fix(usb-bridge): limit send bandwidth

Desktop OS serial drivers often have very small incoming data buffers on
their serial connections because well serial is typically slow... unless
you've actually got usb on both sides.

The default value of 2048/.01s (204.8kB/s) should be further developed
but it's going to be a good idea to keep this smaller than we think
necessary to support slow client hardware.

usb-bridge/ot3usb/serial_thread.py

@@ -12,15 +12,17 @@
 
 QUEUE_MAX_ITEMS = 100
 
+DEFAULT_PACKET_LIMIT = 2048
 
-def _try_write_all_data(serial: serial.Serial, data: bytes) -> None:
+
+def _try_write_all_data(serial: serial.Serial, data: bytes, packet_limit: int) -> None:
  sent = 0
  tries = 0
  if len(data) == 0:
  return
  while sent < len(data):
  try:
- sent += serial.write(data[sent:])
+ sent += serial.write(data[sent:min(sent+packet_limit, len(data))])
  except Exception as e:
  # Any exception means we need to quit
  print(f"Failed to write: {e}")
@@ -31,17 +33,24 @@ def _try_write_all_data(serial: serial.Serial, data: bytes) -> None:
  time.sleep(0.01)
 
 
-def _worker(queue: QUEUE_TYPE) -> None:
+def _worker(queue: QUEUE_TYPE, packet_limit: int) -> None:
  while True:
  ser, data = queue.get()
- _try_write_all_data(ser, data)
+ _try_write_all_data(ser, data, packet_limit)
+
+
+def create_worker_thread(packet_limit: int = DEFAULT_PACKET_LIMIT) -> Tuple[threading.Thread, QUEUE_TYPE]:
+ """Create a serial worker thread. Returns the comms queue.
 
+ packet_limit is a maximum size for a single usb packet which is sent no more
+ frequently than every 10 ms, establishing an effective bandwidth limit. This
+ may be required to allow clients to process data in time, since many desktop
+ OS serial drivers do not have very large data buffers.
+ """
 
-def create_worker_thread() -> Tuple[threading.Thread, QUEUE_TYPE]:
- """Create a serial worker thread. Returns the comms queue."""
  queue: QUEUE_TYPE = Queue(QUEUE_MAX_ITEMS)
  thread = threading.Thread(
- target=_worker, name="serial worker", kwargs={"queue": queue}
+ target=_worker, name="serial worker", kwargs={"queue": queue, "packet_limit": packet_limit}
  )
 
  return (thread, queue)