kubernetes: kubectl proxy not working for kube contexts with base path

What happened:

Given that you have a kube context that looks like this:

apiVersion: v1
kind: Config
clusters:
- cluster:
    server: https://my-url-to-kubernetes.com/any-base-path-to-kubernetes
  name: default
contexts:
- context:
    cluster: default
    user: default
  name: default
current-context: default
users:
- name: default
  user:
    client-certificate-data: ANY
    client-key-data: ANY

and you try to run kubectl proxy, the request http://localhost:8081/version fails, because internally the URL is not correctly rewritten to https://my-url-to-kubernetes.com/any-base-path-to-kubernetes/version and instead calls https://my-url-to-kubernetes.com/version. The problem is in file (at least what I think) https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/apimachinery/pkg/util/proxy/upgradeaware.go#L231 , where the path needs to be passed as well (also for upgrade connections at https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/apimachinery/pkg/util/proxy/upgradeaware.go#L273)

You can see the problem via kubectl proxy --v 10:

I0827 14:18:25.042177   30678 loader.go:375] Config loaded from file:  /Users/fabiankramm/.kube/config
Starting to serve on 127.0.0.1:8001
I0827 14:18:28.414766   30678 proxy_server.go:88] /version matched ^.*
I0827 14:18:28.415391   30678 proxy_server.go:88] 127.0.0.1 matched ^127\.0\.0\.1$
I0827 14:18:28.415610   30678 proxy_server.go:128] Filter accepting GET /version 127.0.0.1
I0827 14:18:28.416002   30678 upgradeaware.go:253] Request was not an upgrade
I0827 14:18:28.416841   30678 round_trippers.go:423] curl -k -v -XGET  -H "Cache-Control: max-age=0" -H "Accept-Language: de-DE,de;q=0.9,en-US;q=0.8,en;q=0.7" -H "Sec-Fetch-Mode: navigate" -H "User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.135 Safari/537.36" -H "Accept-Encoding: gzip, deflate, br" -H "X-Forwarded-For: 127.0.0.1" -H "Sec-Fetch-Site: none" -H "Sec-Fetch-User: ?1" -H "Sec-Fetch-Dest: document" -H "Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9" -H "Upgrade-Insecure-Requests: 1" 'https://test.localhost/version'
I0827 14:18:28.654599   30678 round_trippers.go:443] GET https://test.localhost/version 404 Not Found in 237 milliseconds
I0827 14:18:28.654637   30678 round_trippers.go:449] Response Headers:
I0827 14:18:28.654644   30678 round_trippers.go:452]     Date: Thu, 27 Aug 2020 12:18:29 GMT
I0827 14:18:28.654649   30678 round_trippers.go:452]     Content-Type: text/plain; charset=utf-8
I0827 14:18:28.654654   30678 round_trippers.go:452]     Content-Length: 21
I0827 14:18:28.654659   30678 round_trippers.go:452]     Server: nginx/1.19.1

As you can see the request is made to ‘https://test.localhost/version’, even though the kube config has a base path of ‘/kubernetes’:

apiVersion: v1
kind: Config
clusters:
- cluster:
    certificate-authority-data: ANY
    server: https://test.localhost/kubernetes
  name: default
contexts:
- context:
    cluster: default
    user: default
  name: default
current-context: default
users:
- name: default
  user:
    client-certificate-data: ANY
    client-key-data: ANY

What you expected to happen:

The request http://localhost:8081/version works, when using kubectl proxy

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

• Spin up a cluster with api root at a different base path like /kubernetes
• Run kubectl proxy
• Try http://localhost:8081/version in the webbrowser

Environment:

• Kubernetes version (use kubectl version):

Client Version: version.Info{Major:"1", Minor:"19", GitVersion:"v1.19.0", GitCommit:"e19964183377d0ec2052d1f1fa930c4d7575bd50", GitTreeState:"clean", BuildDate:"2020-08-26T14:30:33Z", GoVersion:"go1.15", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"18", GitVersion:"v1.18.3", GitCommit:"2e7996e3e2712684bc73f0dec0200d64eec7fe40", GitTreeState:"clean", BuildDate:"2020-05-20T12:43:34Z", GoVersion:"go1.13.9", Compiler:"gc", Platform:"linux/amd64"}

• Cloud provider or hardware configuration:

minikube version: v1.12.3
commit: 2243b4b97c131e3244c5f014faedca0d846599f5-dirty