Fix panic for some invalid custom metric requests

[olivierlemasle]

Currently, the invalid custom metric request /apis/custom.metrics.k8s.io/v1beta2/namespaces/default/foo causes a panic of the adapter implementing custom-metrics-apiserver instead of a normal failure.

kubectl get --raw "/apis/custom.metrics.k8s.io/v1beta2/namespaces/default/foo"

The panic is caused by the error runtime error: slice bounds out of range [2:1] here:

custom-metrics-apiserver/pkg/apiserver/endpoints/handlers/get.go

Line 146 in e15f12b

p := strings.Join(requestInfo.Parts[startingIndex:], "/")

In that case, /namespaces/default/foo has three parts so it is handled as a root resource metric (/resource/name/subresource). However, the API server RequestInfoResolver handles the request as a namespaced resource request, and removes the /namespaces/default part.

This commit refactors the handling of metrics related to namespaces, by moving the logic from the storage part to the handler. This allows the handler to perform validation of the request. It then rejects (with a HTTP 400 error) the problematic request.

I'll add unit tests when #139 is merged.

/kind bug

[logicalhan]

/triage accepted
/assign @dgrisonnet

[dgrisonnet]

It will take a bit of time for me to review this since I would have to dive into the code again. I opened #145 to make it easier to understand the codebase and will most likely tackle it while diving into the code.

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/lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all PRs.

This bot triages PRs according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 30d of inactivity since lifecycle/rotten was applied, the PR is closed

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/lifecycle rotten

[olivierlemasle]

/remove-lifecycle rotten

[dgrisonnet]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dgrisonnet, olivierlemasle

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