Merge pull request #39226 from luksa/kubectl_proxy_empty_path

Automatic merge from submit-queue (batch tested with PRs 39311, 39226, 39445)

Make kubectl proxy accept empty path

**What this PR does / why we need it**:
The kubectl proxy previously returned 403 Forbidden: Unauthorized when receiving a request from e.g. "curl localhost:8001" or "curl localhost:8001/".  The previous DefaultPathAcceptRE regex was wrong as it assumed the path in this case would be "/" (but it is actually "").

After someone runs kubectl proxy and tries accessing it with curl, they will probably just try hitting localhost:8001  (which returns an "Unauthorized" response) instead of say localhost:8001/api (which returns a proper response from the API server). Also, whoever previously modified the DefaultPathAcceptRE regex was obviously expecting the regex to accept requests for localhost:8001/

```release-note
fix issue with kubectl proxy so that it will proxy an empty path - e.g. http://localhost:8001
```

pkg/kubectl/proxy_server.go

@@ -34,7 +34,7 @@ import (
 
 const (
 	DefaultHostAcceptRE   = "^localhost$,^127\\.0\\.0\\.1$,^\\[::1\\]$"
-	DefaultPathAcceptRE   = "^/.*"
+	DefaultPathAcceptRE   = "^.*"
 	DefaultPathRejectRE   = "^/api/.*/pods/.*/exec,^/api/.*/pods/.*/attach"
 	DefaultMethodRejectRE = "POST,PUT,PATCH"
 )

pkg/kubectl/proxy_server_test.go

@@ -40,6 +40,15 @@ func TestAccept(t *testing.T) {
 		expectAccept bool
 	}{
 
+		{
+			acceptPaths:  DefaultPathAcceptRE,
+			rejectPaths:  DefaultPathRejectRE,
+			acceptHosts:  DefaultHostAcceptRE,
+			path:         "",
+			host:         "127.0.0.1",
+			method:       "GET",
+			expectAccept: true,
+		},
 		{
 			acceptPaths:  DefaultPathAcceptRE,
 			rejectPaths:  DefaultPathRejectRE,