Allow decoding top level into interface{}

GODRIVER-710

Change-Id: Id67ea6d4e8a6ee2968dc8b6acf282618715d1e03

Author: skriptble

bson/bsoncodec/default_value_decoders.go

@@ -99,7 +99,8 @@ func (dvd DefaultValueDecoders) RegisterDefaultDecoders(rb *RegistryBuilder) {
 		RegisterTypeMapEntry(bsontype.Timestamp, tTimestamp).
 		RegisterTypeMapEntry(bsontype.Decimal128, tDecimal).
 		RegisterTypeMapEntry(bsontype.MinKey, tMinKey).
-		RegisterTypeMapEntry(bsontype.MaxKey, tMaxKey)
+		RegisterTypeMapEntry(bsontype.MaxKey, tMaxKey).
+		RegisterTypeMapEntry(bsontype.Type(0), tD)
 }
 
 // BooleanDecodeValue is the ValueDecoderFunc for bool types.

bson/bsoncodec/default_value_decoders_test.go

@@ -2769,6 +2769,18 @@ func TestDefaultValueDecoders(t *testing.T) {
 				t.Errorf("Errors are not equal. got %v; want %v", got, want)
 			}
 		})
+		t.Run("top level document", func(t *testing.T) {
+			data := bsoncore.BuildDocument(nil, bsoncore.AppendDoubleElement(nil, "pi", 3.14159))
+			vr := bsonrw.NewBSONDocumentReader(data)
+			want := primitive.D{{"pi", 3.14159}}
+			var got interface{}
+			val := reflect.ValueOf(&got).Elem()
+			err := dvd.EmptyInterfaceDecodeValue(DecodeContext{Registry: buildDefaultRegistry()}, vr, val)
+			noerr(t, err)
+			if !cmp.Equal(got, want) {
+				t.Errorf("Did not get correct result. got %v; want %v", got, want)
+			}
+		})
 	})
 }