[kube-client] fix: handle typed nil discovery results

[fuldaxxx]

Overview

Fixes a nil pointer dereference panic in APIResourceList that occurred when the discovery backend returned an error with a nil result (e.g. for an API group that isn't yet registered, such as custom.metrics.k8s.io before the metrics adapter is deployed).

What this PR does / why we need it

Go's typed nil trap: apiResourceListUncached returns (*apiResourceListResult, error). When it returns (nil, err), the nil is a typed nil — a *apiResourceListResult with value nil. When this typed nil is stored in an any interface (as singleflight.Do does), the interface itself is not nil — it holds a type descriptor. As a result, the existing v == nil guard after Do() was bypassed, and the subsequent type assertion v.(*apiResourceListResult).lists dereferenced the nil pointer and panicked.

Fix: wrap the singleflight.Do body to explicitly convert a typed nil result into an untyped nil before it's stored in the any interface, so the guard works as intended.

Reproduces in practice when a Kubernetes cluster has API groups registered in discovery but no server backing them (e.g. custom.metrics.k8s.io before metrics-adapter is running), causing ServerResourcesForGroupVersion to return nil, err.

A regression test (TestAPIResourceListTypedNilNoPanic) is added that panicked before the fix and passes after.

[Copilot]

Pull request overview

Fixes a nil-pointer panic in the client’s discovery path when singleflight.Do() returns an any containing a typed-nil *apiResourceListResult, by ensuring typed-nil results are converted to an untyped nil before being stored in the interface.

Changes:

• Wrap singleflight.Do() callback in apiResourceList() to convert (*apiResourceListResult)(nil) into a true nil interface value.
• Add a regression test reproducing the typed-nil discovery error scenario to ensure APIResourceList no longer panics.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
client/client.go	Converts typed-nil discovery results to untyped nil before returning from singleflight.Do() to prevent nil-deref panics.
client/client_concurrent_test.go	Adds a regression test for the typed-nil discovery failure case to verify APIResourceList doesn’t panic.

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