feat(mapper): reject configs with unknown fields

pkg/mapper/mapper.go

@@ -83,7 +83,10 @@ var defaultQuantiles = []MetricObjective{
 func (m *MetricMapper) InitFromYAMLString(fileContents string) error {
 	var n MetricMapper
 
-	if err := yaml.Unmarshal([]byte(fileContents), &n); err != nil {
+	// Use strict decoding so unknown or misspelled keys in a mapping
+	// config surface as an error instead of being silently dropped
+	// (see https://github.com/prometheus/statsd_exporter/issues/546).
+	if err := yaml.UnmarshalStrict([]byte(fileContents), &n); err != nil {
 		return err
 	}
 

pkg/mapper/mapper_test.go

@@ -14,6 +14,7 @@
 package mapper
 
 import (
+	"strings"
 	"testing"
 	"time"
 
@@ -1760,3 +1761,28 @@ mappings:
 		}
 	}
 }
+
+// TestStrictConfigUnknownField covers the fix for
+// https://github.com/prometheus/statsd_exporter/issues/546: mapping
+// configs with keys the struct tags don't know about must fail to load
+// rather than silently drop the value.
+func TestStrictConfigUnknownField(t *testing.T) {
+	// `nmae` (misspelled `name`) is a field that MetricMapping doesn't
+	// expose; with permissive YAML it would be dropped, leaving the
+	// mapping without a metric name and producing a confusing runtime
+	// error much later (or none at all).
+	const badConfig = `---
+mappings:
+- match: aa.bb.*.*
+  nmae: "aa_bb_${1}_total"
+`
+
+	mapper := MetricMapper{}
+	err := mapper.InitFromYAMLString(badConfig)
+	if err == nil {
+		t.Fatal("expected InitFromYAMLString to reject unknown field `nmae`, got nil")
+	}
+	if !strings.Contains(err.Error(), "not found") && !strings.Contains(err.Error(), "field nmae") {
+		t.Fatalf("expected error to mention the unknown field, got: %v", err)
+	}
+}