gistfile1.py

# -*- coding: utf-8 -*-
import gevent.monkey
gevent.monkey.patch_all()

import collections
import threading
import time
import random
import sys

import logging
logging.basicConfig()
log = logging.getLogger('foo')
log.setLevel(logging.DEBUG)

#import pymysql as dbapi
from mysql import connector as dbapi


class SimplePool(object):
    def __init__(self):
        self.checkedin = collections.deque([
            self._connect() for i in range(50)
        ])
        self.checkout_lock = threading.Lock()
        self.checkin_lock = threading.Lock()

    def _connect(self):
        return dbapi.connect(
            user="scott", passwd="tiger",
            host="localhost", db="test")

    def get(self):
        with self.checkout_lock:
            while not self.checkedin:
                time.sleep(.1)
            return self.checkedin.pop()

    def return_conn(self, conn):
        try:
            conn.rollback()
        except:
            log.error("Exception during rollback", exc_info=True)
            try:
                conn.close()
            except:
                log.error("Exception during close", exc_info=True)

            # recycle to a new connection
            conn = self._connect()
        with self.checkin_lock:
            self.checkedin.append(conn)


def verify_connection_id(conn):
    cursor = conn.cursor()
    try:
        cursor.execute("select connection_id()")
        row = cursor.fetchone()
        return row[0]
    except:
        return None
    finally:
        cursor.close()


def execute_sql(conn, sql, params=()):
    cursor = conn.cursor()
    cursor.execute(sql, params)
    lastrowid = cursor.lastrowid
    cursor.close()
    return lastrowid


pool = SimplePool()

# SELECT * FROM table_b WHERE a_id not in
# (SELECT id FROM table_a) ORDER BY a_id DESC;

PREPARE_SQL = [
    "DROP TABLE IF EXISTS table_b",
    "DROP TABLE IF EXISTS table_a",
    """CREATE TABLE table_a (
        id INT NOT NULL AUTO_INCREMENT,
        data VARCHAR (256) NOT NULL,
        PRIMARY KEY (id)
    ) engine='InnoDB'""",
    """CREATE TABLE table_b (
        id INT NOT NULL AUTO_INCREMENT,
        a_id INT NOT NULL,
        data VARCHAR (256) NOT NULL,
        -- uncomment this to illustrate where the driver is attempting
        -- to INSERT the row during ROLLBACK
        -- FOREIGN KEY (a_id) REFERENCES table_a(id),
        PRIMARY KEY (id)
    )  engine='InnoDB'
    """]

connection = pool.get()
for sql in PREPARE_SQL:
    execute_sql(connection, sql)
connection.commit()
pool.return_conn(connection)
print("Table prepared...")


def transaction_kill_worker():
    while True:
        try:
            connection = None
            with gevent.Timeout(0.1):
                connection = pool.get()
                rowid = execute_sql(
                    connection,
                    "INSERT INTO table_a (data) VALUES (%s)", ("a",))
                gevent.sleep(random.random() * 0.2)

                try:
                    execute_sql(
                        connection,
                        "INSERT INTO table_b (a_id, data) VALUES (%s, %s)",
                        (rowid, "b",))

                    # this version prevents the commit from
                    # proceeding on a bad connection
                    # if verify_connection_id(connection):
                    #    connection.commit()

                    # this version does not.  It will commit the
                    # row for table_b without the table_a being present.
                    connection.commit()

                    pool.return_conn(connection)
                except Exception:
                    connection.rollback()
                    pool.return_conn(connection)
                    sys.stdout.write("$")
        except gevent.Timeout:
            # try to return the connection anyway
            if connection is not None:
                pool.return_conn(connection)
            sys.stdout.write("#")
        except Exception:
            # logger.exception(e)
            sys.stdout.write("@")
        else:
            sys.stdout.write(".")
        finally:
            if connection is not None:
                pool.return_conn(connection)


def main():
    for i in range(50):
        gevent.spawn(transaction_kill_worker)

    gevent.sleep(3)

    while True:
        gevent.sleep(5)


if __name__ == "__main__":
    main()

this is with both pymysql and mysql-connector. Some discussion about pymysql is at PyMySQL/PyMySQL#275. I'm also appealing to openstack-dev for clues at http://lists.openstack.org/pipermail/openstack-dev/2014-December/052344.html.

Unrelated, but fyi - you can clean up your main block by using a gevent.pool.Group to wait on.

import gevent.pool

def main():
    group = gevent.pool.Group()
    for i in range(50):
        group.spawn(transaction_kill_worker)
    group.join()