Handle a Docker daemon without registry info

The current implementation of the ElectAuthServer doesn't handle well when the
default Registry server is not included in the response from the daemon Info
endpoint.

That leads to the storage and usage of the credentials for the default registry
(`https://index.docker.io/v1/`) under an empty string on the client config file.

Sample config file after a login via a Docker Daemon without Registry
information:
```json
{
	"auths": {
		"": {
			"auth": "***"
		}
	}
}
```

That can lead to duplication of the password for the default registry and
authentication failures against the default registry if a pull/push is performed
without first authenticating via the misbehaving daemon.

Also, changes the output of the warning message from stdout to sdterr as
per dnephin suggestion.

Signed-off-by: Marcus Martins <marcus@docker.com>

cli/command/registry.go

@@ -28,7 +28,9 @@ func ElectAuthServer(ctx context.Context, cli Cli) string {
 	// the default registry URL might be Windows specific.
 	serverAddress := registry.IndexServer
 	if info, err := cli.Client().Info(ctx); err != nil {
-		fmt.Fprintf(cli.Out(), "Warning: failed to get default registry endpoint from daemon (%v). Using system default: %s\n", err, serverAddress)
+		fmt.Fprintf(cli.Err(), "Warning: failed to get default registry endpoint from daemon (%v). Using system default: %s\n", err, serverAddress)
+	} else if info.IndexServerAddress == "" {
+		fmt.Fprintf(cli.Err(), "Warning: Empty registry endpoint from daemon. Using system default: %s\n", serverAddress)
 	} else {
 		serverAddress = info.IndexServerAddress
 	}

cli/command/registry_test.go

@@ -0,0 +1,79 @@
+package command_test
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/pkg/errors"
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/net/context"
+
+	// Prevents a circular import with "github.com/docker/cli/cli/internal/test"
+	. "github.com/docker/cli/cli/command"
+	"github.com/docker/cli/cli/internal/test"
+	"github.com/docker/docker/api/types"
+	"github.com/docker/docker/client"
+)
+
+type fakeClient struct {
+	client.Client
+	infoFunc func() (types.Info, error)
+}
+
+func (cli *fakeClient) Info(_ context.Context) (types.Info, error) {
+	if cli.infoFunc != nil {
+		return cli.infoFunc()
+	}
+	return types.Info{}, nil
+}
+
+func TestElectAuthServer(t *testing.T) {
+	testCases := []struct {
+		expectedAuthServer string
+		expectedWarning    string
+		infoFunc           func() (types.Info, error)
+	}{
+		{
+			expectedAuthServer: "https://index.docker.io/v1/",
+			expectedWarning:    "",
+			infoFunc: func() (types.Info, error) {
+				return types.Info{IndexServerAddress: "https://index.docker.io/v1/"}, nil
+			},
+		},
+		{
+			expectedAuthServer: "https://index.docker.io/v1/",
+			expectedWarning:    "Empty registry endpoint from daemon",
+			infoFunc: func() (types.Info, error) {
+				return types.Info{IndexServerAddress: ""}, nil
+			},
+		},
+		{
+			expectedAuthServer: "https://foo.bar",
+			expectedWarning:    "",
+			infoFunc: func() (types.Info, error) {
+				return types.Info{IndexServerAddress: "https://foo.bar"}, nil
+			},
+		},
+		{
+			expectedAuthServer: "https://index.docker.io/v1/",
+			expectedWarning:    "failed to get default registry endpoint from daemon",
+			infoFunc: func() (types.Info, error) {
+				return types.Info{}, errors.Errorf("error getting info")
+			},
+		},
+	}
+	for _, tc := range testCases {
+		buf := new(bytes.Buffer)
+		cli := test.NewFakeCli(&fakeClient{infoFunc: tc.infoFunc}, buf)
+		errBuf := new(bytes.Buffer)
+		cli.SetErr(errBuf)
+		server := ElectAuthServer(context.Background(), cli)
+		assert.Equal(t, tc.expectedAuthServer, server)
+		actual := errBuf.String()
+		if tc.expectedWarning == "" {
+			assert.Empty(t, actual)
+		} else {
+			assert.Contains(t, actual, tc.expectedWarning)
+		}
+	}
+}