Pick registry to login from full image name as well

podman login reg.io/username/image works as well now. It picks
the registry and checks for authentication, if none exist it
will prompt for username and password.
If the credentials exist but are not valid, it will prompt the
user for new valid credentials.

Signed-off-by: Urvashi Mohnani <umohnani@redhat.com>

cmd/podman/login.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"bufio"
-	"context"
 	"fmt"
 	"os"
 	"strings"
@@ -60,27 +59,48 @@ func loginCmd(c *cli.Context) error {
 	if len(args) == 0 {
 		return errors.Errorf("registry must be given")
 	}
-	server := scrubServer(args[0])
+	server := registryFromFullName(scrubServer(args[0]))
 	authfile := getAuthFile(c.String("authfile"))
 
 	sc := common.GetSystemContext("", authfile, false)
 
 	// username of user logged in to server (if one exists)
-	userFromAuthFile, err := config.GetUserLoggedIn(sc, server)
+	userFromAuthFile, passFromAuthFile, err := config.GetAuthentication(sc, server)
 	if err != nil {
 		return errors.Wrapf(err, "error getting logged-in user")
 	}
-	username, password, err := getUserAndPass(c.String("username"), c.String("password"), userFromAuthFile)
+
+	ctx := getContext()
+
+	var (
+		username string
+		password string
+	)
+
+	if userFromAuthFile != "" {
+		username = userFromAuthFile
+		password = passFromAuthFile
+		fmt.Println("Authenticating with existing credentials...")
+		if err := docker.CheckAuth(ctx, sc, username, password, server); err == nil {
+			fmt.Println("Existing credentials are valid. Already logged in to", server)
+			return nil
+		}
+		fmt.Println("Existing credentials are invalid, please enter valid username and password")
+	}
+
+	username, password, err = getUserAndPass(c.String("username"), c.String("password"), userFromAuthFile)
 	if err != nil {
 		return errors.Wrapf(err, "error getting username and password")
 	}
+
 	sc.DockerInsecureSkipTLSVerify = !c.BoolT("tls-verify")
 	if c.String("cert-dir") != "" {
 		sc.DockerCertPath = c.String("cert-dir")
 	}
 
-	if err = docker.CheckAuth(context.TODO(), sc, username, password, server); err == nil {
-		if err := config.SetAuthentication(sc, server, username, password); err != nil {
+	if err = docker.CheckAuth(ctx, sc, username, password, server); err == nil {
+		// Write the new credentials to the authfile
+		if err = config.SetAuthentication(sc, server, username, password); err != nil {
 			return err
 		}
 	}
@@ -126,3 +146,14 @@ func getUserAndPass(username, password, userFromAuthFile string) (string, string
 	}
 	return strings.TrimSpace(username), password, err
 }
+
+// registryFromFullName gets the registry from the input. If the input is of the form
+// quay.io/myuser/myimage, it will parse it and just return quay.io
+// It also returns true if a full image name was given
+func registryFromFullName(input string) string {
+	split := strings.Split(input, "/")
+	if len(split) > 1 {
+		return split[0]
+	}
+	return split[0]
+}