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.

Kubernetes-commit: d5651948cf1a14ed284b4708e2057e4cbc72bcbe

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 {
@@ -228,15 +231,28 @@ func ModifyConfig(configAccess ConfigAccess, newConfig clientcmdapi.Config, rela
 				destinationFile = configAccess.GetDefaultFilename()
 			}
 
-			configToWrite, err := getConfigFromFile(destinationFile)
-			if err != nil {
-				return err
+			// 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
+				}
+				seenConfigs[destinationFile] = configToWrite
 			}
-			configToWrite.Contexts[key] = context
 
-			if err := WriteToFile(*configToWrite, destinationFile); err != nil {
-				return err
-			}
+			configToWrite.Contexts[key] = context
+		}
+	}
+
+	// actually persist config object changes
+	for destinationFile, configToWrite := range seenConfigs {
+		if err := WriteToFile(*configToWrite, destinationFile); err != nil {
+			return err
 		}
 	}