Use absolute path of template to decide if we need to remove it

CHANGELOG.md

@@ -7,6 +7,7 @@
 * [ENHANCEMENT] Ruler: Added `-ruler.alertmanager-client.tls-enabled` configuration for alertmanager client. #3432 #3597
 * [ENHANCEMENT] Activity tracker logs now have `component=activity-tracker` label. #3556
 * [BUGFIX] Log the names of services that are not yet running rather than `unsupported value type` when calling `/ready` and some services are not running. #3625
+* [BUGFIX] Alertmanager: Fix template spurious deletion with relative data dir. #3604
 
 ### Mixin
 * [BUGFIX] Alerts: Fixed `MimirCompactorSkippedBlocksWithOutOfOrderChunks` matching on non-existent label. #3628

pkg/alertmanager/multitenant.go

@@ -635,7 +635,11 @@ func (am *MultitenantAlertmanager) setConfig(cfg alertspb.AlertConfigDesc) error
 	// List existing files to keep track of the ones to be removed
 	if oldTemplateFiles, err := os.ReadDir(userTemplateDir); err == nil {
 		for _, file := range oldTemplateFiles {
-			pathsToRemove[filepath.Join(userTemplateDir, file.Name())] = struct{}{}
+			templateFilePath, err := safeTemplateFilepath(userTemplateDir, file.Name())
+			if err != nil {
+				return err
+			}
+			pathsToRemove[templateFilePath] = struct{}{}
 		}
 	}