Slow performance in RedisScheduleSource.get_schedules() leads to missed scheduled jobs

[AzikDeveloper]

I added print statements to measure how long get_schedules() takes and noticed that it sometimes takes over 2 minutes to complete. This causes the "loop that runs every 1 minute" in run_scheduler_loop to miss some intervals.

[AzikDeveloper]

In the get_schedules() method, this line seems to be causing the issue:
async for key in redis.scan_iter(f"{self.prefix}:*"):

scan_iter appears to never yield any keys.

After debugging, I found that AsyncScanCommands.scan_iter() looks like this:

AsyncScanCommands.scan_iter(self, ...):
    cursor = "0"
    while cursor != 0:
        cursor, data = await self.scan(
            cursor=cursor, match=match, count=count, _type=_type, **kwargs
        )
        for d in data:
            yield d

The issue seems to be that self.scan() returns a non-zero cursor but with empty data, causing the loop to never terminate.

[danfimov]

Try to use ListRedisScheduleSource instead of RedisScheduleSource:

• RedisScheduleSource is deprecated and will be removed in future releases;
• As far as I know ListRedisScheduleSource has performance improvements in comparison with RedisScheduleSource.