GitLab utilities

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

`MergeHash`

Refer to: https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/utils/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 https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/utils/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 will only happen 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.

`StrongMemoize`

Refer to https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/utils/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 https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/cache/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`

The ReactiveCaching concern is used to fetch some data in the background and store it in the Rails cache, keeping it up-to-date for as long as it is being requested. If the data hasn't been requested for reactive_cache_lifetime, it will stop being refreshed, and then be removed.

Example of use:

class Foo < ApplicationRecord
  include ReactiveCaching

  after_save :clear_reactive_cache!

  def calculate_reactive_cache
    # Expensive operation here. The return value of this method is cached
  end

  def result
    with_reactive_cache do |data|
      # ...
    end
  end
end

In this example, the first time #result is called, it will return nil. However, it will enqueue a background worker to call #calculate_reactive_cache and set an initial cache lifetime of ten minutes.

The background worker needs to find or generate the object on which with_reactive_cache was called. The default behaviour can be overridden by defining a custom reactive_cache_worker_finder. Otherwise, the background worker will use the class name and primary key to get the object using the ActiveRecord find_by method.

class Bar
  include ReactiveCaching

  self.reactive_cache_key = ->() { ["bar", "thing"] }
  self.reactive_cache_worker_finder = ->(_id, *args) { from_cache(*args) }

  def self.from_cache(var1, var2)
    # This method will be called by the background worker with "bar1" and
    # "bar2" as arguments.
    new(var1, var2)
  end

  def initialize(var1, var2)
    # ...
  end

  def calculate_reactive_cache
    # Expensive operation here. The return value of this method is cached
  end

  def result
    with_reactive_cache("bar1", "bar2") do |data|
      # ...
    end
  end
end

Each time the background job completes, it stores the return value of #calculate_reactive_cache. It is also re-enqueued to run again after reactive_cache_refresh_interval, therefore, it will keep the stored value up to date. Calculations are never run concurrently.

Calling #result while a value is cached will call the block given to #with_reactive_cache, yielding the cached value. It will also extend the lifetime by the reactive_cache_lifetime value.

Once the lifetime has expired, no more background jobs will be enqueued and calling #result will again return nil - starting the process all over again.