fixup! WIP - Add Upgrader unit tests

Author: pchila

internal/pkg/agent/application/upgrade/upgrade.go

@@ -61,6 +61,7 @@ var (
 	ErrUpgradeSameVersion = errors.New("upgrade did not occur because it is the same version")
 	ErrNonFipsToFips      = errors.New("cannot switch to fips mode when upgrading")
 	ErrFipsToNonFips      = errors.New("cannot switch to non-fips mode when upgrading")
+	ErrNilUpdateMarker    = errors.New("loaded a nil update marker")
 )
 
 func init() {
@@ -443,6 +444,11 @@ func (u *Upgrader) PersistManualRollback(ctx context.Context, topDir string) err
 	if err != nil {
 		return fmt.Errorf("loading marker: %w", err)
 	}
+
+	if updateMarker == nil {
+		return ErrNilUpdateMarker
+	}
+
 	updateMarker.DesiredOutcome = OUTCOME_ROLLBACK
 	err = SaveMarker(paths.Data(), updateMarker, true)
 	if err != nil {

internal/pkg/agent/application/upgrade/upgrade_test.go

@@ -33,6 +33,7 @@ import (
 	"github.com/elastic/elastic-agent/pkg/core/logger"
 	"github.com/elastic/elastic-agent/pkg/core/logger/loggertest"
 	infomocks "github.com/elastic/elastic-agent/testing/mocks/internal_/pkg/agent/application/info"
+	upgrademocks "github.com/elastic/elastic-agent/testing/mocks/internal_/pkg/agent/application/upgrade"
 	ackermocks "github.com/elastic/elastic-agent/testing/mocks/internal_/pkg/fleetapi/acker"
 	clientmocks "github.com/elastic/elastic-agent/testing/mocks/pkg/control/v2/client"
 )
@@ -1035,7 +1036,7 @@ func TestIsSameReleaseVersion(t *testing.T) {
 func TestManualRollback(t *testing.T) {
 	log, _ := loggertest.New(t.Name())
 	mockAgentInfo := infomocks.NewAgent(t)
-	upgrader, err := NewUpgrader(log, &artifact.Config{}, &configuration.UpgradeConfig{}, mockAgentInfo, new(AgentWatcherHelper))
+	upgrader, err := NewUpgrader(log, &artifact.Config{}, &configuration.UpgradeConfig{}, mockAgentInfo, upgrademocks.NewWatcherHelper(t))
 	require.NoError(t, err, "error creating upgrader")
 
 	upgrader.Upgrade(t.Context(), "", true, "", nil, nil, true, false)