Prometheus scaler throws an error when a query returns empty

[jeffhollan]

When using prometheus to do something like scale an HTTP workload, if the requests are nil for that period of time the scaler throws an error every time it polls:

{"level":"error","ts":1583710650.0702486,"logger":"prometheus_scaler","msg":"error executing prometheus query","error":"Prometheus query sum(rate(nginx_ingress_controller_requests{service="java-azure-function-http"}[5s])) returned empty","stacktrace":"github.com/go-logr/zapr.(*zapLogger).Error\n\t/go/pkg/mod/github.com/go-logr/zapr@v0.1.1/zapr.go:128\ngithub.com/kedacore/keda/pkg/scalers.(*prometheusScaler).IsActive\n\tkeda/pkg/scalers/prometheus.go:101\ngithub.com/kedacore/keda/pkg/handler.(*ScaleHandler).handleScaleDeployment\n\tkeda/pkg/handler/scale_loop.go:119\ngithub.com/kedacore/keda/pkg/handler.(*ScaleHandler).handleScale\n\tkeda/pkg/handler/scale_loop.go:45\ngithub.com/kedacore/keda/pkg/handler.(*ScaleHandler).HandleScaleLoop\n\tkeda/pkg/handler/scale_loop.go:28"}

Would be good to have the scaler handle an empty query as a 0

[droessmj]

are you just looking for this line to be a 0,nil return instead of it's current case or is there a complexity I'm missing?

keda/pkg/scalers/prometheus.go

Line 149 in 3692eca

return -1, fmt.Errorf("Prometheus query %s returned empty", s.metadata.query)

Edit: I did see that I'd need to bubble up zero as a valid return value higher in call stack and did so in the PR.