiveSupport::Concern that includes class methods, you do not get expected results because ActiveSupport::Concern doesn't work like a regular Ruby module.

Since we already have Prependable as a patch for ActiveSupport::Concern to enable prepend, it has consequences with how it would interact with override and class_methods. As a workaround, extend ClassMethods into the defining Prependable module.

This allows us to use override to verify class_methods used in the context mentioned above. This workaround only applies when we run the verification, not when running the application itself.

Here are example code blocks that demonstrate the effect of this workaround: following codes:

module Base
  extend ActiveSupport::Concern

  class_methods do
    def f
    end
  end
end

module Derived
  include Base
end

# Without the workaround
Base.f    # => NoMethodError
Derived.f # => nil

# With the workaround
Base.f    # => nil
Derived.f # => nil

StrongMemoize

Refer to strong_memoize.rb:

• 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 you don't want to compute again. Instead you could use defined? to check if the value is set or not. It's tedious to write such pattern, and StrongMemoize would help you use such pattern.

  Instead of writing patterns like this:

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

  You 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

Refer to request_cache.rb.

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 is:

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, and 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

ReactiveCaching

Read the documentation on ReactiveCaching.