fix(kuma-cp): don't panic in webhook if k8s object can't convert to core resource

[michaelbeaumont]

Summary

Trying to set, for example, a negative weight on a TrafficRoute split results in a confusing error for users:

error: trafficroutes.kuma.io "route-all-default" could not be patched: Internal error occurred: failed calling webhook "validator.kuma-admission.kuma.io": failed to call webhook: Post "https://kuma-control-plane.kuma-system.svc:443/validate-kuma-io-v1alpha1?timeout=10s": EOF

and a panic in the webhook goroutine in kuma-cp:

2022/06/10 06:42:31 http: panic serving 172.17.0.1:8216: failed to unmarshal *v1alpha1.TrafficRoute: json: cannot unmarshal number -1 into Go value of type uint32
goroutine 1247 [running]:
net/http.(*conn).serve.func1()
	/usr/lib/go/src/net/http/server.go:1825 +0xbf
panic({0x1e1dde0, 0xc0013482b0})
	/usr/lib/go/src/runtime/panic.go:844 +0x258
github.com/kumahq/kuma/pkg/util/proto.MustUnmarshalJSON({0xc0006ca833?, 0xc001c9e1e8?, 0x40d987?}, {0x3031080?, 0xc00163f560?})
	/home/mike/projects/kuma/pkg/util/proto/proto.go:67 +0xc7
github.com/kumahq/kuma/pkg/plugins/resources/k8s/native/api/v1alpha1.(*TrafficRoute).GetSpec(0xc0006ca480?)
	/home/mike/projects/kuma/pkg/plugins/resources/k8s/native/api/v1alpha1/zz_generated.mesh.go:1689 +0x58
github.com/kumahq/kuma/pkg/plugins/resources/k8s.(*SimpleConverter).ToCoreResource(0xc001421530?, {0x3057300, 0xc001644b40}, {0x3042890, 0xc000651d40})
	/home/mike/projects/kuma/pkg/plugins/resources/k8s/converter.go:53 +0x112
github.com/kumahq/kuma/pkg/plugins/runtime/k8s/webhooks.(*validatingHandler).decode(_, {{{0xc001421530, 0x24}, {{0xc000ce2930, 0x7}, {0xc000ce2938, 0x8}, {0xc000ce2940, 0xc}}, {{0xc000ce2950, ...}, ...}, ...}})
	/home/mike/projects/kuma/pkg/plugins/runtime/k8s/webhooks/validation.go:105 +0x16a

Here we can't assume that GetSpec succeeds.

Now the user gets:

error: trafficroutes.kuma.io "route-all-default" could not be patched: admission webhook "validator.kuma-admission.kuma.io" denied the request: json: cannot unmarshal number -2 into Go value of type uint32

[codecov-commenter]

Codecov Report

Merging #4455 (62e1e4b) into master (260d813) will decrease coverage by 0.06%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master    #4455      +/-   ##
==========================================
- Coverage   55.42%   55.36%   -0.07%     
==========================================
  Files         947      947              
  Lines       57938    57976      +38     
==========================================
- Hits        32113    32097      -16     
- Misses      23284    23326      +42     
- Partials     2541     2553      +12     

Impacted Files	Coverage Δ
...donothingpolicy/k8s/v1alpha1/zz_generated.types.go	0.00% <0.00%> (ø)
pkg/plugins/resources/k8s/caching_converter.go	0.00% <0.00%> (ø)
...lugins/resources/k8s/native/pkg/model/resources.go	100.00% <ø> (ø)
...ime/k8s/controllers/gateway_instance_controller.go	0.00% <0.00%> (ø)
tools/policy-gen/protoc-gen-kumapolicy/crd.go	0.00% <ø> (ø)
tools/resource-gen/main.go	0.00% <ø> (ø)
...urces/k8s/native/api/v1alpha1/zz_generated.mesh.go	45.73% <23.95%> (-0.39%)	⬇️
...ces/k8s/native/api/v1alpha1/zz_generated.system.go	44.31% <37.50%> (+0.13%)	⬆️
pkg/plugins/resources/k8s/converter.go	76.47% <50.00%> (-4.78%)	⬇️
...e/test/api/sample/v1alpha1/sample_types_helpers.go	82.75% <100.00%> (+0.61%)	⬆️
... and 15 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 260d813...62e1e4b. Read the comment docs.

[jakubdyszkiewicz]

Uhhh, can we recover from the panic in webhook server?

We recover from panics in API Server. We should never crash a CP for invalid request. Otherwise, it's very easy to do DDoS a CP.

[michaelbeaumont]

Uhhh, can we recover from the panic in webhook server?

Yeah it doesn't crash kuma-cp. It just results in the request failing inexplicably.

[jakubdyszkiewicz]

I misunderstand the description then, great!