CircuitPython asyncio TCP multi-server multi-client

code.py

import time
import random
import os
import wifi
import socketpool
import asyncio


NUM_CLIENTS = 4
NUM_SERVERS = 2
BASE_PORT = 5000
MAXBUF = 64


class Socket:
    """ https://github.com/adafruit/circuitpython/pull/7173 """
    def __init__(self, s):
        self.s = s
        s.setblocking(False)

    async def recv_into(self, buf):
        await asyncio.core._io_queue.queue_read(self.s)
        return self.s.recv_into(buf)

    async def send(self, buf):
        await asyncio.core._io_queue.queue_write(self.s)
        return self.s.send(buf)


async def accepted_conn(server_num, conn, event):
    """handles a single server connection from a client"""

    inbuf = bytearray(MAXBUF)

    sconn = Socket(conn)

    size = False
    while not size:
        try:
            size = await(sconn.recv_into(inbuf))  # OSError: [Errno 128] ENOTCONN
        except OSError as ex:
            await asyncio.sleep(0)
    print(f"{time.monotonic():.1f}s SERVER {server_num} received {size} bytes {inbuf[:size]}")

    outbuf = f"{time.monotonic():.1f}s Hello client from server {server_num}".encode()
    await(sconn.send(outbuf))
    print(f"{time.monotonic():.1f}s SERVER {server_num} sent {len(outbuf)} bytes {outbuf}")

    # sconn.close()  # AttributeError: 'Socket' object has no attribute 'close'
    conn.close()
    event.set()

async def server(server_num):
    """persistent TCP server"""

    BACKLOG = NUM_CLIENTS

    s = pool.socket(pool.AF_INET, pool.SOCK_STREAM)
    s.setsockopt(pool.SOL_SOCKET, pool.SO_REUSEADDR, 1)  # 
    port = BASE_PORT + server_num
    s.bind((host, port))
    s.listen(BACKLOG)
    print(f"{time.monotonic():.1f}s SERVER {server_num} listening on port {port}")
    total = 0
    while True:
        print(f"{time.monotonic():.1f}s SERVER {server_num} accepting connections")
        s.setblocking(False)
        addr = False
        while not addr:
            try:
                conn, addr = s.accept()  # OSError: [Errno 11] EAGAIN
                print(f"{time.monotonic():.1f}s SERVER {server_num} accepted connection from {addr}")
                event = asyncio.Event()
                t = asyncio.create_task(accepted_conn(server_num, conn, event))
                await event.wait()
                t.cancel()
                total += 1
                print(f"{time.monotonic():.1f}s SERVER {server_num} has processed {total} total connections")
            except OSError as ex:
                await asyncio.sleep(0)
        await asyncio.sleep(0)

async def connected_client(client_num, s, event):
    """handles a single client connection to a server"""

    inbuf = bytearray(MAXBUF)

    ss = Socket(s)

    outbuf = f"{time.monotonic():.1f}s Hello server from client {client_num}".encode()
    size = await(ss.send(outbuf))
    print(f"{time.monotonic():.1f}s CLIENT {client_num} sent {size} bytes {outbuf}")

    size = False
    while not size:
        try:
            size = await(ss.recv_into(inbuf))  # OSError: [Errno 11] EAGAIN
        except OSError as ex:
            await asyncio.sleep(0)
    print(f"{time.monotonic():.1f}s CLIENT {client_num} received {size} bytes {inbuf[:size]}")

    # ss.close()  # AttributeError: 'Socket' object has no attribute 'close'
    event.set()

async def client(client_num):
    """TCP client wrapper"""

    total = 0
    while True:
        with pool.socket(pool.AF_INET, pool.SOCK_STREAM) as s:
            port = random.randint(BASE_PORT, BASE_PORT + NUM_SERVERS)
            print(f"{time.monotonic():.1f}s CLIENT {client_num} connecting to ({host}, {port})")
            try:
                s.connect((host, port))  # OSError: [Errno 119] EINPROGRESS
                print(f"{time.monotonic():.1f}s CLIENT {client_num} connected to ({host}, {port})")
                event = asyncio.Event()
                t = asyncio.create_task(connected_client(client_num, s, event))
                await event.wait()
                t.cancel
                total += 1
                print(f"{time.monotonic():.1f}s CLIENT {client_num} has processed {total} total connections")
            except OSError as ex:
                await asyncio.sleep(0)
        await asyncio.sleep(random.random())


async def main():
    print(f"{time.monotonic():.1f}s creating {NUM_SERVERS} servers, {NUM_CLIENTS} clients")
    tasks = []
    for server_num in range(NUM_SERVERS):
        tasks.append(asyncio.create_task(server(server_num)))
    for client_num in range(NUM_CLIENTS):
        tasks.append(asyncio.create_task(client(client_num)))
    print(f"{time.monotonic():.1f}s {len(tasks)} tasks created")
    while True:
        await asyncio.sleep(0)


time.sleep(3)  # wait for serial after reset
wifi.radio.connect(os.getenv('WIFI_SSID'), os.getenv('WIFI_PASSWORD'))
host = str(wifi.radio.ipv4_address)
pool = socketpool.SocketPool(wifi.radio)

asyncio.run(main())

A few key things are as-yet unimplemented here and in the CircuitPython core / asyncio library, and it eventually hard faults into safe mode (Hard fault: memory access or instruction error.). I'm sure there are more graceful ways of doing much of this.

Tested on Adafruit CircuitPython 9.2.8 on 2025-05-28; Adafruit QT Py ESP32-S3 4MB Flash 2MB PSRAM with ESP32S3

ESP32-S3 has 8 sockets. Each server uses 2 sockets. Each client uses 1 socket.

Hard fault was due to using 9.2.8, which still had this issue that had been largely attributed to socket servers. addendum: The minimal example was fixed by 10.0.0-beta.3, but the full code still hard faults... back to the drawing board.

Note that in CP 10 (10.0.0-beta.3), there are now only 4 TCP sockets available on espressif, rather than 8:

import time, wifi, socketpool
pool = socketpool.SocketPool(wifi.radio)
socks = []
while True:
    socks.append(pool.socket(pool.AF_INET, pool.SOCK_STREAM))
    print(f"{len(socks)}")  # 8 in CP 9; 4 in CP 10

Update: back to 8 sockets with adafruit/circuitpython#10609.

Retested with Adafruit CircuitPython 10.1.0-beta.1 on 2025-11-06; Adafruit QT Py ESP32-S3 4MB Flash 2MB PSRAM with ESP32S3 and issue reported here: adafruit/circuitpython#10775