Alertmanager: Fix config validation gap around unreferenced templates (#9207)

* Create test covering an unreferenced invalid template

* Validate all templates, not just referenced ones

* changelog

Author: alexweav

CHANGELOG.md

@@ -106,6 +106,7 @@
 * [BUGFIX] Querier: Fix invalid query results when multiple chunks are being merged. #8992
 * [BUGFIX] Query-frontend: return annotations generated during evaluation of sharded queries. #9138
 * [BUGFIX] Querier: Support optional start and end times on `/prometheus/api/v1/labels`, `/prometheus/api/v1/label/<label>/values`, and `/prometheus/api/v1/series` when `max_query_into_future: 0`. #9129
+* [BUGFIX] Alertmanager: Fix config validation gap around unreferenced templates. #9207
 
 ### Mixin
 

pkg/alertmanager/api.go

@@ -11,7 +11,6 @@ import (
 	"io"
 	"net/http"
 	"os"
-	"path/filepath"
 	"reflect"
 
 	"github.com/go-kit/log"
@@ -246,6 +245,7 @@ func validateUserConfig(logger log.Logger, cfg alertspb.AlertConfigDesc, limits
 	}
 	defer os.RemoveAll(userTempDir)
 
+	templateFiles := make([]string, 0, len(cfg.Templates))
 	for _, tmpl := range cfg.Templates {
 		templateFilepath, err := safeTemplateFilepath(userTempDir, tmpl.Filename)
 		if err != nil {
@@ -257,11 +257,8 @@ func validateUserConfig(logger log.Logger, cfg alertspb.AlertConfigDesc, limits
 			level.Error(logger).Log("msg", "unable to store template file", "err", err, "user", cfg.User)
 			return fmt.Errorf("unable to store template file '%s'", tmpl.Filename)
 		}
-	}
 
-	templateFiles := make([]string, len(amCfg.Templates))
-	for i, t := range amCfg.Templates {
-		templateFiles[i] = filepath.Join(userTempDir, t)
+		templateFiles = append(templateFiles, templateFilepath)
 	}
 
 	_, err = template.FromGlobs(templateFiles, WithCustomFunctions(user))

pkg/alertmanager/api_test.go

@@ -746,6 +746,23 @@ alertmanager_config: |
     - "*.tmpl"
 template_files:
   "test.tmpl": "{{ invalid Go template }}"
+`,
+			err: fmt.Errorf(`error validating Alertmanager config: template: test.tmpl:1: function "invalid" not defined`),
+		},
+		{
+			name: "should return error if template is wrong even when not referenced by config",
+			cfg: `
+alertmanager_config: |
+  route:
+    receiver: 'default-receiver'
+    group_wait: 30s
+    group_interval: 5m
+    repeat_interval: 4h
+    group_by: [cluster, alertname]
+  receivers:
+    - name: default-receiver
+template_files:
+  "test.tmpl": "{{ invalid Go template }}"
 `,
 			err: fmt.Errorf(`error validating Alertmanager config: template: test.tmpl:1: function "invalid" not defined`),
 		},