Attach connection errors to socket

Raising the errors would crash the current thread. As this thread
contains the Eventmachine, that would influence other connections
as well. Attaching the errors to the connection and reading them
after the connection was closed ensures that the thread stays
alive while handling the errors in the main thread of the current
request.

lib/runner/connection.rb

@@ -10,6 +10,7 @@ class Runner::Connection
   BACKEND_OUTPUT_SCHEMA = JSONSchemer.schema(JSON.parse(File.read('lib/runner/backend-output.schema.json')))
 
   attr_writer :status
+  attr_reader :error
 
   def initialize(url, strategy, event_loop)
     @socket = Faye::WebSocket::Client.new(url, [], ping: 5)
@@ -73,7 +74,8 @@ class Runner::Connection
     if WEBSOCKET_MESSAGE_TYPES.include?(message_type)
       __send__("handle_#{message_type}", event)
     else
-      raise Runner::Error::UnexpectedResponse.new("Unknown WebSocket message type: #{message_type}")
+      @error = Runner::Error::UnexpectedResponse.new("Unknown WebSocket message type: #{message_type}")
+      close(:error)
     end
   end
 
@@ -87,17 +89,16 @@ class Runner::Connection
     Rails.logger.debug { "#{Time.zone.now.getutc}: Closing connection to #{@socket.url} with status: #{@status}" }
     case @status
       when :timeout
-        raise Runner::Error::ExecutionTimeout.new('Execution exceeded its time limit')
+        @error = Runner::Error::ExecutionTimeout.new('Execution exceeded its time limit')
       when :terminated_by_codeocean, :terminated_by_management
         @exit_callback.call @exit_code
-        @event_loop.stop
-      when :terminated_by_client
-        @event_loop.stop
+      when :terminated_by_client, :error
       else # :established
         # If the runner is killed by the DockerContainerPool after the maximum allowed time per user and
         # while the owning user is running an execution, the command execution stops and log output is incomplete.
-        raise Runner::Error::Unknown.new('Execution terminated with an unknown reason')
+        @error = Runner::Error::Unknown.new('Execution terminated with an unknown reason')
     end
+    @event_loop.stop
   end
 
   def handle_exit(event)

lib/runner/strategy/docker_container_pool.rb

@@ -68,6 +68,7 @@ class Runner::Strategy::DockerContainerPool < Runner::Strategy
       socket.close(:timeout)
       destroy_at_management
     end
+    socket
   end
 
   private

lib/runner/strategy/poseidon.rb

@@ -89,6 +89,7 @@ class Runner::Strategy::Poseidon < Runner::Strategy
     websocket_url = execute_command(command)
     socket = Connection.new(websocket_url, self, event_loop)
     yield(socket)
+    socket
   end
 
   def destroy_at_management
@@ -130,7 +131,8 @@ class Runner::Strategy::Poseidon < Runner::Strategy
     def decode(raw_event)
       JSON.parse(raw_event.data)
     rescue JSON::ParserError => e
-      raise Runner::Error::UnexpectedResponse.new("The WebSocket message from Poseidon could not be decoded to JSON: #{e.inspect}")
+      @error = Runner::Error::UnexpectedResponse.new("The WebSocket message from Poseidon could not be decoded to JSON: #{e.inspect}")
+      close(:error)
     end
 
     def encode(data)