illustrate pymysql DB connection causing errors which resolve when we make sure they are recycled in greenletexit

test.py

import eventlet
eventlet.monkey_patch()

import itertools
import pymysql
import random
from eventlet.green import time
import greenlet

def connect():
    return pymysql.connect(user="scott", passwd="tiger", host="127.0.0.1", db="test")


greenlet_id = itertools.count(1)


def do_work(connections, conn_index):
    our_id = next(greenlet_id)
    conn = connections[conn_index]
    while True:
        try:
            print("greenlet %d working..." % our_id)
            cursor = conn.cursor()
            for i in range(10):
                cursor.execute(
                    "insert into stuff (data) values (%s)",
                    (("some_data_%f" % random.random()), )
                )
            cursor.close()
            conn.commit()

            cursor = conn.cursor()
            cursor.execute("select sleep(%s)", (random.random(), ))
            time.sleep(random.random())
            if cursor.description is None:
                raise Exception("cursor.description not supposed to be none")
            cursor.fetchall()
            cursor.close()
            conn.rollback()
            time.sleep(random.random())
        except pymysql.Error as err:
            print("error occurred, invalidating connection: %s" % err)
            conn.rollback()
            conn.close()
            conn = connections[conn_index] = connect()
        except Exception as err:
            print("totally unexpected error occurred: %r" % err)
        except greenlet.GreenletExit as ex:
            print("exit exception: %r" % ex)
            if ensure_greenlet_exit_handled:
                conn.close()
                conn = connections[conn_index] = connect()
            break

if __name__ == '__main__':
    ensure_greenlet_exit_handled = False

    num = 10

    conn = connect()
    cursor = conn.cursor()
    cursor.execute("drop table if exists stuff")
    cursor.execute(
        "create table stuff(id integer primary key auto_increment, "
        "data varchar(200))")
    cursor.close()
    conn.commit()

    connections = [connect() for i in range(num)]

    greenlets = [
        eventlet.spawn(do_work, connections, idx)
        for idx in range(num)]

    while True:
        for idx in range(num):
            time.sleep(random.random())
            print("kill...")
            eventlet.greenthread.kill(greenlets[idx])
            time.sleep(.5)
            greenlets[idx] = eventlet.spawn(do_work, connections, idx)

see also https://bitbucket.org/zzzeek/sqlalchemy/issues/3803/dbapi-connections-go-invalid-on