JobMutationWebhook nil pointer crash

[irvinlim]

Encountered panic in nil pointer exception:

2022/04/17 15:42:29 http: panic serving 10.244.0.1:8270: runtime error: invalid memory address or nil pointer dereference
goroutine 29143 [running]:
net/http.(*conn).serve.func1()
        /opt/hostedtoolcache/go/1.17.8/x64/src/net/http/server.go:1802 +0xb9
panic({0x17b32c0, 0x299acd0})
        /opt/hostedtoolcache/go/1.17.8/x64/src/runtime/panic.go:1047 +0x266
github.com/furiko-io/furiko/pkg/execution/mutation.(*Mutator).MutateJob(0xc000837630, 0xc00000a780)
        /home/runner/work/furiko/furiko/pkg/execution/mutation/mutation.go:131 +0xd8
github.com/furiko-io/furiko/pkg/execution/webhooks/jobmutatingwebhook.(*Webhook).Patch(0xc00000a5a0, 0xc000479040, 0x7fe542fa45b8)
        /home/runner/work/furiko/furiko/pkg/execution/webhooks/jobmutatingwebhook/webhook.go:176 +0x57
github.com/furiko-io/furiko/pkg/execution/webhooks/jobmutatingwebhook.(*Webhook).Handle(0x1c40778, {0xc00045bf40, 0x1c1e0b8}, 0xc000479040)
        /home/runner/work/furiko/furiko/pkg/execution/webhooks/jobmutatingwebhook/webhook.go:140 +0x26c
github.com/furiko-io/furiko/pkg/runtime/httphandler.HandleAdmissionWebhook.func1({0x1c1e0b8, 0xc00011c8c0}, 0xc00052ef00)
        /home/runner/work/furiko/furiko/pkg/runtime/httphandler/webhooks.go:67 +0x123
net/http.HandlerFunc.ServeHTTP(0xc00013ad00, {0x1c1e0b8, 0xc00011c8c0}, 0xc00011c8c0)
        /opt/hostedtoolcache/go/1.17.8/x64/src/net/http/server.go:2047 +0x2f
net/http.(*ServeMux).ServeHTTP(0xc0001a5928, {0x1c1e0b8, 0xc00011c8c0}, 0xc00052ef00)
        /opt/hostedtoolcache/go/1.17.8/x64/src/net/http/server.go:2425 +0x149
net/http.serverHandler.ServeHTTP({0x1c109d0}, {0x1c1e0b8, 0xc00011c8c0}, 0xc00052ef00)
        /opt/hostedtoolcache/go/1.17.8/x64/src/net/http/server.go:2879 +0x43b
net/http.(*conn).serve(0xc00037c960, {0x1c22878, 0xc000600720})
        /opt/hostedtoolcache/go/1.17.8/x64/src/net/http/server.go:1930 +0xb08
created by net/http.(*Server).Serve
        /opt/hostedtoolcache/go/1.17.8/x64/src/net/http/server.go:3034 +0x4e8

[irvinlim]

#5 should be prioritized to improve the stability of handling such issues.

EDIT: Actually, it seems that the particular HTTP handler goroutine will crash and return EOF, rather than crashing the whole service:

Error from server (InternalError): error when creating "job-signal-demo.yaml": Internal error occurred: failed calling webhook "mutating.webhook.jobs.execution.furiko.io": failed to call webhook: Post "https://execution-webhook-service.furiko-system.svc:443/mutating/jobs.execution.furiko.io?timeout=10s": EOF

Rather than let kube-apiserver return an EOF error, we could recover from such panics, and ALWAYS return an InternalError to fail validation/mutation.