fix(installer): Support leftover /opt/datadog-agent dir (#29443)

pkg/fleet/installer/service/systemd.go

@@ -73,6 +73,13 @@ func disableUnit(ctx context.Context, unit string) (err error) {
 	span, _ := tracer.StartSpanFromContext(ctx, "disable_unit")
 	defer func() { span.Finish(tracer.WithError(err)) }()
 	span.SetTag("unit", unit)
+
+	enabledErr := exec.CommandContext(ctx, "systemctl", "is-enabled", "--quiet", unit).Run()
+	if enabledErr != nil {
+		// unit is already disabled or doesn't exist, we can return fast
+		return nil
+	}
+
 	err = exec.CommandContext(ctx, "systemctl", "disable", unit).Run()
 	exitErr := &exec.ExitError{}
 	if !errors.As(err, &exitErr) {

test/new-e2e/tests/installer/package_agent_test.go

@@ -423,6 +423,18 @@ func (s *packageAgentSuite) TestUpgrade_DisabledAgentDebRPM_to_OCI() {
 	s.host.Run("sudo systemctl show datadog-agent -p ExecStart | grep /opt/datadog-packages")
 }
 
+func (s *packageAgentSuite) TestInstallWithLeftoverDebDir() {
+	// create /opt/datadog-agent to simulate a disabled agent
+	s.host.Run("sudo mkdir -p /opt/datadog-agent")
+
+	// install OCI agent
+	s.RunInstallScript(envForceInstall("datadog-agent"))
+
+	state := s.host.State()
+	s.assertUnits(state, false)
+	s.host.Run("sudo systemctl show datadog-agent -p ExecStart | grep /opt/datadog-packages")
+}
+
 func (s *packageAgentSuite) purgeAgentDebInstall() {
 	pkgManager := s.host.GetPkgManager()
 	switch pkgManager {