Monkey-patch Rails ActionCable for deterministic behavior in tests

action_cable_sync.rb

# frozen_string_literal: true

# Make ActionCable stream delivery synchronous so that Turbo broadcast_replace_to
# calls complete before Capybara assertions run in system tests.
#
# There are two async hops between broadcast_replace_to and the DOM update:
#
# Hop 1 — AsyncSubscriberMap#invoke_callback (the test adapter extends async)
#   posts each subscriber callback onto the StreamEventLoop's thread pool
#   ("ActionCable-streamer") via @event_loop.post { super }. This fires before
#   worker_pool_stream_handler runs, so the entire delivery is off-thread.
#   Fix: override invoke_callback to call super directly, skipping the post.
#
# Hop 2 — worker_pool_stream_handler dispatches the decoded message via
#   connection.worker_pool.async_invoke, posting onto another ThreadPoolExecutor.
#   Fix: redefine to call invoke (synchronous) instead — invoke calls work()
#   directly on the calling thread, bypassing the executor entirely.
#
# With this setup, the entire broadcast_replace_to flow runs synchronously in the test thread,
# so that Capybara assertions can reliably observe the DOM updates:
#
ActionCable::SubscriptionAdapter::Async::AsyncSubscriberMap.module_eval do
  def invoke_callback(*)
    super
  end
end

ActionCable::Channel::Streams.module_eval do
  private

  def worker_pool_stream_handler(broadcasting, user_handler, coder: nil)
    handler = stream_handler(broadcasting, user_handler, coder: coder)

    lambda do |message|
      now_ns = Process.clock_gettime(Process::CLOCK_REALTIME, :nanosecond)
      now_iso = Time.at(now_ns / 1_000_000_000r).in_time_zone.iso8601(9)
      Rails.logger.debug { "Invoking ActionCable stream handler synchronously for message: #{message.inspect} at #{now_iso} (epoch_ns=#{now_ns})" }
      connection.worker_pool.invoke handler, :call, message, connection: connection
    end
  end
end