core: Show JSON error offsets where possible (#7437)

admin.go

@@ -1110,7 +1110,10 @@ func unsyncedConfigAccess(method, path string, body []byte, out io.Writer) error
 	if len(body) > 0 {
 		err = json.Unmarshal(body, &val)
 		if err != nil {
-			return fmt.Errorf("decoding request body: %v", err)
+			if jsonErr, ok := err.(*json.SyntaxError); ok {
+				return fmt.Errorf("decoding request body: %w, at offset %d", jsonErr, jsonErr.Offset)
+			}
+			return fmt.Errorf("decoding request body: %w", err)
 		}
 	}
 

caddyconfig/configadapters.go

@@ -81,7 +81,11 @@ func JSONModuleObject(val any, fieldName, fieldVal string, warnings *[]Warning)
 	err = json.Unmarshal(enc, &tmp)
 	if err != nil {
 		if warnings != nil {
-			*warnings = append(*warnings, Warning{Message: err.Error()})
+			message := err.Error()
+			if jsonErr, ok := err.(*json.SyntaxError); ok {
+				message = fmt.Sprintf("%v, at offset %d", jsonErr.Error(), jsonErr.Offset)
+			}
+			*warnings = append(*warnings, Warning{Message: message})
 		}
 		return nil
 	}

cmd/main.go

@@ -231,7 +231,10 @@ func loadConfigWithLogger(logger *zap.Logger, configFile, adapterName string) ([
 		// validate that the config is at least valid JSON
 		err = json.Unmarshal(config, new(any))
 		if err != nil {
-			return nil, "", "", fmt.Errorf("config is not valid JSON: %v; did you mean to use a config adapter (the --adapter flag)?", err)
+			if jsonErr, ok := err.(*json.SyntaxError); ok {
+				return nil, "", "", fmt.Errorf("config is not valid JSON: %w, at offset %d; did you mean to use a config adapter (the --adapter flag)?", err, jsonErr.Offset)
+			}
+			return nil, "", "", fmt.Errorf("config is not valid JSON: %w; did you mean to use a config adapter (the --adapter flag)?", err)
 		}
 	}
 

modules.go

@@ -342,7 +342,11 @@ func ParseStructTag(tag string) (map[string]string, error) {
 func StrictUnmarshalJSON(data []byte, v any) error {
 	dec := json.NewDecoder(bytes.NewReader(data))
 	dec.DisallowUnknownFields()
-	return dec.Decode(v)
+	err := dec.Decode(v)
+	if jsonErr, ok := err.(*json.SyntaxError); ok {
+		return fmt.Errorf("%w, at offset %d", jsonErr, jsonErr.Offset)
+	}
+	return err
 }
 
 var JSONRawMessageType = reflect.TypeFor[json.RawMessage]()