Add buffering to output received from runner

app/errors/runner/connection/buffer/error.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class Runner::Connection::Buffer
+  class Error < ApplicationError
+    class NotEmpty < Error; end
+  end
+end

lib/runner/connection.rb

@@ -18,6 +18,7 @@ class Runner::Connection
     @status = :established
     @event_loop = event_loop
     @locale = locale
+    @buffer = Buffer.new
 
     # For every event type of Faye WebSockets, the corresponding
     # RunnerConnection method starting with `on_` is called.
@@ -78,25 +79,27 @@ class Runner::Connection
 
   def on_message(raw_event)
     Rails.logger.debug { "#{Time.zone.now.getutc}: Receiving from #{@socket.url}: #{raw_event.data.inspect}" }
-    # The WebSocket connection might group multiple lines. For further processing, we require all lines
+    @buffer.store raw_event.data
-    # to be processed separately. Therefore, we split the lines by each newline character not part of an enclosed
+    @buffer.events.each do |event_data|
-    # substring either in single or double quotes (e.g., within a JSON)
+      forward_message event_data
-    # Inspired by https://stackoverflow.com/questions/13040585/split-string-by-spaces-properly-accounting-for-quotes-and-backslashes-ruby
-    raw_event.data.scan(/(?:"(?:\\.|[^"])*"|'(?:\\.|[^'])*'|[^"\n])+/).each do |event_data|
-      event = decode(event_data)
-      next unless BACKEND_OUTPUT_SCHEMA.valid?(event)
-
-      event = event.deep_symbolize_keys
-      message_type = event[:type].to_sym
-      if WEBSOCKET_MESSAGE_TYPES.include?(message_type)
-        __send__("handle_#{message_type}", event)
-      else
-        @error = Runner::Error::UnexpectedResponse.new("Unknown WebSocket message type: #{message_type}")
-        close(:error)
-      end
     end
   end
 
+  def forward_message(event_data)
+    event = decode(event_data)
+    return unless BACKEND_OUTPUT_SCHEMA.valid?(event)
+
+    event = event.deep_symbolize_keys
+    message_type = event[:type].to_sym
+    if WEBSOCKET_MESSAGE_TYPES.include?(message_type)
+      __send__("handle_#{message_type}", event)
+    else
+      @error = Runner::Error::UnexpectedResponse.new("Unknown WebSocket message type: #{message_type}")
+      close(:error)
+    end
+  end
+  private :forward_message
+
   def on_open(_event)
     @start_callback.call
   end
@@ -105,6 +108,7 @@ class Runner::Connection
 
   def on_close(_event)
     Rails.logger.debug { "#{Time.zone.now.getutc}: Closing connection to #{@socket.url} with status: #{@status}" }
+    forward_message @buffer.flush
     case @status
       when :timeout
         @error = Runner::Error::ExecutionTimeout.new('Execution exceeded its time limit')

lib/runner/connection/buffer.rb

@@ -0,0 +1,96 @@
+# frozen_string_literal: true
+
+class Runner::Connection::Buffer
+  # The WebSocket connection might group multiple lines. For further processing, we require all lines
+  # to be processed separately. Therefore, we split the lines by each newline character not part of an enclosed
+  # substring either in single or double quotes (e.g., within a JSON)
+  # Inspired by https://stackoverflow.com/questions/13040585/split-string-by-spaces-properly-accounting-for-quotes-and-backslashes-ruby
+  SPLIT_INDIVIDUAL_LINES = Regexp.compile(/(?:"(?:\\.|[^"])*"|'(?:\\.|[^'])*'|[^\n])+/)
+
+  def initialize
+    @global_buffer = +''
+    @buffering = false
+    @line_buffer = Queue.new
+    super
+  end
+
+  def store(event_data)
+    # First, we append the new data to the existing `@global_buffer`.
+    # Either, the `@global_buffer` is empty and this is a NO OP.
+    # Or, the `@global_buffer` contains an incomplete string and thus requires the new part.
+    @global_buffer += event_data
+    # We process the full `@global_buffer`. Valid parts are removed from the buffer and
+    # the remaining invalid parts are still stored in `@global_buffer`.
+    @global_buffer = process_and_split @global_buffer
+  end
+
+  def events
+    # Return all items from `@line_buffer` in an array (which is iterable) and clear the queue
+    Array.new(@line_buffer.size) { @line_buffer.pop }
+  end
+
+  def flush
+    raise Error::NotEmpty unless @line_buffer.empty?
+
+    remaining_buffer = @global_buffer
+    @buffering = false
+    @global_buffer = +''
+    remaining_buffer
+  end
+
+  private
+
+  def process_and_split(message_parts, stop: false)
+    # We need a temporary buffer to operate on
+    buffer = +''
+    message_parts.scan(SPLIT_INDIVIDUAL_LINES).each do |line|
+      # Same argumentation as above: We can always append (previous empty or invalid)
+      buffer += line
+
+      if buffering_required_for? buffer
+        @buffering = true
+        # Check the existing substring `buffer` if it contains a valid message.
+        # The remaining buffer is stored for further processing.
+        buffer = process_and_split buffer, stop: true unless stop
+      else
+        add_to_line_buffer(buffer)
+        # Clear the current buffer.
+        buffer = +''
+      end
+    end
+    # Return the remaining buffer which might become the `@global_buffer`
+    buffer
+  end
+
+  def add_to_line_buffer(message)
+    @buffering = false
+    @global_buffer = +''
+    @line_buffer.push message
+  end
+
+  def buffering_required_for?(message)
+    # First, check if the message is very short and start with {
+    return true if message.size <= 5 && message.start_with?(/\s*{/)
+
+    invalid_json = !valid_json?(message)
+    # Second, if we have the beginning of a valid command but an invalid JSON
+    return true if invalid_json && message.start_with?(/\s*{"cmd/)
+    # Third, global_buffer the message if it contains long messages (e.g., an image or turtle batch commands)
+    return true if invalid_json && (message.include?('<img') || message.include?('"turtlebatch"'))
+
+    # If nothing applies, we don't want to global_buffer the current message
+    false
+  end
+
+  def currently_buffering?
+    @buffering
+  end
+
+  def valid_json?(data)
+    # Try parsing the JSON. If that is successful, we have a valid JSON (otherwise not)
+    JSON.parse(data)
+    true
+  rescue JSON::ParserError
+    false
+  end
+end

lib/runner/strategy/docker_container_pool.rb

@@ -124,6 +124,9 @@ class Runner::Strategy::DockerContainerPool < Runner::Strategy
     def decode(event_data)
       case event_data
         when /(@#{@strategy.container_id[0..11]}|#exit)/
+          # TODO: The whole message line is kept back. If this contains the remaining buffer, this buffer is also lost.
+          # Example: A Java program prints `{` and then exists (with `#exit`). The `event_data` processed here is `{#exit`
+
           # Assume correct termination for now and return exit code 0
           # TODO: Can we use the actual exit code here?
           @exit_code = 0