[ActionCable] Add sentinel support to ActionCable Redis subscription adapter

[aisrael]

When attempting to use ActionCable with Redis Sentinel, it fails to establish a connection. Even if config/cable.yml is configured similarly to the Rails cache and session stores, the Redis connector in the subscription adapter doesn't recognize the sentinel configuration.

Steps to reproduce

1. Deploy Redis for HA using Redis Sentinel (see https://redis.io/topics/sentinel).
2. Configure Rails cache and session store with Redis Sentinel (https://github.com/redis-store/redis-rails#usage-with-redis-sentinel). Works.
3. Configure ActionCable Redis with sentinels by supplying the following to config/cable.yml:

development:
  adapter: redis
  url: redis://mymaster
  sentinels:
  - host: redis-sentinel
    port: 26379

Expected behavior

ActionCable will be able to connect to the Redis instance(s) through the Redis sentinel(s).

Actual behavior

SocketError in TweetsController#create
getaddrinfo: nodename nor servname provided, or not known

...

actioncable (5.1.6) lib/action_cable/subscription_adapter/redis.rb:22:in `broadcast'
actioncable (5.1.6) lib/action_cable/subscription_adapter/channel_prefix.rb:6:in `broadcast'
actioncable (5.1.6) lib/action_cable/server/broadcasting.rb:46:in `block in broadcast'
activesupport (5.1.6) lib/active_support/notifications.rb:168:in `instrument'
actioncable (5.1.6) lib/action_cable/server/broadcasting.rb:44:in `broadcast'
actioncable (5.1.6) lib/action_cable/server/broadcasting.rb:23:in `broadcast'

Workaround

Add config/initializers/action_cable.rb as follows:

require 'action_cable/subscription_adapter/redis'

ActionCable::SubscriptionAdapter::Redis.redis_connector = lambda do |config|
  redis_opts = { url: config[:url] }
  redis_opts[:sentinels] = config[:sentinels] if config.key?(:sentinels)
  ::Redis.new(redis_opts)
end

While this is a suitable workaround, it should be easy enough to modify the Redis connector such that it recognizes Redis sentinel configuration if present. For example:

      # Overwrite this factory method for redis connections if you want to use a different Redis library than Redis.
      # This is needed, for example, when using Makara proxies for distributed Redis.
      cattr_accessor(:redis_connector) do ->(config) {
        redis_opts = { url: config[:url] }
        redis_opts[:sentinels] = config[:sentinels] if config.key?(:sentinels)
        ::Redis.new(redis_opts)
      end

If desired, I can gladly create a PR.

System configuration

Rails version: Rails 5.1.6

Ruby version: ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]

Redis version: Redis server v=4.0.9 sha=00000000:0 malloc=libc bits=64 build=e0c8d37381c486c6

[rails-bot]

This issue has been automatically marked as stale because it has not been commented on for at least three months.
The resources of the Rails team are limited, and so we are asking for your help.
If you can still reproduce this error on the 5-2-stable branch or on master, please reply with all of the information you have about it in order to keep the issue open.
Thank you for all your contributions.

[georgeclaghorn]

This was fixed in #36053.