Use strict unmarshalling of rules (#7767)

* Fix unmarshalling of rules

Fix #7042

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

pkg/rulefmt/rulefmt.go

@@ -14,7 +14,9 @@
 package rulefmt
 
 import (
+	"bytes"
 	"context"
+	"io"
 	"io/ioutil"
 	"strings"
 	"time"
@@ -267,8 +269,11 @@ func Parse(content []byte) (*RuleGroups, []error) {
 		errs   []error
 	)
 
-	err := yaml.Unmarshal(content, &groups)
-	if err != nil {
+	decoder := yaml.NewDecoder(bytes.NewReader(content))
+	decoder.KnownFields(true)
+	err := decoder.Decode(&groups)
+	// Ignore io.EOF which happens with empty input.
+	if err != nil && err != io.EOF {
 		errs = append(errs, err)
 	}
 	err = yaml.Unmarshal(content, &node)

pkg/rulefmt/rulefmt_test.go

@@ -67,6 +67,10 @@ func TestParseFileFailure(t *testing.T) {
 			filename: "invalid_record_name.bad.yaml",
 			errMsg:   "invalid recording rule name",
 		},
+		{
+			filename: "bad_field.bad.yaml",
+			errMsg:   "field annotation not found",
+		},
 	}
 
 	for _, c := range table {

pkg/rulefmt/testdata/bad_field.bad.yaml

@@ -0,0 +1,10 @@
+groups:
+  - name: yolo
+    rules:
+      - alert: hola
+        expr: 1
+        labels:
+          instance: localhost
+        annotation:
+          summary: annonations is written without s above
+