Logical operation query return null when comes two empty vectors in prometheus 2.0

[ZeeShen]

What did you do?
query

up{not_match="1"} or up{not_match="2"}

What did you expect to see?
return {"status":"success","data":{"resultType":"vector","result":[]}}

What did you see instead? Under which circumstances?
{"status":"success","data":{"resultType":"vector","result":null}}

prometheus graph js running into error Uncaught TypeError: Cannot read property 'length' of null

Environment

• Prometheus version:

  build user:       root@615b82cb36b6
  build date:       20171108-07:11:59
  go version:       go1.9.2

Actually, my situation is: I had some old alerting rules that allow users to disable some alerts by setting a NoAlert label.

node_memory_usage{NoAlert!~".*mem.*",job="nodes"} > 0.95
node_cpu_usage{NoAlert!~".*cpu.*",job="nodes"} > 0.95

Because of #3575, prometheus 2.0 doesn't return metrics that NoAlert label is not set. So I tried and this null situation happened.

(node_memory_usage{NoAlert="",job="nodes"})
 or (node_memory_usage{NoAlert!~".*mem.*",job="nodes"})

I will put my NoAlert rules in alertmanager as it persists silences on disk now. But this null result maybe not a supposed return?

[brian-brazil]

Dupe of #3515, fix will be in the next release.

[lock]

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