systemd: implement 'Start()' function

Implement and test the 'Start()' function for a systemd-managed daemon
service. As in the 'launchd' daemon provider, the 'Start()' function does
not check whether the service is already running; if it is, the operation is
a no-op.

Signed-off-by: Victoria Dye <[email protected]>

Author: vdye

internal/daemon/systemd.go

@@ -82,7 +82,17 @@ func (s *systemd) Create(config *DaemonConfig, force bool) error {
 }
 
 func (s *systemd) Start(label string) error {
-	return fmt.Errorf("not implemented")
+	// TODO: warn user if already running
+	exitCode, err := s.cmdExec.Run("systemctl", "--user", "start", label)
+	if err != nil {
+		return err
+	}
+
+	if exitCode != 0 {
+		return fmt.Errorf("'systemctl stop' exited with status %d", exitCode)
+	}
+
+	return nil
 }
 
 func (s *systemd) Stop(label string) error {

internal/daemon/systemd_test.go

@@ -162,3 +162,45 @@ func TestSystemd_Create(t *testing.T) {
 		assert.Contains(t, fileContents, fmt.Sprintf("Description=%s", basicDaemonConfig.Description))
 	})
 }
+
+func TestSystemd_Start(t *testing.T) {
+	// Set up mocks
+	testUser := &user.User{
+		Uid:      "123",
+		Username: "testuser",
+		HomeDir:  "/my/test/dir",
+	}
+	testUserProvider := &mockUserProvider{}
+	testUserProvider.On("CurrentUser").Return(testUser, nil)
+
+	testCommandExecutor := &mockCommandExecutor{}
+
+	systemd := daemon.NewSystemdProvider(testUserProvider, testCommandExecutor, nil)
+
+	// Test #1: systemctl succeeds
+	t.Run("Calls correct systemctl command", func(t *testing.T) {
+		testCommandExecutor.On("Run",
+			"systemctl",
+			[]string{"--user", "start", basicDaemonConfig.Label},
+		).Return(0, nil).Once()
+
+		err := systemd.Start(basicDaemonConfig.Label)
+		assert.Nil(t, err)
+		mock.AssertExpectationsForObjects(t, testCommandExecutor)
+	})
+
+	// Reset the mock structure between tests
+	testCommandExecutor.Mock = mock.Mock{}
+
+	// Test #2: systemctl fails
+	t.Run("Returns error when systemctl fails", func(t *testing.T) {
+		testCommandExecutor.On("Run",
+			mock.AnythingOfType("string"),
+			mock.AnythingOfType("[]string"),
+		).Return(1, nil).Once()
+
+		err := systemd.Start(basicDaemonConfig.Label)
+		assert.NotNil(t, err)
+		mock.AssertExpectationsForObjects(t, testCommandExecutor)
+	})
+}