tracer: Check for nil interface types in span.Finish(WithError())

ddtrace/tracer/option.go

@@ -15,6 +15,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"reflect"
 	"runtime"
 	"strconv"
 	"strings"
@@ -941,6 +942,15 @@ func FinishTime(t time.Time) FinishOption {
 // err to set tags such as the error message, error type and stack trace. It has
 // no effect if the error is nil.
 func WithError(err error) FinishOption {
+	if err == nil {
+		return func(_ *ddtrace.FinishConfig) {}
+	}
+	v := reflect.ValueOf(err)
+	// Here be dragons: https://go.dev/doc/faq#nil_error
+	// https://github.com/DataDog/dd-trace-go/issues/2029
+	if !v.IsValid() || (v.Kind() == reflect.Ptr && v.IsNil()) {
+		return func(_ *ddtrace.FinishConfig) {}
+	}
 	return func(cfg *ddtrace.FinishConfig) {
 		cfg.Error = err
 	}

ddtrace/tracer/span_test.go

@@ -230,6 +230,28 @@ func TestSpanFinishWithError(t *testing.T) {
 	assert.NotEmpty(span.Meta[ext.ErrorStack])
 }
 
+type MyError struct {
+	msg string
+}
+
+func (e *MyError) Error() string {
+	return e.msg
+}
+
+func TestSpanFinishWithErrorNilCustomError(t *testing.T) {
+	assert := assert.New(t)
+
+	var err error
+	var myError *MyError
+	myError = nil
+	err = myError
+
+	span := newBasicSpan("web.request")
+	span.Finish(WithError(err))
+
+	assert.Equal(int32(0), span.Error)
+}
+
 func TestSpanFinishWithErrorNoDebugStack(t *testing.T) {
 	assert := assert.New(t)