Return results from runCommand in error cases

GODRIVER-832

Change-Id: I5dc892d7cab5e3d8cf03dc3acda3bc77787a0a90

Author: connieyu

mongo/single_result.go

@@ -48,11 +48,11 @@ func (sr *SingleResult) Decode(v interface{}) error {
 
 // DecodeBytes will return a copy of the document as a bson.Raw. If there was an
 // error from the operation that created this SingleResult then the error
-// will be returned. If there were no returned documents, ErrNoDocuments is
+// will be returned along with the result. If there were no returned documents, ErrNoDocuments is
 // returned.
 func (sr *SingleResult) DecodeBytes() (bson.Raw, error) {
 	if sr.err != nil {
-		return nil, sr.err
+		return sr.rdr, sr.err
 	}
 
 	if sr.err = sr.setRdrContents(); sr.err != nil {

mongo/single_result_test.go

@@ -8,6 +8,7 @@ package mongo
 
 import (
 	"bytes"
+	"errors"
 	"testing"
 
 	"go.mongodb.org/mongo-driver/bson"
@@ -53,4 +54,16 @@ func TestSingleResult(t *testing.T) {
 			t.Fatalf("Error returned by SingleResult.Err() was %v when ErrNoDocuments was expected", err)
 		}
 	})
+
+	t.Run("TestDecodeWithErr", func(t *testing.T) {
+		r := []byte("foo")
+		sr := &SingleResult{rdr: r, err: errors.New("DecodeBytes error")}
+		res, err := sr.DecodeBytes()
+		if !bytes.Equal(res, r) {
+			t.Fatalf("DecodeBytes contents do not match; expected %v, returned %v", res, r)
+		}
+		if err != sr.err {
+			t.Fatalf("Error returned by DecodeBytes was %v when %v was expected", err, sr.err)
+		}
+	})
 }