ActionCable: use find method when unsubscribing

[rmachielse]

Summary

If a frontend for some reason tries to unsubscribe from a non existing action cable subscription, the following error is logged:

Could not execute command from ({"command"=>"unsubscribe", "identifier"=>"{\"channel\":\"SomeChannel\"}"}) [NoMethodError - undefined method `unsubscribe_from_channel' for nil:NilClass]:
/app/vendor/bundle/ruby/2.4.0/gems/actioncable-5.1.4/lib/action_cable/connection/subscriptions.rb:48:in `remove_subscription'
/app/vendor/bundle/ruby/2.4.0/gems/actioncable-5.1.4/lib/action_cable/connection/subscriptions.rb:44:in `remove'
/app/vendor/bundle/ruby/2.4.0/gems/actioncable-5.1.4/lib/action_cable/connection/subscriptions.rb:16:in `execute_command'
/app/vendor/bundle/ruby/2.4.0/gems/actioncable-5.1.4/lib/action_cable/connection/base.rb:85:in `dispatch_websocket_message' /app/vendor/bundle/ruby/2.4.0/gems/actioncable-5.1.4/lib/action_cable/server/worker.rb:58:in `block in invoke'

Instead, it will now properly log:

Could not execute command from ({"command"=>"unsubscribe", "identifier"=>"{\"channel\":\"SomeChannel\"}"}) [RuntimeError - Unable to find subscription with identifier: {"channel":"SomeChannel"}]:
/app/vendor/bundle/ruby/2.4.0/gems/actioncable-5.1.4/lib/action_cable/connection/subscriptions.rb:76:in `find'
/app/vendor/bundle/ruby/2.4.0/gems/actioncable-5.1.4/lib/action_cable/connection/subscriptions.rb:44:in `remove'
/app/vendor/bundle/ruby/2.4.0/gems/actioncable-5.1.4/lib/action_cable/connection/subscriptions.rb:16:in `execute_command'
/app/vendor/bundle/ruby/2.4.0/gems/actioncable-5.1.4/lib/action_cable/connection/base.rb:85:in `dispatch_websocket_message'
/app/vendor/bundle/ruby/2.4.0/gems/actioncable-5.1.4/lib/action_cable/server/worker.rb:58:in `block in invoke

It's a subtle difference but it helps a lot when debugging.

[rails-bot]

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[rmachielse]

@pixeltrix can you or anyone else review this?

[kamipo]

Thanks!