Repository: kubernetes
Issue: bug: kubelet panic & crash if `--config-dir` is used
Labels: kind/bug,sig/node,triage/accepted

### What happened?

I created a v1.30.0 k8s cluster with kubeadm and created a  drop-in directory for kubelet configuration under `/etc/kubernetes/kubelet.conf.d`. The normal conf file created by kubeadm is in ` /var/lib/kubelet/config.yaml`

According to the [docs](https://kubernetes.io/docs/tasks/administer-cluster/kubelet-config-file/#kubelet-conf-d) the kubelet should merge all configs in specified order and start. But the kubelet crashes under two setups:

**Setup 1**:

The `/etc/kubernetes/kubelet.conf.d/20-kubelet.conf` is completely empty the kubelet crashes with:
```
>kubelet --config-dir /etc/kubernetes/kubelet.conf.d --config /var/lib/kubelet/config.yaml
E0510 13:47:28.749138    9050 run.go:74] "command failed" err="failed to merge kubelet configs: failed to walk through kubelet dropin directory \"/etc/kubernetes/kubelet.conf.d\": failed to load kubelet dropin file, path: /etc/kubernetes/kubelet.conf.d/20-kubelet.conf, error: kubelet config file \"/etc/kubernetes/kubelet.conf.d/20-kubelet.conf\" was empty"
```
If there is just one space or an newline in that file it crashes with:
```
>kubelet --config-dir /etc/kubernetes/kubelet.conf.d --config /var/lib/kubelet/config.yaml
E0510 13:49:26.243517    9062 run.go:74] "command failed" err=<
        failed to merge kubelet configs: failed to walk through kubelet dropin directory "/etc/kubernetes/kubelet.conf.d": failed to load kubelet dropin file, path: /etc/kubernetes/kubelet.conf.d/20-kubelet.conf, error: Object 'Kind' is missing in '
        '
```
Even when the docs say:

> These files may contain partial configurations and might not be valid config files by themselves. Validation is only performed on the final resulting configuration structure stored internally in the kubelet. 

If there is nothing or only whitspaces in  `/etc/kubernetes/kubelet.conf.d` the merged config should not be invalid and not every file in `/etc/kubernetes/kubelet.conf.d` must contain a `kind` property.

**Setup 2**:
The `/etc/kubernetes/kubelet.conf.d/20-kubelet.conf` is the same as the `/var/lib/kubelet/config.yaml` the kubelet panics and crashes with:
```
> kubelet --config-dir /etc/kubernetes/kubelet.conf.d --config /var/lib/kubelet/config.yaml
panic: non-positive interval for NewTicker

goroutine 1 [running]:
time.NewTicker(0xc00098f7a0?)
        time/tick.go:22 +0xe5
k8s.io/klog/v2/internal/clock.RealClock.NewTicker(...)
        k8s.io/klog/v2@v2.120.1/internal/clock/clock.go:111
k8s.io/klog/v2.(*flushDaemon).run(0xc000229650, 0x0)
        k8s.io/klog/v2@v2.120.1/klog.go:1169 +0x10f
k8s.io/klog/v2.StartFlushDaemon(0x0)
        k8s.io/klog/v2@v2.120.1/klog.go:1220 +0x36
k8s.io/component-base/logs/api/v1.apply(0xc0008c8a38, 0x0, {0x7f1ede53b2f0, 0xc000557130})
        k8s.io/component-base/logs/api/v1/options.go:285 +0x890
k8s.io/component-base/logs/api/v1.validateAndApply(0xc0008c8a38, 0x0, {0x7f1ede53b2f0, 0xc000557130}, 0x4?)
        k8s.io/component-base/logs/api/v1/options.go:132 +0x71
k8s.io/component-base/logs/api/v1.ValidateAndApplyAsField(...)
        k8s.io/component-base/logs/api/v1/options.go:124
k8s.io/kubernetes/cmd/kubelet/app.NewKubeletCommand.func1(0xc0000d8908, {0xc000100060, 0x4, 0x4})
        k8s.io/kubernetes/cmd/kubelet/app/server.go:238 +0x4bc
github.com/spf13/cobra.(*Command).execute(0xc0000d8908, {0xc000100060, 0x4, 0x4})
        github.com/spf13/cobra@v1.7.0/command.go:940 +0x882
github.com/spf13/cobra.(*Command).ExecuteC(0xc0000d8908)
        github.com/spf13/cobra@v1.7.0/command.go:1068 +0x3a5
github.com/spf13/cobra.(*Command).Execute(...)
        github.com/spf13/cobra@v1.7.0/command.go:992
k8s.io/component-base/cli.run(0xc0000d8908)
        k8s.io/component-base/cli/run.go:146 +0x290
k8s.io/component-base/cli.Run(0xc0000061c0?)
        k8s.io/component-base/cli/run.go:46 +0x17
main.main()
        k8s.io/kubernetes/cmd/kubelet/kubelet.go:36 +0x18
```
Using the same config only via `--config` works.

### What did you expect to happen?

The kubelet should start with empty files in `/etc/kubernetes/kubelet.conf.d` or if the files in `/var/lib/kubelet/config.yaml` and `/etc/kubernetes/kubelet.conf.d` are the same.

If this is not the expected usage, the docs have to be adjusted. 

### How can we reproduce it (as minimally and precisely as possible)?

Create a cluster with kubeadm v1.30 with kublet config in `/var/lib/kubelet/config.yaml` and `/etc/kubernetes/kubelet.conf.d` or setup kubelet by hand. 

```yaml
# kubeadm config
apiVersion: kubeadm.k8s.io/v1beta3
kind: InitConfiguration

nodeRegistration:
  kubeletExtraArgs:
    config: /var/lib/kubelet/config.yaml
    config-dir: /etc/kubernetes/kubelet.conf.d
``` 


### Anything else we need to know?

The kublet conf api version is `apiVersion: kubelet.config.k8s.io/v1beta1` and is was with and without setting `KUBELET_CONFIG_DROPIN_DIR_ALPHA`

Functionality was introduced in #119390

### Kubernetes version

<details>

```console
$ kubectl version
Client Version: v1.30.0
Kustomize Version: v5.0.4-0.20230601165947-6ce0bf390ce3
$ kubeadm version
kubeadm version: &version.Info{Major:"1", Minor:"30", GitVersion:"v1.30.0", GitCommit:"7c48c2bd72b9bf5c44d21d7338cc7bea77d0ad2a", GitTreeState:"clean", BuildDate:"2024-04-17T17:34:08Z", GoVersion:"go1.22.2", Compiler:"gc", Platform:"linux/amd64"}
```

</details>


### Cloud provider

<details>
Hetzner/None
</details>


### OS version

<details>

```console
# On Linux:
$ cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 12 (bookworm)"
NAME="Debian GNU/Linux"
VERSION_ID="12"
VERSION="12 (bookworm)"
VERSION_CODENAME=bookworm
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
$ uname -a
Linux controlplane-node-amd64-vcxkzu 6.1.0-21-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.90-1 (2024-05-03) x86_64 GNU/Linux

# On Windows:
C:\> wmic os get Caption, Version, BuildNumber, OSArchitecture
# paste output here
```

</details>


### Install tools

<details>
kubeadm
</details>


### Container runtime (CRI) and version (if applicable)

<details>
containerd github.com/containerd/containerd v1.7.16
</details>


### Related plugins (CNI, CSI, ...) and versions (if applicable)

<details>
not relevant
</details>