[Elastic Agent] Remove symlink.prev from previously failed upgrade

x-pack/elastic-agent/CHANGELOG.next.asciidoc

@@ -74,6 +74,7 @@
 - Fix add support for Logstash output. {pull}24305[24305]
 - Do not log Elasticsearch configuration for monitoring output when running with debug. {pull}26583[26583]
 - Fix issue where proxy enrollment options broke enrollment command. {pull}26749[26749]
+- Remove symlink.prev from previously failed upgrade {pull}26785[26785]
 
 ==== New features
 

x-pack/elastic-agent/pkg/agent/application/upgrade/rollback.go

@@ -73,6 +73,9 @@ func Cleanup(currentHash string, removeMarker bool) error {
 		return err
 	}
 
+	// remove symlink to avoid upgrade failures, ignore error
+	_ = os.Remove(prevSymlinkPath())
+
 	dirPrefix := fmt.Sprintf("%s-", agentName)
 	currentDir := fmt.Sprintf("%s-%s", agentName, currentHash)
 	for _, dir := range subdirs {

x-pack/elastic-agent/pkg/agent/application/upgrade/step_relink.go

@@ -21,22 +21,37 @@ func ChangeSymlink(ctx context.Context, targetHash string) error {
 	// create symlink to elastic-agent-{hash}
 	hashedDir := fmt.Sprintf("%s-%s", agentName, targetHash)
 
-	agentPrevName := agentName + ".prev"
 	symlinkPath := filepath.Join(paths.Top(), agentName)
 	newPath := filepath.Join(paths.Top(), "data", hashedDir, agentName)
 
 	// handle windows suffixes
 	if runtime.GOOS == "windows" {
-		agentPrevName = agentName + ".exe.prev"
 		symlinkPath += ".exe"
 		newPath += ".exe"
 	}
 
-	prevNewPath := filepath.Join(paths.Top(), agentPrevName)
+	prevNewPath := prevSymlinkPath()
+
+	// remove symlink to avoid upgrade failures
+	if err := os.Remove(prevNewPath); !os.IsNotExist(err) {
+		return err
+	}
+
 	if err := os.Symlink(newPath, prevNewPath); err != nil {
 		return errors.New(err, errors.TypeFilesystem, "failed to update agent symlink")
 	}
 
 	// safely rotate
 	return file.SafeFileRotate(symlinkPath, prevNewPath)
 }
+
+func prevSymlinkPath() string {
+	agentPrevName := agentName + ".prev"
+
+	// handle windows suffixes
+	if runtime.GOOS == "windows" {
+		agentPrevName = agentName + ".exe.prev"
+	}
+
+	return filepath.Join(paths.Top(), agentPrevName)
+}