kubectl: Fix current-context being written to wrong file

This is what happens when writing back a OIDC refresh token:

 - plugin/pkg/client/auth/oidc/oidc.go:282
   Calls `Persist` to save the new refresh token.

 - tools/clientcmd/config.go:372
   Calls `ModifyConfig` to save the config.

 - tools/clientcmd/config.go:167
   Calls `configAccess.GetLoadingPrecedence()` to get the files
   listed from the `KUBECONFIG` environment variable.

 - tools/clientcmd/loader.go:334
   If the `ConfigAccess` was a `ClientConfigLoadingRules`, it
   directly returns the `Precedence` slice from its `rules`
   field.

   THE PROBLEM:
     The slice can be modified by the caller, unintentionally
     changing the value of the `ClientConfigLoadingRules`'
     `Precedence` field.

 - tools/clientcmd/config.go:170
   Then proceeds to in-place sort the slice returned by the
   `ConfigAccess`. This is the same slice (by identity)
   as the `ClientConfigLoadingRules`' `Precedence` field,
   destroying its intended order.

 - tools/clientcmd/config.go:179
   Calls `configAccess.GetStartingConfig` to read the original
   config so it can be compared with the new config.

 - tools/clientcmd/loader.go:339
   Calls `NewNonInteractiveDeferredLoadingClientConfig` with
   itself as a parameter.

   CONSEQUENCE:
     At this point, its the `Precedence` has been
     unintentionally sorted. When it loads the
     config again, it gives precedence to whichever
     file comes first in ascending alphabetical order.

 - tools/clientcmd/config.go:192
   If the file returned by `GetStartingConfig` has a
   different `current-context` than the new config,
   it calls `writeCurrentContext` to update the first
   kubeconfig file in the `KUBECONFIG` environment
   variable.

 - tools/clientcmd/config.go:403
   Calls `configAccess.GetDefaultFilename` to find the
   destination kubeconfig file.

 - tools/clientcmd/loader.go:358
   Iterates through the kubeconfig files returned by
   `GetLoadingPreferences` to find the first file
   that exists.

   CONSEQUENCE:
     With the slice being sorted earlier, the files
     returned by this call of `GetLoadingPreferences`
     will be sorted alphabetically, rather than by
     their intended order.

staging/src/k8s.io/client-go/tools/clientcmd/config_test.go

@@ -0,0 +1,119 @@
+/*
+Copyright 2025 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package clientcmd
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestModifyConfigWritesToFirstKubeconfigFile(t *testing.T) {
+	const (
+		contextNameA   = "context-a"
+		contextNameB   = "context-b"
+		newContextName = "new-context"
+	)
+
+	tempdir := t.TempDir()
+	configFile1, _ := os.Create(filepath.Join(tempdir, "kubeconfig-a"))
+	configFile2, _ := os.Create(filepath.Join(tempdir, "kubeconfig-b"))
+
+	// The first kubeconfig has everything.
+	err := os.WriteFile(configFile1.Name(), []byte(`
+kind: Config
+apiVersion: v1
+clusters:
+- cluster:
+    api-version: v1
+    server: https://kubernetes.default.svc:443
+    certificate-authority: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
+  name: kubeconfig-cluster
+contexts:
+- context:
+    cluster: kubeconfig-cluster
+    namespace: default
+    user: kubeconfig-user
+  name: `+contextNameA+`
+current-context: `+contextNameA+`
+users:
+- name: kubeconfig-user
+  user:
+    tokenFile: /var/run/secrets/kubernetes.io/serviceaccount/token
+`), os.FileMode(0755))
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	// The second kubeconfig declares a new context and activates it.
+	err = os.WriteFile(configFile2.Name(), []byte(`
+kind: Config
+apiVersion: v1
+contexts:
+- context:
+    cluster: kubeconfig-cluster
+    namespace: a-different-namespace
+    user: kubeconfig-user
+  name: `+contextNameB+`
+current-context: `+contextNameB+`
+`), os.FileMode(0755))
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	// Set KUBECONFIG to the files, in descending alphabetical order.
+	// This will be used to check that they don't get sorted.
+	envVarValue := fmt.Sprintf("%s%c%s", configFile2.Name(), filepath.ListSeparator, configFile1.Name())
+	t.Setenv(RecommendedConfigPathEnvVar, envVarValue)
+
+	// Load the kubeconfigs, change the active context, and call ModifyConfig.
+	loadingRules := NewDefaultClientConfigLoadingRules()
+	config, err := loadingRules.Load()
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	newConfig := config.DeepCopy()
+	newConfig.CurrentContext = newContextName
+	err = ModifyConfig(loadingRules, *newConfig, false)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	// Load the files again and check that only configFile2 was changed.
+	config1, err := LoadFromFile(configFile1.Name()) // file sorts first, but was specified last
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	if config1.CurrentContext != contextNameA {
+		t.Errorf("Config should not be modified, but was. Expected %q, got %q", contextNameA, config1.CurrentContext)
+	}
+
+	config2, err := LoadFromFile(configFile2.Name()) // file sorts last, but was specified first
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	if config2.CurrentContext != newContextName {
+		t.Errorf("Config should be modified, but was not. Expected %q, got %q", newContextName, config2.CurrentContext)
+	}
+}

staging/src/k8s.io/client-go/tools/clientcmd/loader.go

@@ -331,7 +331,10 @@ func (rules *ClientConfigLoadingRules) GetLoadingPrecedence() []string {
 		return []string{rules.ExplicitPath}
 	}
 
-	return rules.Precedence
+	// Create a copy in case something tries to sort the returned slice.
+	precedence := make([]string, len(rules.Precedence))
+	copy(precedence, rules.Precedence)
+	return precedence
 }
 
 // GetStartingConfig implements ConfigAccess