login: warn about --password on command line

cli/command/registry/login.go

@@ -2,6 +2,7 @@ package registry
 
 import (
 	"fmt"
+	"io/ioutil"
 
 	"golang.org/x/net/context"
 
@@ -16,6 +17,7 @@ type loginOptions struct {
 	serverAddress string
 	user          string
 	password      string
+	passwordFile  string
 }
 
 // NewLoginCommand creates a new `docker login` command
@@ -39,14 +41,64 @@ func NewLoginCommand(dockerCli command.Cli) *cobra.Command {
 
 	flags.StringVarP(&opts.user, "username", "u", "", "Username")
 	flags.StringVarP(&opts.password, "password", "p", "", "Password")
+	flags.StringVarP(&opts.passwordFile, "password-file", "", "", "Password file whose contents are the password itself")
 
 	return cmd
 }
 
+// getPasswordFromFile reads the contents of the file, or stdin if the file is
+// == "-".
+//
+// It also trims off the last \n in the file, if it exists. Most people don't
+// have \ns in their password, and this allows stuff like echo "password" >
+// foo, without having to remember to pass -n, or vi, which can be configured
+// to automatically append newlines, etc.
+//
+// For users that do have a \n as the last character of their password, they
+// need to store it as \n\n. I think this conforms to the principle of least
+// surprise, but I could be wrong :)
+func getPasswordFromFile(dockerCli command.Cli, name string) (string, error) {
+	var err error
+	var raw []byte
+
+	if name == "-" {
+		raw, err = ioutil.ReadAll(dockerCli.In())
+	} else {
+		raw, err = ioutil.ReadFile(name)
+	}
+
+	if err != nil {
+		return "", err
+	}
+
+	contents := string(raw)
+	if contents[len(contents)-1] == '\n' {
+		contents = contents[:len(contents)-1]
+	}
+
+	return contents, nil
+}
+
 func runLogin(dockerCli command.Cli, opts loginOptions) error {
 	ctx := context.Background()
 	clnt := dockerCli.Client()
 
+	if opts.password != "" {
+		fmt.Fprintf(dockerCli.Err(), "Using --password via the CLI is insecure. Please use --password-file.\n")
+		if opts.passwordFile != "" {
+			return errors.Errorf("--password and --password-file are mutually exclusive")
+		}
+	}
+
+	if opts.passwordFile != "" {
+		contents, err := getPasswordFromFile(dockerCli, opts.passwordFile)
+		if err != nil {
+			return err
+		}
+
+		opts.password = contents
+	}
+
 	var (
 		serverAddress string
 		authServer    = command.ElectAuthServer(ctx, dockerCli)