[Upgrade Watcher] Try restarting Agent in multiple ways during rollback

changelog/fragments/1692323065-bugfix-rollback-restart-agent-no-process.yaml

@@ -0,0 +1,32 @@
+# Kind can be one of:
+# - breaking-change: a change to previously-documented behavior
+# - deprecation: functionality that is being removed in a later release
+# - bug-fix: fixes a problem in a previous version
+# - enhancement: extends functionality but does not break or fix existing behavior
+# - feature: new functionality
+# - known-issue: problems that we are aware of in a given version
+# - security: impacts on the security of a product or a user’s deployment.
+# - upgrade: important information for someone upgrading from a prior version
+# - other: does not fit into any of the other categories
+kind: bug-fix
+
+# Change summary; a 80ish characters long description of the change.
+summary: Ensure that Elastic Agent is restarted during rollback
+
+# Long description; in case the summary is not enough to describe the change
+# this field accommodate a description without length limits.
+# NOTE: This field will be rendered only for breaking-change and known-issue kinds at the moment.
+#description:
+
+# Affected component; usually one of "elastic-agent", "fleet-server", "filebeat", "metricbeat", "auditbeat", "all", etc.
+component: elastic-agent
+
+# PR URL; optional; the PR number that added the changeset.
+# If not present is automatically filled by the tooling finding the PR where this changelog fragment has been added.
+# NOTE: the tooling supports backports, so it's able to fill the original PR number instead of the backport PR number.
+# Please provide it if you are adding a fragment for a different PR.
+pr: https://github.com/elastic/elastic-agent/pull/3268
+
+# Issue URL; optional; the GitHub issue related to this changeset (either closes or is part of).
+# If not present is automatically filled by the tooling with the issue linked to the PR number.
+#issue: https://github.com/owner/repo/1234

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

@@ -127,7 +127,7 @@ func InvokeWatcher(log *logger.Logger) error {
 }
 
 func restartAgent(ctx context.Context, log *logger.Logger) error {
-	restartFn := func(ctx context.Context) error {
+	restartViaDaemonFn := func(ctx context.Context) error {
 		c := client.New()
 		err := c.Connect(ctx)
 		if err != nil {
@@ -143,24 +143,43 @@ func restartAgent(ctx context.Context, log *logger.Logger) error {
 		return nil
 	}
 
+	restartViaServiceFn := func(ctx context.Context) error {
+		topPath := paths.Top()
+		err := install.RestartService(topPath)
+		if err != nil {
+			return fmt.Errorf("failed to restart agent via service: %w", err)
+		}
+
+		return nil
+	}
+
 	signal := make(chan struct{})
 	backExp := backoff.NewExpBackoff(signal, restartBackoffInit, restartBackoffMax)
 
 	for restartAttempt := 1; restartAttempt <= maxRestartCount; restartAttempt++ {
 		backExp.Wait()
 		log.Infof("Restarting Agent via control protocol; attempt %d of %d", restartAttempt, maxRestartCount)
+		// First, try to restart Agent by sending a restart command
+		// to its daemon (via GRPC).
+		err := restartViaDaemonFn(ctx)
+		if err == nil {
+			break
+		}
+		log.Warnf("Failed to restart agent via control protocol: %s", err.Error())
 
-		err := restartFn(ctx)
+		// Next, try to restart Agent via the service.
+		log.Infof("Restarting Agent via service; attempt %d of %d", restartAttempt, maxRestartCount)
+		err = restartViaServiceFn(ctx)
 		if err == nil {
 			break
 		}
 
 		if restartAttempt == maxRestartCount {
-			log.Error("Failed to restart agent via control protocol after final attempt")
+			log.Error("Failed to restart agent after final attempt")
 			return err
 		}
 
-		log.Warnf("Failed to restart agent via control protocol: %s; will try again in %v", err.Error(), backExp.NextWait())
+		log.Warnf("Failed to restart agent via service: %s; will try again in %v", err.Error(), backExp.NextWait())
 	}
 
 	close(signal)

internal/pkg/agent/install/install.go

@@ -172,6 +172,22 @@ func StopService(topPath string) error {
 	return nil
 }
 
+// RestartService restarts the installed service.
+func RestartService(topPath string) error {
+	svc, err := newService(topPath)
+	if err != nil {
+		return err
+	}
+	err = svc.Restart()
+	if err != nil {
+		return errors.New(
+			err,
+			fmt.Sprintf("failed to restart service (%s)", paths.ServiceName),
+			errors.M("service", paths.ServiceName))
+	}
+	return nil
+}
+
 // FixPermissions fixes the permissions on the installed system.
 func FixPermissions(topPath string) error {
 	return fixPermissions(topPath)