agent: use mapstructure's Metadata.Unused to detect extraneous config

command/agent/config.go

@@ -335,13 +335,6 @@ type Config struct {
 
 	// WatchPlans contains the compiled watches
 	WatchPlans []*watch.WatchPlan `mapstructure:"-" json:"-"`
-
-	// Allow the following fields to be present in configuration files without
-	// mapstructure erroring on them.
-	_ interface{} `mapstructure:"services"`
-	_ interface{} `mapstructure:"checks"`
-	_ interface{} `mapstructure:"service"`
-	_ interface{} `mapstructure:"check"`
 }
 
 type dirEnts []os.FileInfo
@@ -472,7 +465,6 @@ func DecodeConfig(r io.Reader) (*Config, error) {
 	msdec, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
 		Metadata: &md,
 		Result:   &result,
-		ErrorUnused: true,
 	})
 	if err != nil {
 		return nil, err
@@ -482,6 +474,20 @@ func DecodeConfig(r io.Reader) (*Config, error) {
 		return nil, err
 	}
 
+	// Check unused fields and verify that no bad configuration options were
+	// passed to Consul. There are a few additional fields which don't directly
+	// use mapstructure decoding, so we need to account for those as well.
+	allowedKeys := []string{"service", "services", "check", "checks"}
+	var unused []string
+	for _, field := range md.Unused {
+		if !strContains(allowedKeys, field) {
+			unused = append(unused, field)
+		}
+	}
+	if len(unused) > 0 {
+		return nil, fmt.Errorf("Config has invalid keys: %s", strings.Join(unused, ","))
+	}
+
 	// Handle time conversions
 	if raw := result.DNSConfig.NodeTTLRaw; raw != "" {
 		dur, err := time.ParseDuration(raw)