在试图锁定时发现'死锁;尝试重启事务'

Question

我有多处理的应用程序，每个线程执行以下python代码：

while True:
    db_cursor.execute("SET autocommit=0;")
    db_cursor.execute("LOCK TABLES prqueue WRITE;")
    db_cursor.execute("SELECT MAX(id) FROM prqueue;")

    for record in db_cursor.fetchall():
        if record[0]:
            db_cursor.execute("DELETE FROM prqueue WHERE id='%s';" % record[0]);
            #db_connector.commit()
            db_cursor.execute("COMMIT;")
            db_cursor.execute("UNLOCK TABLES;")

            # some work that takes time

        else:
            db_cursor.execute("COMMIT;")
            db_cursor.execute("UNLOCK TABLES;")

有时我的进程因OperationalError异常而死：'Deadlock found when trying to get lock; try restarting transaction'。

这里最奇怪的是流程在不同的行上死亡。例如，来自其中3个的错误消息：

Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/home/crawler/Worker.py", line 122, in do_crawling
    db_cursor.execute("COMMIT;")
  File "/usr/lib/python2.7/dist-packages/MySQLdb/cursors.py", line 174, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorclass, errorvalue
OperationalError: (1213, 'Deadlock found when trying to get lock; try restarting transaction')

Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/home/crawler/Worker.py", line 122, in do_crawling
    db_cursor.execute("COMMIT;")
  File "/usr/lib/python2.7/dist-packages/MySQLdb/cursors.py", line 174, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorclass, errorvalue
OperationalError: (1213, 'Deadlock found when trying to get lock; try restarting transaction')

Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/home/crawler/Worker.py", line 109, in do_crawling
    db_cursor.execute("SELECT MAX(id) FROM prqueue;")
  File "/usr/lib/python2.7/dist-packages/MySQLdb/cursors.py", line 174, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorclass, errorvalue
OperationalError: (1213, 'Deadlock found when trying to get lock; try restarting transaction')

我在这段代码中做错了什么？完全厌倦了所有这些僵局。

谢谢。