enhancement(5235): added deferred error handler in upgrade function to add insufficient disk space error into the error chain

enhancement(5235): added comments for the deferred error handler in upgrade func

Author: Kaan Yalti

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

@@ -23,6 +23,7 @@ import (
 	"github.com/elastic/elastic-agent/internal/pkg/agent/application/paths"
 	"github.com/elastic/elastic-agent/internal/pkg/agent/application/reexec"
 	"github.com/elastic/elastic-agent/internal/pkg/agent/application/upgrade/artifact"
+	upgradeErrors "github.com/elastic/elastic-agent/internal/pkg/agent/application/upgrade/artifact/download/errors"
 	"github.com/elastic/elastic-agent/internal/pkg/agent/application/upgrade/details"
 	"github.com/elastic/elastic-agent/internal/pkg/agent/configuration"
 	"github.com/elastic/elastic-agent/internal/pkg/agent/errors"
@@ -197,6 +198,16 @@ func checkUpgrade(log *logger.Logger, currentVersion, newVersion agentVersion, m
 func (u *Upgrader) Upgrade(ctx context.Context, version string, sourceURI string, action *fleetapi.ActionUpgrade, det *details.Details, skipVerifyOverride bool, skipDefaultPgp bool, pgpBytes ...string) (_ reexec.ShutdownCallbackFn, err error) {
 	u.log.Infow("Upgrading agent", "version", version, "source_uri", sourceURI)
 
+	defer func() {
+		if err != nil {
+			// Add the disk space error to the error chain if it is a disk space error
+			// so that we can use errors.Is to check for it
+			if upgradeErrors.IsDiskSpaceError(err) {
+				err = goerrors.Join(err, upgradeErrors.ErrInsufficientDiskSpace)
+			}
+		}
+	}()
+
 	currentVersion := agentVersion{
 		version:  release.Version(),
 		snapshot: release.Snapshot(),