🐛 fix nil Decoder in multiValidating and multiMutating handlers by implementing DecoderInjector

[d-honeybadger]

TL;DR
Webhooks with multiValidating and multiMutating handlers currently fail to inject decoders into their sub-handlers, resulting in nil pointer panics. This PR is fixing it.

Full description
Before the change in #1307, webhook method webhook.InjectScheme was called before webhook.InjectFunc, meaning that decoder intstantiation was happening before injecting it by webhook.InjectFunc.
After that PR (specifically, the changes to controllerManager.SetFields in pkg/manager/internal.go) , the order has changed, and now webhook.InjectFunc gets called before webhook.InjectScheme, meaning that we webhook.InjectFunc injects a nil decoder.
It appears that such re-ordering was anticipated, because in the webhook.InjectScheme it tries to inject decoder again, just in case, with this comment:

// inject the decoder here too, just in case the order of calling this is not
// scheme first, then inject func
if w.Handler != nil {
	if _, err := InjectDecoderInto(w.GetDecoder(), w.Handler); err != nil {
		return err
	}
}

This second attempt at decoder injection works just fine for regular webhooks, but fails if the weebhook's handler is a multiValidating or multiMutating handler, because those handlers do not implement DecoderInjector interface.
Note that for regular handlers, it is the responsibility of the user to implement DecoderInjector interface, but because both multiValidating and multiMutating are unexported types, a user cannot do so for them.

The fix proposed in this PR is not the only possible solution. Another one could be to switch back the order of inject.InjectorInto and cm.cluster.SetFields in

controller-runtime/pkg/manager/internal.go

Line 227 in 2c238de

func (cm *controllerManager) SetFields(i interface{}) error {

. And yet another one is proposed in #1437

Link to the issue that was filed for this bug: #1487

Tagging @porridge since you were looking at that issue and @alvaroaleman as the author of the PR that re-ordered webhook.InjectScheme and webhook.InjectFunc.

Happy to make any changes or implement a different fix for this bug.

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[alvaroaleman]

The fix proposed in this PR is not the only possible solution. Another one could be to switch back the order of inject.InjectorInto and cm.cluster.SetFields in

@d-honeybadger would you mind to also do that, just on the off chance that something else also relied on the order there?

/ok-to-test

[alvaroaleman]

Please check the return

[d-honeybadger]

@d-honeybadger would you mind to also do that, just on the off chance that something else also relied on the order there?

👍 done!
And fixed a linting problem.

[alvaroaleman]

/lgtm
/approve

thanks!

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alvaroaleman, d-honeybadger

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