⚠️ Default selectors to Nothing if it is nil for MatchingLabelsSelector and MatchingFieldsSelector

[acumino]

If the selector is not set for MatchingLabelsSelector and MatchingFieldsSelector, then the list call panics.
This PR updates MatchingLabelsSelector and MatchingFieldsSelector to default their Selector fields to labels.Nothing() and fields.Nothing() respectively if unset in ApplyToList.

[k8s-ci-robot]

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[acumino]

/cc @sbueringer

[troy0820]

/ok-to-test

[alvaroaleman]

Why would a label or field selector be set if it has no value?

[acumino]

Two reasons -

• When a list call happens with empty/nil label selector, it lists everything, since MatchingLabelsSelector also wants to achieve a similar thing; therefore, in case of a nil label selector, it should list all the objects.
• Currently if nil is passed for selector in MatchingLabelsSelector the list call panics, which should not happen

[sbueringer]

For what it's worth, this is the official godoc on metav1.LabelSelector

// A label selector is a label query over a set of resources. The result of matchLabels and
// matchExpressions are ANDed. An *empty* label selector matches *all objects*. A *null*
// label selector matches *no objects*.

If we follow that semantic here as well null would mean match no objects

[acumino]

If I use -

client.List(ctx, nodeList, &client.ListOptions{LabelSelector: nil})

It lists all objects. What should be the way to go here?

Also

A null label selector matches no objects.

If we follow this, then I can modify the PR to use Nothing instead of Everything when MatchingLabelsSelector's selector is not set so it doesn't panic.

[alvaroaleman]

Could you explain why you have code that ends up with a nil selector and why that is not considered to be a bug?

[acumino]

WDYT about the below changes -

// ApplyToList applies this configuration to the given list options.
func (m MatchingLabelsSelector) ApplyToList(opts *ListOptions) {
	if m.Selector == nil {
		m.Selector = labels.SelectorFromSet(labels.Set{})
	}
	opts.LabelSelector = m
}

[alvaroaleman]

Having a nil selector in there is incorrect usage and we can not error. We do recover panics though so it seems all right to me.

Your suggestion makes a nil selector match everything, which as mentioned earlier we do not want.

[acumino]

recover panics though so it seems all right to me.

This doesn't allow us to let the panic happen at the first place.

Also, IMO we should allow list all, because as per the doc string MatchingLabelsSelector is supposed to behave the same as LabelSelector, which it doesn't currently.

By my main concern for this PR was that we should not let the panic happen, we should stop it one way or another and not let the user figure it out after trial and error. I am open to suggestions for that.
If we really don't want to list anything in case of nil Selector in MatchingLabelsSelector, I can use labels.Nothing instead of Everything.
This will fix both of our concerns.

[alvaroaleman]

Also, IMO we should allow list all, because as per the doc string MatchingLabelsSelector is supposed to behave the same as LabelSelector, which it doesn't currently.

Which could also be fixed by changing LabelSelector rather than MatchingLabelSelector.

I don't personally mind the panic, given our default is to recover panics and this is basically incorrect usage and we don't have a way to report that, as the listOpts can not error. I am okay changing it to labels.Nothing() if you want to do that, but I would veto changing it to labels.Everything() due to potential security implications.

[acumino]

My main issue was nil pointer panic, I am ok with nothing also.
Update the PR, PTAL.

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 16a47334870891aed9482e6966ba44660880fdb8

[alvaroaleman]

/hold

in case @sbueringer wants to check

[sbueringer]

Thx!

/lgtm
/approve
/hold cancel

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: acumino, alvaroaleman, sbueringer

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