Panic when using empty relabel config

[xocasdashdash]

Bug Report

What did you do?
I added an empty relabel config like so:

relabel_configs:
            -   source_labels: [ '__meta_dns_name' ]
                regex: 'node-exporter\.(.+?).+'
                target_label: 'az'
                replacement: '$1'
            -

What did you expect to see?
Prometheus not to panic.
What did you see instead? Under which circumstances?
It panics. ie: https://imgur.com/a/aFSiz5V
Environment

• System information:

Linux 3.10.0-862.el7.x86_64 x86_64

• Prometheus version:

  prometheus, version 2.5.0 (branch: HEAD, revision: 67dc912)
  build user: root@578ab108d0b9
  build date: 20181106-11:40:44
  go version: go1.11.1

I also found this bug on 2.4.3

[juliusv]

Thanks for the report, looking into it!

[juliusv]

https://github.com/prometheus/prometheus/blob/master/relabel/relabel.go#L41 accesses cfg.SourceLabels, but cfg is a pointer that can actually become nil when initialized from an empty YAML object (an empty relabeling rule in this case). Either the list elements should not be pointers or we should check for nil values.

[simonpasquier]

I would lean towards rejecting any empty relabeling rule at load time since it isn't a useful configuration AFAICT.

[juliusv]

@simonpasquier Good point, that would be even better :)

[infoverload]

I am interested in working on this!

[simonpasquier]

@infoverload go ahead!

[juliusv]

Heads-up: I'm fixing all the slice-of-pointers config nil pointer crashes in one go, so this will be covered by that.