Ignore ivar set by graphql-anycable during compatibility check

[alexeisersun-mixbook]

What is the purpose of this pull request?

Fix compatibility between graphql-anycable and anycable/rails/compatibility module.

Minimal steps to reproduce:

In development environment:

# in Gemfile
gem "anycable-rails" # 1.6.1
gem "graphql-anycable" # 1.3.1

# in config/application.rb
require "anycable/rails/compatibility"

Then configure GraphQL channel as per official README.

In AnyCable WS logs it will show:

 WRN failed to handle incoming message [...] data="{\"id\":\"...\",\"type\":\"subscribe\",\"payload\":{\"variables\":{},\"operation...(133)" error="perform failed for {\"channel\":\"GraphqlChannel\",\"channelId\":\"...\"}, cause: Application error: Channel instance variables are not supported by AnyCable, but were set: @__sid__"

A similar error will be shown in AnyCable RPC logs as well.

What changes did you make? (overview)

Ignore @__sid__ instance variables, which is used internally by Grahpql::Subscriptions::AnyCableSubscriptions for connection management, when AnyCable::Compatibility runs.

Is there anything you'd like reviewers to focus on?

Checklist

• I've added tests for this change
• I've added a Changelog entry
• I've updated documentation