frontend: aggregate functions on missing label-values return a `null` result instead of `[]`

[faevourite]

Hi folks.

I noticed a small difference between the behaviour of OSS Prometheus and the frontend component in the query_range API:

• Given a metric foo with label values bar="0" and bar="1"
• If I run an aggregate query on a non-existent label-value like count(foo{bar="2"}),
• The result in OSS Prometheus is:
  • {"status":"success","data":{"resultType":"matrix","result":[]}}
• The result in prometheus-engine's frontend is:
  • {"status":"success","data":{"resultType":"matrix","result":null}}

This causes the frontend to break (query ui disappears; screen becomes empty) and causes issues for some prometheus tools like pint.

Is it possible to preserve compatibility and return an empty array in this case instead of null?

[lyanco]

I understand the problem, but the front end is no longer the recommended way to auth queries and we're not really developing for it any more. Do you see this issue when querying the cloud monitoring API directly (using the data source syncer or otherwise)? If so we can look into fixing it there.