query-frontend: Allow separate label cache

[craigfurman]

Instead of using the cache config from the query-range.

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Fixed a bug in which the cache config from the query frontend's query range cache was passed to the label cache. This made it impossible to use a label cache on its own, because invalid empty yaml (from the query range cache) would be passed to the label cache config builder.

Any users who are attempting to use separate label and query range caches today should find that the query range one is being used for both. If this is merged and upgraded to, it will cause the label cache to become used instead.

Verification

I've only done some very basic testing, by running the thanos binary locally and verifying whether the different config combinations cause an error on startup with or without this fix.

The error I see from master is:

{"caller":"main.go:131","err":"response cache with type  is not supported\ngithub.com/thanos-io/thanos/pkg/queryfrontend.NewCacheConfig\n\t/go/src/github.com/thanos-io/thanos/pkg/queryfrontend/config.go:139\nmain.runQueryFrontend\n\t/go/src/github.com/thanos-io/thanos/cmd/thanos/query_frontend.go:161\nmain.registerQueryFrontend.func1\n\t/go/src/github.com/thanos-io/thanos/cmd/thanos/query_frontend.go:129\nmain.main\n\t/go/src/github.com/thanos-io/thanos/cmd/thanos/main.go:129\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:204\nruntime.goexit\n\t/usr/local/go/src/runtime/asm_amd64.s:1374\ninitializing the labels cache config\nmain.runQueryFrontend\n\t/go/src/github.com/thanos-io/thanos/cmd/thanos/query_frontend.go:163\nmain.registerQueryFrontend.func1\n\t/go/src/github.com/thanos-io/thanos/cmd/thanos/query_frontend.go:129\nmain.main\n\t/go/src/github.com/thanos-io/thanos/cmd/thanos/main.go:129\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:204\nruntime.goexit\n\t/usr/local/go/src/runtime/asm_amd64.s:1374\npreparing query-frontend command failed\nmain.main\n\t/go/src/github.com/thanos-io/thanos/cmd/thanos/main.go:131\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:204\nruntime.goexit\n\t/usr/local/go/src/runtime/asm_amd64.s:1374","level":"error","ts":"2020-12-10T11:20:32.8017198Z"}

[yeya24]

LGTM! Thanks for the fix.

[GiedriusS]

LGTM, thank you!

[craigfurman]

Thanks! Any chance this can make it into a patch release soon? I'm not sure what thanos' release cadence is.