Buffer writes to the cable sockets

Otherwise, they can sometimes block, leading to reduced system
throughput.

actioncable/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Buffer writes to websocket connections, to avoid blocking threads
+    that could be doing more useful things.
+
+    *Matthew Draper*, *Tinco Andringa*
+
 *   Protect against concurrent writes to a websocket connection from
     multiple threads; the underlying OS write is not always threadsafe.
 

actioncable/lib/action_cable/connection/stream.rb

@@ -14,6 +14,9 @@ def initialize(event_loop, socket)
 
         @rack_hijack_io = nil
         @write_lock = Mutex.new
+
+        @write_head = nil
+        @write_buffer = Queue.new
       end
 
       def each(&callback)
@@ -30,14 +33,62 @@ def shutdown
       end
 
       def write(data)
-        @write_lock.synchronize do
+        if @stream_send
-          return @rack_hijack_io.write(data) if @rack_hijack_io
+          return @stream_send.call(data)
-          return @stream_send.call(data) if @stream_send
         end
+
+        if @write_lock.try_lock
+          begin
+            if @write_head.nil? && @write_buffer.empty?
+              written = @rack_hijack_io.write_nonblock(data, exception: false)
+
+              case written
+              when :wait_writable
+                # proceed below
+              when data.bytesize
+                return data.bytesize
+              else
+                @write_head = data.byteslice(written, data.bytesize)
+                @event_loop.writes_pending @rack_hijack_io
+
+                return data.bytesize
+              end
+            end
+          ensure
+            @write_lock.unlock
+          end
+        end
+
+        @write_buffer << data
+        @event_loop.writes_pending @rack_hijack_io
+
+        data.bytesize
       rescue EOFError, Errno::ECONNRESET
         @socket_object.client_gone
       end
 
+      def flush_write_buffer
+        @write_lock.synchronize do
+          loop do
+            if @write_head.nil?
+              return true if @write_buffer.empty?
+              @write_head = @write_buffer.pop
+            end
+
+            written = @rack_hijack_io.write_nonblock(@write_head, exception: false)
+            case written
+            when :wait_writable
+              return false
+            when @write_head.bytesize
+              @write_head = nil
+            else
+              @write_head = @write_head.byteslice(written, @write_head.bytesize)
+              return false
+            end
+          end
+        end
+      end
+
       def receive(data)
         @socket_object.parse(data)
       end

actioncable/lib/action_cable/connection/stream_event_loop.rb

@@ -5,7 +5,7 @@ module ActionCable
   module Connection
     class StreamEventLoop
       def initialize
-        @nio = @thread = nil
+        @nio = @executor = @thread = nil
         @map = {}
         @stopping = false
         @todo = Queue.new
@@ -20,13 +20,14 @@ def timer(interval, &block)
       def post(task = nil, &block)
         task ||= block
 
-        Concurrent.global_io_executor << task
+        spawn
+        @executor << task
       end
 
       def attach(io, stream)
         @todo << lambda do
-          @map[io] = stream
+          @map[io] = @nio.register(io, :r)
-          @nio.register(io, :r)
+          @map[io].value = stream
         end
         wakeup
       end
@@ -39,6 +40,15 @@ def detach(io, stream)
         wakeup
       end
 
+      def writes_pending(io)
+        @todo << lambda do
+          if monitor = @map[io]
+            monitor.interests = :rw
+          end
+        end
+        wakeup
+      end
+
       def stop
         @stopping = true
         wakeup if @nio
@@ -52,6 +62,13 @@ def spawn
             return if @thread && @thread.status
 
             @nio ||= NIO::Selector.new
+
+            @executor ||= Concurrent::ThreadPoolExecutor.new(
+              min_threads: 1,
+              max_threads: 10,
+              max_queue: 0,
+            )
+
             @thread = Thread.new { run }
 
             return true
@@ -77,12 +94,25 @@ def run
 
             monitors.each do |monitor|
               io = monitor.io
-              stream = @map[io]
+              stream = monitor.value
 
               begin
-                stream.receive io.read_nonblock(4096)
+                if monitor.writable?
-              rescue IO::WaitReadable
+                  if stream.flush_write_buffer
-                next
+                    monitor.interests = :r
+                  end
+                  next unless monitor.readable?
+                end
+
+                incoming = io.read_nonblock(4096, exception: false)
+                case incoming
+                when :wait_readable
+                  next
+                when nil
+                  stream.close
+                else
+                  stream.receive incoming
+                end
               rescue
                 # We expect one of EOFError or Errno::ECONNRESET in
                 # normal operation (when the client goes away). But if

actioncable/test/stubs/test_server.rb

@@ -28,7 +28,9 @@ def event_loop
     @event_loop ||= if @config.use_faye
       ActionCable::Connection::FayeEventLoop.new
     else
-      ActionCable::Connection::StreamEventLoop.new
+      ActionCable::Connection::StreamEventLoop.new.tap do |loop|
+        loop.instance_variable_set(:@executor, Concurrent.global_io_executor)
+      end
     end
   end