using usocket seems to halt the loop (micropython) - loops

Question

I'm trying to code a simple program for a ESP32 board.
My main program is fairly simple and it has to run on a loop.
On the side, the device also needs to be able to respond to HTTP requests with a very simple response.
This is my attempt (a rework of https://randomnerdtutorials.com/micropython-esp32-esp8266-bme280-web-server/):
try:
import usocket as socket
except:
import socket
from micropython import const
import time
REFRESH_DELAY = const(60000) #millisecondi
def do_connect():
import network
wlan = network.WLAN(network.STA_IF)
wlan.active(True)
if not wlan.isconnected():
print('connecting to network...')
wlan.config(dhcp_hostname=HOST)
wlan.connect('SSID', 'PSWD')
while not wlan.isconnected():
pass
print('network config:', wlan.ifconfig())
import json
import esp
esp.osdebug(None)
import gc
gc.collect()
do_connect()
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, SENSOR_SCKT_PORT))
s.listen(5)
prevRun = 0
i = 0
while True:
print("iteration #"+str(i))
i += 1
# run every 60 seconds
curRun = int(round(time.time() * 1000))
if curRun - prevRun >= REFRESH_DELAY:
prevRun = curRun
# MAIN PROGRAM
# ......
# whole bunch of code
# ....
# run continuously:
try:
if gc.mem_free() < 102000:
gc.collect()
conn, addr = s.accept()
conn.settimeout(3.0)
print('Got a connection from %s' % str(addr))
request = conn.recv(1024)
conn.settimeout(None)
request = str(request)
#print('Content = %s' % request)
measurements = 'some json stuff'
conn.send('HTTP/1.1 200 OK\n')
conn.send('Content-Type: text/html\n')
conn.send('Connection: close\n\n')
conn.send(measurements)
conn.close()
except OSError as e:
conn.close()
print('Connection closed')
what happens is I only get the iteration #0, and then the while True loop halts.
If I ping this server with a HTTP request, I get a correct response, AND the loop advances to iteration #1 and #2 (no idea why it thinks I pinged it with 2 requests).
So it seems that socket.listen(5) is halting the while loop.
Is there any way to avoid this?
Any other solution?
I don't think that threading is an option here.

Answer 1

The problem is that s.accept() is a blocking call...it won't return until it receives a connection. This is why it pauses your loop.
The easiest solution is probably to check whether or not a connection is waiting before calling s.accept(); you can do this using either select.select or select.poll. I prefer the select.poll API, which would end up looking something like this:
import esp
import gc
import json
import machine
import network
import select
import socket
import time
from micropython import const
HOST = '0.0.0.0'
SENSOR_SCKT_PORT = const(1234)
REFRESH_DELAY = const(60000) # milliseconds
def wait_for_connection():
print('waiting for connection...')
wlan = network.WLAN(network.STA_IF)
while not wlan.isconnected():
machine.idle()
print('...connected. network config:', wlan.ifconfig())
esp.osdebug(None)
gc.collect()
wait_for_connection()
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, SENSOR_SCKT_PORT))
s.listen(5)
poll = select.poll()
poll.register(s, select.POLLIN)
prevRun = 0
i = 0
while True:
print("iteration #"+str(i))
i += 1
# run every 60 seconds
curRun = int(round(time.time() * 1000))
if curRun - prevRun >= REFRESH_DELAY:
prevRun = curRun
# MAIN PROGRAM
# ......
# whole bunch of code
# ....
# run continuously:
try:
if gc.mem_free() < 102000:
gc.collect()
events = poll.poll(100)
if events:
conn, addr = s.accept()
conn.settimeout(3.0)
print('Got a connection from %s' % str(addr))
request = conn.recv(1024)
conn.settimeout(None)
request = str(request)
# print('Content = %s' % request)
measurements = 'some json stuff'
conn.send('HTTP/1.1 200 OK\n')
conn.send('Content-Type: text/html\n')
conn.send('Connection: close\n\n')
conn.send(measurements)
conn.close()
except OSError:
conn.close()
print('Connection closed')
You'll note that I've taken a few liberties with your code to get it running on my device and to appease my sense of style; primarily, I've excised most of your do_connect method and put all the imports at the top of the file.
The only real changes are:
We create a select.poll() object:
poll = select.poll()
We ask it to monitor the s variable for POLLIN events:
poll.register(s, select.POLLIN)
We check if any connections are pending before attempting to handle a connection:
events = poll.poll(100)
if events:
conn, addr = s.accept()
conn.settimeout(3.0)
[...]
With these changes in place, running your code and making a request looks something like this:
iteration #0
iteration #1
iteration #2
iteration #3
iteration #4
iteration #5
iteration #6
Got a connection from ('192.168.1.169', 54392)
iteration #7
iteration #8
iteration #9
iteration #10
Note that as written here, your loop will iterate at least once every 100ms (and you can control that by changing the timeout on our call to poll.poll()).
Note: the above was tested on an esp8266 device (A Wemos D1 clone) running MicroPython v1.13-268-gf7aafc062).