"kubectl logs" ignores "--tail" option when "--selector" is given

[jpetazzo]

What happened:

I do kubectl logs --tail 1 --selector run=foo.

I see the last 10 lines of output of each pod matching run=foo.

What you expected to happen:

I was expecting to see only the last line of output of each pod.

How to reproduce it (as minimally and precisely as possible):

kubectl run clock --image=jpetazzo/clock --replicas=2
sleep 3
kubectl logs --tail 1 --selector run=clock

The jpetazzo/clock image prints the date every second.

Normally we should just see two lines of output; instead we see 20.

Anything else we need to know?:

This seems to be a regression in Kubernetes 1.12, as this worked correctly from 1.8 to 1.11.

(I observed this problem starting with k8s 1.12.0 and it's still present in 1.12.2)

Environment:

• Kubernetes version (use kubectl version): 1.12.2
• Cloud provider or hardware configuration: AWS
• OS (e.g. from /etc/os-release): Ubuntu
• Kernel (e.g. uname -a): 4.4.0
• Install tools: kubeadm
• Others:

/kind bug

[jpetazzo]

It looks like the issue was introduced by commit 3a3633a, specifically here.

If I understand that code correctly, it forces the number of lines to selectorTail (10) even if o.Tail is set.

Since that seems to be in kubectl:

/sig cli

[WanLinghao]

/assign

[liggitt]

It looks like the issue was introduced by commit 3a3633a, specifically here.

didn't that commit just move the code from 3a3633a#diff-0fabd226d454fe805719db043ac4e46bL203 ?

[jpetazzo]

didn't that commit just move the code from 3a3633a#diff-0fabd226d454fe805719db043ac4e46bL203 ?

Not quite; the old code was:

		if logOptions.TailLines == nil && tail != -1 {
			logOptions.TailLines = &selectorTail
		}

The new code is:

	if len(o.Selector) > 0 && o.Tail != -1 {
		logOptions.TailLines = &selectorTail
	} else if o.Tail != -1 {
		logOptions.TailLines = &o.Tail
	}

I don't know why it was changed, but the new logic reads as "if the user specified a selector and a number of lines different from -1, then set the number of lines to 10".