kubectl should error when namespace doesn't match file for update

A user who runs `kubectl update -f foo.json` where foo.json is a
resource in a namespace that does not match $(kubectl namespace)
may not intend to update the resource in that other namespace.

For now, return an error when the user does not explicitly set
the namespace via the CLI:

    # foo.json in 'one', current is 'two'
    $ kubectl update -f foo.json # FAILS

    $ kubectl update --namespace=one -f foo.json # SUCCEEDS

pkg/kubectl/cmd/cmd.go

@@ -196,6 +196,18 @@ func getKubeNamespace(cmd *cobra.Command) string {
 	return result
 }
 
+// getExplicitKubeNamespace returns the value of the namespace a
+// user explicitly provided on the command line, or false if no
+// such namespace was specified.
+func getExplicitKubeNamespace(cmd *cobra.Command) (string, bool) {
+	if ns := getFlagString(cmd, "namespace"); len(ns) > 0 {
+		return ns, true
+	}
+	// TODO: determine when --ns-path is set but equal to the default
+	// value and return its value and true.
+	return "", false
+}
+
 func getKubeConfig(cmd *cobra.Command) *client.Config {
 	config := &client.Config{}
 

pkg/kubectl/cmd/resource.go

@@ -145,3 +145,14 @@ func ResourceFromFile(filename string, typer runtime.ObjectTyper, mapper meta.RE
 
 	return
 }
+
+// CompareNamespaceFromFile returns an error if the namespace the user has provided on the CLI
+// or via the default namespace file does not match the namespace of an input file. This
+// prevents a user from unintentionally updating the wrong namespace.
+func CompareNamespaceFromFile(cmd *cobra.Command, namespace string) error {
+	defaultNamespace := getKubeNamespace(cmd)
+	if defaultNamespace != namespace {
+		return fmt.Errorf("The namespace from the provided file %q does not match the namespace %q. You must pass '--namespace=%s' to perform this operation.", namespace, defaultNamespace, namespace)
+	}
+	return nil
+}

pkg/kubectl/cmd/update.go

@@ -47,6 +47,9 @@ Examples:
 			client, err := f.Client(cmd, mapping)
 			checkErr(err)
 
+			err = CompareNamespaceFromFile(cmd, namespace)
+			checkErr(err)
+
 			err = kubectl.NewRESTHelper(client, mapping).Update(namespace, name, true, data)
 			checkErr(err)
 			fmt.Fprintf(out, "%s\n", name)