Cannot read data from serial when no longer connected

[bbilger]

CircuitPython version

Adafruit CircuitPython 9.1.3 on 2024-08-29; Adafruit Feather RP2040 USB Host with rp2040

Code/REPL

import time
import usb_cdc

serial = usb_cdc.data
# serial.reset_input_buffer() # ensure a clean initial state during testing
# serial.timeout = 0 # just to be sure but doesn't really change anything
while True:
    in_waiting = serial.in_waiting
    if in_waiting > 0:
        print(f'time={time.time()} connected={serial.connected} in_waiting={in_waiting} data={serial.readline()}')
    time.sleep(1)

Behavior

Sending data like this to the device works (note the sleep after the write!)

python -c 'import serial; import time; ser = serial.Serial("/dev/ttyACM1"); ser.write(b"hi\n"); time.sleep(2); ser.close();'
# time=1577838411 connected=True in_waiting=3 data=b'hi\n'                                                                                                                                                                                                                                                               

Sending data like this does NOT work (empty bytes). The data is there (in_waiting > 0) but it can't be read

python -c 'import serial; import time; ser = serial.Serial("/dev/ttyACM1"); ser.write(b"hi\n"); ser.close();'
# time=1577838507 connected=False in_waiting=3 data=b''                                                                                                                                                                                                                                                                
# time=1577838508 connected=False in_waiting=3 data=b''
# ...

As soon as one does establish a connection (without writing new data), the data from the previous command can be read

python -c 'import serial; import time; ser = serial.Serial("/dev/ttyACM1"); time.sleep(2); ser.close();'
# time=1577838525 connected=True in_waiting=3 data=b'hi\n'      

Description

It seems like the read* functions only return data when a sender is connected. The data that was received (in_waiting > 0) should, however, be readable even if the sender disconnected before the device was able to read it.

Additional information

It's irrelevant whether readline() or read(in_waiting) is used. The current behavior is especially problematic since one cannot send data doing something like this: echo 'hi' > /dev/ttyACM1.

[bbilger]

In case anyone else runs into this, my current workaround is to write something back from the device (serial.write(b'ok\n')) after read, and make the sender wait and as such keep the connection open until it receives the response:

python -c 'import serial; import time; ser = serial.Serial("/dev/ttyACM1"); ser.write(b"hi\n"); print(ser.readline()); ser.close();'

[incense]

Same observation here (no surprise). Would also prefer being able to read from serial even after connection has been closed by host.