login1: Add NewWithConnection method

This method allows passing existing D-Bus connection, which allows to
re-use connection between clients and to mock D-Bus connection for
testing purposes.

Signed-off-by: Mateusz Gozdek <mgozdek@microsoft.com>

login1/dbus.go

@@ -32,8 +32,33 @@ const (
 
 // Conn is a connection to systemds dbus endpoint.
 type Conn struct {
-	conn   *dbus.Conn
-	object dbus.BusObject
+	conn        Connection
+	connManager connectionManager
+	object      Caller
+}
+
+// Objector describes functionality required from a given D-Bus connection.
+type Connection interface {
+	Object(string, dbus.ObjectPath) dbus.BusObject
+	Signal(ch chan<- *dbus.Signal)
+	// TODO: This should be replaced with AddMatchSignal.
+	// See https://github.com/coreos/go-systemd/issues/388 for details.
+	BusObject() dbus.BusObject
+}
+
+// ConnectionManager explicitly wraps dependencies on established D-Bus connection.
+type connectionManager interface {
+	Hello() error
+	Auth(authMethods []dbus.Auth) error
+	Close() error
+
+	Connection
+}
+
+// Caller describes required functionality from D-Bus object.
+type Caller interface {
+	// TODO: This method should eventually be removed, as it provides no context support.
+	Call(method string, flags dbus.Flags, args ...interface{}) *dbus.Call
 }
 
 // New establishes a connection to the system bus and authenticates.
@@ -47,20 +72,32 @@ func New() (*Conn, error) {
 	return c, nil
 }
 
+// NewWithConnection creates new login1 client using given D-Bus connection.
+func NewWithConnection(connection Connection) (*Conn, error) {
+	if connection == nil {
+		return nil, fmt.Errorf("no connection given")
+	}
+
+	return &Conn{
+		conn:   connection,
+		object: connection.Object(dbusDest, dbusPath),
+	}, nil
+}
+
 // Close closes the dbus connection
 func (c *Conn) Close() {
 	if c == nil {
 		return
 	}
 
-	if c.conn != nil {
-		c.conn.Close()
+	if c.conn != nil && c.connManager != nil {
+		c.connManager.Close()
 	}
 }
 
 func (c *Conn) initConnection() error {
 	var err error
-	c.conn, err = dbus.SystemBusPrivate()
+	c.connManager, err = dbus.SystemBusPrivate()
 	if err != nil {
 		return err
 	}
@@ -70,18 +107,19 @@ func (c *Conn) initConnection() error {
 	// libc)
 	methods := []dbus.Auth{dbus.AuthExternal(strconv.Itoa(os.Getuid()))}
 
-	err = c.conn.Auth(methods)
+	err = c.connManager.Auth(methods)
 	if err != nil {
-		c.conn.Close()
+		c.connManager.Close()
 		return err
 	}
 
-	err = c.conn.Hello()
+	err = c.connManager.Hello()
 	if err != nil {
-		c.conn.Close()
+		c.connManager.Close()
 		return err
 	}
 
+	c.conn = c.connManager
 	c.object = c.conn.Object("org.freedesktop.login1", dbus.ObjectPath(dbusPath))
 
 	return nil

login1/dbus_test.go

@@ -20,6 +20,8 @@ import (
 	"regexp"
 	"testing"
 
+	"github.com/godbus/dbus/v5"
+
 	"github.com/coreos/go-systemd/v22/login1"
 )
 
@@ -88,3 +90,91 @@ func TestListUsers(t *testing.T) {
 		}
 	}
 }
+
+func Test_Creating_new_connection_with_custom_connection(t *testing.T) {
+	t.Parallel()
+
+	t.Run("connects_to_global_login1_path_and_interface", func(t *testing.T) {
+		t.Parallel()
+
+		objectConstructorCalled := false
+
+		connectionWithContextCheck := &mockConnection{
+			ObjectF: func(dest string, path dbus.ObjectPath) dbus.BusObject {
+				objectConstructorCalled = true
+
+				expectedDest := "org.freedesktop.login1"
+
+				if dest != expectedDest {
+					t.Fatalf("Expected D-Bus destination %q, got %q", expectedDest, dest)
+				}
+
+				expectedPath := dbus.ObjectPath("/org/freedesktop/login1")
+
+				if path != expectedPath {
+					t.Fatalf("Expected D-Bus path %q, got %q", expectedPath, path)
+				}
+
+				return nil
+			},
+		}
+
+		if _, err := login1.NewWithConnection(connectionWithContextCheck); err != nil {
+			t.Fatalf("Unexpected error creating connection: %v", err)
+		}
+
+		if !objectConstructorCalled {
+			t.Fatalf("Expected object constructor to be called")
+		}
+	})
+
+	t.Run("returns_error_when_no_custom_connection_is_given", func(t *testing.T) {
+		t.Parallel()
+
+		testConn, err := login1.NewWithConnection(nil)
+		if err == nil {
+			t.Fatalf("Expected error creating connection with no connector")
+		}
+
+		if testConn != nil {
+			t.Fatalf("Expected connection to be nil when New returns error")
+		}
+	})
+}
+
+// mockConnection is a test helper for mocking dbus.Conn.
+type mockConnection struct {
+	ObjectF func(string, dbus.ObjectPath) dbus.BusObject
+}
+
+// Auth ...
+func (m *mockConnection) Auth(authMethods []dbus.Auth) error {
+	return nil
+}
+
+// Hello ...
+func (m *mockConnection) Hello() error {
+	return nil
+}
+
+// Signal ...
+func (m *mockConnection) Signal(ch chan<- *dbus.Signal) {}
+
+// Object ...
+func (m *mockConnection) Object(dest string, path dbus.ObjectPath) dbus.BusObject {
+	if m.ObjectF == nil {
+		return nil
+	}
+
+	return m.ObjectF(dest, path)
+}
+
+// Close ...
+func (m *mockConnection) Close() error {
+	return nil
+}
+
+// BusObject ...
+func (m *mockConnection) BusObject() dbus.BusObject {
+	return nil
+}