Add GraphQL channel for handling subscriptions

This ActionCable channel will be used to execute GraphQL subscriptions

app/channels/graphql_channel.rb

+# frozen_string_literal: true
+
+# This is based on https://github.com/rmosolgo/graphql-ruby/blob/v1.11.8/lib/graphql/subscriptions/action_cable_subscriptions.rb#L19-L82
+# modified to work with our own ActionCableLink client
+
+class GraphqlChannel < ApplicationCable::Channel # rubocop:disable Gitlab/NamespacedClass
+  def subscribed
+    @subscription_ids = []
+
+    query = params['query']
+    variables = Gitlab::Graphql::Variables.new(params['variables']).to_h
+    operation_name = params['operationName']
+
+    result = GitlabSchema.execute(
+      query,
+      context: context,
+      variables: variables,
+      operation_name: operation_name
+    )
+
+    payload = {
+      result: result.to_h,
+      more: result.subscription?
+    }
+
+    # Track the subscription here so we can remove it
+    # on unsubscribe.
+    if result.context[:subscription_id]
+      @subscription_ids << result.context[:subscription_id]
+    end
+
+    transmit(payload)
+  end
+
+  def unsubscribed
+    @subscription_ids.each do |sid|
+      GitlabSchema.subscriptions.delete_subscription(sid)
+    end
+  end
+
+  rescue_from Gitlab::Graphql::Variables::Invalid do |exception|
+    transmit({ errors: [{ message: exception.message }] })
+  end
+
+  private
+
+  # When modifying the context, also update GraphqlController#context if needed
+  # so that we have similar context when executing queries, mutations, and subscriptions
+  #
+  # Objects added to the context may also need to be reloaded in
+  # `Subscriptions::BaseSubscription` so that they are not stale
+  def context
+    # is_sessionless_user is always false because we only support cookie auth in ActionCable
+    { channel: self, current_user: current_user, is_sessionless_user: false }
+  end
+end

app/controllers/graphql_controller.rb

@@ -109,6 +109,8 @@ class GraphqlController < ApplicationController
     end
   end
 
+  # When modifying the context, also update GraphqlChannel#context if needed
+  # so that we have similar context when executing queries, mutations, and subscriptions
   def context
     @context ||= { current_user: current_user, is_sessionless_user: !!sessionless_user?, request: request }
   end

app/graphql/gitlab_schema.rb

@@ -10,6 +10,7 @@ class GitlabSchema < GraphQL::Schema
   DEFAULT_MAX_DEPTH = 15
   AUTHENTICATED_MAX_DEPTH = 20
 
+  use GraphQL::Subscriptions::ActionCableSubscriptions
   use GraphQL::Pagination::Connections
   use BatchLoader::GraphQL
   use Gitlab::Graphql::Pagination::Connections
@@ -24,6 +25,7 @@ class GitlabSchema < GraphQL::Schema
 
   query Types::QueryType
   mutation Types::MutationType
+  subscription Types::SubscriptionType
 
   default_max_page_size 100
 

app/graphql/subscriptions/base_subscription.rb

+# frozen_string_literal: true
+
+module Subscriptions
+  class BaseSubscription < GraphQL::Schema::Subscription
+    object_class Types::BaseObject
+    field_class Types::BaseField
+
+    def initialize(object:, context:, field:)
+      super
+
+      # Reset user so that we don't use a stale user for authorization
+      current_user.reset if current_user
+    end
+
+    def authorized?(*)
+      raise NotImplementedError
+    end
+
+    private
+
+    def current_user
+      context[:current_user]
+    end
+  end
+end