opentracing: set Span error when adding error tags

This change allows setting a tag of name Error ("error") having a value
of error (but covers for other types to) which will in turn set the
error properties of the Span internally.

Fixes #144

opentracing/span.go

@@ -64,6 +64,15 @@ func (s *Span) SetTag(key string, value interface{}) ot.Span {
 		s.Span.Lock()
 		defer s.Span.Unlock()
 		s.Span.Type = fmt.Sprint(value)
+	case Error:
+		switch v := value.(type) {
+		case nil:
+			// no error
+		case error:
+			s.Span.SetError(v)
+		default:
+			s.Span.SetError(fmt.Errorf("%v", v))
+		}
 	default:
 		// NOTE: locking is not required because the `SetMeta` is
 		// already thread-safe

opentracing/span_test.go

@@ -1,6 +1,7 @@
 package opentracing
 
 import (
+	"errors"
 	"testing"
 	"time"
 
@@ -74,3 +75,55 @@ func TestSpanSetDatadogTags(t *testing.T) {
 	assert.Equal("db-cluster", span.Span.Service)
 	assert.Equal("SELECT * FROM users;", span.Span.Resource)
 }
+
+func TestSpanSetErrorTag(t *testing.T) {
+	assert := assert.New(t)
+
+	for _, tt := range []struct {
+		name string      // span name
+		val  interface{} // tag value
+		msg  string      // error message
+		typ  string      // error type
+	}{
+		{
+			name: "error.error",
+			val:  errors.New("some error"),
+			msg:  "some error",
+			typ:  "*errors.errorString",
+		},
+		{
+			name: "error.string",
+			val:  "some string error",
+			msg:  "some string error",
+			typ:  "*errors.errorString",
+		},
+		{
+			name: "error.struct",
+			val:  struct{ N int }{5},
+			msg:  "{5}",
+			typ:  "*errors.errorString",
+		},
+		{
+			name: "error.other",
+			val:  1,
+			msg:  "1",
+			typ:  "*errors.errorString",
+		},
+		{
+			name: "error.nil",
+			val:  nil,
+			msg:  "",
+			typ:  "",
+		},
+	} {
+		span := NewSpan(tt.name)
+		span.SetTag(Error, tt.val)
+
+		assert.Equal(span.Meta["error.msg"], tt.msg)
+		assert.Equal(span.Meta["error.type"], tt.typ)
+
+		if tt.val != nil {
+			assert.NotEqual(span.Meta["error.stack"], "")
+		}
+	}
+}

opentracing/tags.go

@@ -7,10 +7,6 @@ const (
 	ServiceName = "service.name"
 	// ResourceName defines the Resource name for the Span
 	ResourceName = "resource.name"
-	// ErrorMsg defines the error message
-	ErrorMsg = "error.msg"
-	// ErrorType defines the error class
-	ErrorType = "error.type"
-	// ErrorStack defines the stack for the given error or panic
-	ErrorStack = "error.stack"
+	// Error defines an error.
+	Error = "error.error"
 )