Add ability to disable local caching

[GiedriusS]

Currently, "cache size each conn" is set to the default value if 0 is provided: https://github.com/rueian/rueidis/blob/master/rueidis.go#L201-L203, https://github.com/rueian/rueidis/blob/8b21fc0a8614397bbe4beada7292f2b9431a9d18/pipe.go#L63. Perhaps we could skip the LRU entirely if zero has been passed? However, that would be a breaking change. Perhaps a separate flag could exist such as DisableLocalCache?

Ran into this while doing thanos-io/thanos#5593. I think some users might not want to cache items locally if their systems don't have much RAM, for example.

[rueian]

Sure, a separate flag to disable the cache is much easier to implement.

[rueian]

Hi @GiedriusS,

v0.0.71 now has a ClientOption.DisableCache that will fall back Client.DoCache/Client.DoMultiCache to Client.Do/Client.DoMulti and effectively disables the client-side caching.

In terms of the second thing, There's no way to disable the default TTL in case PTTL fails you mentioned in the thanos-io/thanos#5593. Since the PTTL is sent inside the same MULTI EXEC block of the target command, it is guaranteed that PTTL succeeds if you get the target response.

[GiedriusS]

I see. I am reading the code a bit now. So, it doesn't matter what time.Duration we pass to MGetCache? 🤔

[rueian]

Hi @GiedriusS,

The time.Duration you passed will take effect in two cases:

1. The time.Duration is smaller than PTTL response.
2. The key on redis has no TTL

[GiedriusS]

I see, it makes sense! Thank you for the response. I'll continue on with thanos-io/thanos#5593.