Return error on field type mismatch (Graylog2#20)

gelf/message.go

@@ -3,6 +3,7 @@ package gelf
 import (
 	"bytes"
 	"encoding/json"
+	"fmt"
 	"time"
 )
 
@@ -84,21 +85,29 @@ func (m *Message) UnmarshalJSON(data []byte) error {
 			m.Extra[k] = v
 			continue
 		}
+
+		ok := true
 		switch k {
 		case "version":
-			m.Version = v.(string)
+			m.Version, ok = v.(string)
 		case "host":
-			m.Host = v.(string)
+			m.Host, ok = v.(string)
 		case "short_message":
-			m.Short = v.(string)
+			m.Short, ok = v.(string)
 		case "full_message":
-			m.Full = v.(string)
+			m.Full, ok = v.(string)
 		case "timestamp":
-			m.TimeUnix = v.(float64)
+			m.TimeUnix, ok = v.(float64)
 		case "level":
-			m.Level = int32(v.(float64))
+			var level float64
+			level, ok = v.(float64)
+			m.Level = int32(level)
 		case "facility":
-			m.Facility = v.(string)
+			m.Facility, ok = v.(string)
+		}
+
+		if !ok {
+			return fmt.Errorf("invalid type for field %s", k)
 		}
 	}
 	return nil

gelf/message_test.go

@@ -0,0 +1,27 @@
+package gelf
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestWrongFieldTypes(t *testing.T) {
+	msgData := map[string]string{
+		"version":       `{"version": 1.1}`,
+		"host":          `{"host": ["a", "b"]}`,
+		"short_message": `{"short_message": {"a": "b"}}`,
+		"full_message":  `{"full_message": null}`,
+		"timestamp":     `{"timestamp": "12345"}`,
+		"level":         `{"level": false}`,
+		"facility":      `{"facility": true}`,
+	}
+
+	for k, j := range msgData {
+		var msg Message
+		err := json.Unmarshal([]byte(j), &msg)
+		if err == nil {
+			t.Errorf("expected type error on field %s", k)
+		}
+	}
+
+}