improve kubeconfig file modification time

Trades runtime complexity for spacial complexity when modifying
large amounts of contexts on a kubeconfig.

In cases where there are few destination filenames for a given
amount of contexts, but a large amount of contexts, this patch
prevents reading and writing to the same file (or small number
of files) over and over again needlessly.

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

@@ -126,6 +126,46 @@ func TestMergeContext(t *testing.T) {
 	matchStringArg(namespace, actual, t)
 }
 
+func TestModifyContext(t *testing.T) {
+	expectedCtx := map[string]bool{
+		"updated": true,
+		"clean":   true,
+	}
+
+	pathOptions := NewDefaultPathOptions()
+	config := createValidTestConfig()
+
+	// define new context and assign it - our path options config
+	config.Contexts["updated"] = &clientcmdapi.Context{
+		Cluster:  "updated",
+		AuthInfo: "updated",
+	}
+	config.CurrentContext = "updated"
+
+	if err := ModifyConfig(pathOptions, *config, true); err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	startingConfig, err := pathOptions.GetStartingConfig()
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	// make sure the current context was updated
+	matchStringArg("updated", startingConfig.CurrentContext, t)
+
+	// there should now be two contexts
+	if len(startingConfig.Contexts) != len(expectedCtx) {
+		t.Fatalf("unexpected nuber of contexts, expecting %v, but found %v", len(expectedCtx), len(startingConfig.Contexts))
+	}
+
+	for key := range startingConfig.Contexts {
+		if !expectedCtx[key] {
+			t.Fatalf("expected context %q to exist", key)
+		}
+	}
+}
+
 func TestCertificateData(t *testing.T) {
 	caData := []byte("ca-data")
 	certData := []byte("cert-data")

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

@@ -220,6 +220,9 @@ func ModifyConfig(configAccess ConfigAccess, newConfig clientcmdapi.Config, rela
 		}
 	}
 
+	// seenConfigs stores a map of config source filenames to computed config objects
+	seenConfigs := map[string]*clientcmdapi.Config{}
+
 	for key, context := range newConfig.Contexts {
 		startingContext, exists := startingConfig.Contexts[key]
 		if !reflect.DeepEqual(context, startingContext) || !exists {
@@ -228,17 +231,30 @@ func ModifyConfig(configAccess ConfigAccess, newConfig clientcmdapi.Config, rela
 				destinationFile = configAccess.GetDefaultFilename()
 			}
 
-			configToWrite, err := getConfigFromFile(destinationFile)
+			// we only obtain a fresh config object from its source file
+			// if we have not seen it already - this prevents us from
+			// reading and writing to the same number of files repeatedly
+			// when multiple / all contexts share the same destination file.
+			configToWrite, seen := seenConfigs[destinationFile]
+			if !seen {
+				var err error
+				configToWrite, err = getConfigFromFile(destinationFile)
 				if err != nil {
 					return err
 				}
-			configToWrite.Contexts[key] = context
+				seenConfigs[destinationFile] = configToWrite
+			}
 
+			configToWrite.Contexts[key] = context
+		}
+	}
+
+	// actually persist config object changes
+	for destinationFile, configToWrite := range seenConfigs {
 		if err := WriteToFile(*configToWrite, destinationFile); err != nil {
 			return err
 		}
 	}
-	}
 
 	for key, authInfo := range newConfig.AuthInfos {
 		startingAuthInfo, exists := startingConfig.AuthInfos[key]