Stream XREAD last ID '$' has incorrect behavior when blocking forever

[josh-hook]

Hi, I found an issue using the XREAD command when passing in $ as the ID. I'm using version v2.20.0.

I think this is because in the MiniRedis server it's performing the xread callback periodically and each time getting the last ID if the original XREAD command contained $. Instead I think it should be getting the last ID in the initial command and then re-using that instead of $ for future xread polls.

For example this will never return anything:

go func() {
  result, err := client.XRead(ctx, &redis.XReadArgs{
    Streams: []string{"my-stream", "$"},
    Block:   0,
  }).Result()
}()

_, err = client.XAdd(ctx, &redis.XAddArgs{
  Stream: "my-stream",
  Values: map[string]interface{"test": "data"}
}).Result()

Because when we add the new item to the stream, in MiniRedis it's then using the newest item ID in the next poll to xread.

Easiest fix looks like in cmd_stream.go it should fetch IDs before the blocking call:

    s, _ := m.db(getCtx(c).selectedDB).streamKeys[opts.streams[0]]
    opts.ids = []string{s.lastID()}
    blocking(...)

Please correct me if you think this behavior is wrong 😄

[alicebob]

Please correct me if you think this behavior is wrong 😄

Thanks for your issue! I will have a look soon, if no-one beats me to it :)

[alicebob]

That sounds logical. Would you mind making a PR? It's fine if it isn't perfect :)

[josh-hook]

PR here https://github.com/alicebob/miniredis/pull/293 😄

[alicebob]

On Thu, Sep 22, 2022 at 01:51:14AM -0700, Joshua Hook wrote:

PR here https://github.com/alicebob/miniredis/pull/293 😄

Thanks! I'll have a look soon (busy today)