Format errors correctly

Errors included in a logrus.Entry are not formatted correctly. This is caused
by sirupsen/logrus#137

formatter.go

@@ -129,6 +129,22 @@ func (f *Formatter) errorOrigin() (stack.Call, error) {
 	}
 }
 
+// taken from https://github.com/sirupsen/logrus/blob/master/json_formatter.go#L51
+func replaceErrors(source logrus.Fields) logrus.Fields {
+	data := make(logrus.Fields, len(source))
+	for k, v := range source {
+		switch v := v.(type) {
+		case error:
+			// Otherwise errors are ignored by `encoding/json`
+			// https://github.com/sirupsen/logrus/issues/137
+			data[k] = v.Error()
+		default:
+			data[k] = v
+		}
+	}
+	return data
+}
+
 // Format formats a logrus entry according to the Stackdriver specifications.
 func (f *Formatter) Format(e *logrus.Entry) ([]byte, error) {
 	severity := levelsToSeverity[e.Level]
@@ -138,7 +154,7 @@ func (f *Formatter) Format(e *logrus.Entry) ([]byte, error) {
 		Message:  e.Message,
 		Severity: severity,
 		Context: &context{
-			Data: e.Data,
+			Data: replaceErrors(e.Data),
 		},
 	}
 

formatter_test.go

@@ -54,6 +54,24 @@ var formatterTests = []struct {
 			},
 		},
 	},
+	{
+		run: func(logger *logrus.Logger) {
+			logger.
+				WithField("foo", "bar").
+				WithError(errors.New("test error")).
+				Info("my log entry")
+		},
+		out: map[string]interface{}{
+			"severity": "INFO",
+			"message":  "my log entry",
+			"context": map[string]interface{}{
+				"data": map[string]interface{}{
+					"foo":   "bar",
+					"error": "test error",
+				},
+			},
+		},
+	},
 	{
 		run: func(logger *logrus.Logger) {
 			logger.WithField("foo", "bar").Error("my log entry")
@@ -71,8 +89,8 @@ var formatterTests = []struct {
 				},
 				"reportLocation": map[string]interface{}{
 					"filePath":     "github.com/TV4/logrus-stackdriver-formatter/formatter_test.go",
-					"lineNumber":   59.0,
-					"functionName": "glob..func2",
+					"lineNumber":   77.0,
+					"functionName": "glob..func3",
 				},
 			},
 		},
@@ -97,8 +115,8 @@ var formatterTests = []struct {
 				},
 				"reportLocation": map[string]interface{}{
 					"filePath":     "github.com/TV4/logrus-stackdriver-formatter/formatter_test.go",
-					"lineNumber":   85.0,
-					"functionName": "glob..func3",
+					"lineNumber":   103.0,
+					"functionName": "glob..func4",
 				},
 			},
 		},
@@ -130,8 +148,8 @@ var formatterTests = []struct {
 				},
 				"reportLocation": map[string]interface{}{
 					"filePath":     "github.com/TV4/logrus-stackdriver-formatter/formatter_test.go",
-					"lineNumber":   115.0,
-					"functionName": "glob..func4",
+					"lineNumber":   133.0,
+					"functionName": "glob..func5",
 				},
 			},
 		},