Log a Warning in case there is a nil from Fargate

[kang-makes]

We have a ticket for a nil pointer dereference:

[WARN] computing Storage Driver stats: only devicemapper is supported
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x240 pc=0x7f5640]

goroutine 1 [running]:
github.com/newrelic/nri-docker/src/raw/aws.(*FargateFetcher).getFargateContainerMetrics(0xc4202ae880, 0xc42000e430, 0xc42028d480, 0x832901)
  /go/src/github.com/newrelic/nri-docker/src/raw/aws/fargate_fetcher.go:112 +0x4b0
github.com/newrelic/nri-docker/src/raw/aws.(*FargateFetcher).fargateStatsFromCacheOrNew(0xc4202ae880, 0xc42000e430, 0x0, 0x0)
  /go/src/github.com/newrelic/nri-docker/src/raw/aws/fargate_fetcher.go:76 +0x20a
github.com/newrelic/nri-docker/src/raw/aws.(*FargateFetcher).Fetch(0xc4202ae880, 0xc4200ab1e0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
  /go/src/github.com/newrelic/nri-docker/src/raw/aws/fargate_fetcher.go:54 +0x6a
github.com/newrelic/nri-docker/src/biz.(*MetricsFetcher).Process(0xc4202ae940, 0xc4202e2ae0, 0x2b, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
  /go/src/github.com/newrelic/nri-docker/src/biz/metrics.go:145 +0x1e4
github.com/newrelic/nri-docker/src/nri.(*ContainerSampler).SampleAll(0xc4201180c0, 0xb5f7a0, 0xc42001a018, 0xc4200a09a0, 0x0, 0x0)
  /go/src/github.com/newrelic/nri-docker/src/nri/sampler.go:89 +0x21c
main.main()
  /go/src/github.com/newrelic/nri-docker/src/docker.go:92 +0x647

This should fix the issue and warn us from where does this nil come from.

[gsanchezgavier]

LGTM it would be nice to add at least the test case for this failure mode

[kang-makes]

Because of the way it is developed I cannot mock the HTTP Server so I cannot test it properly.

I filed ticket #136 for that.

[sigilioso]

👍