add custom error type for unknown topic or partition (segmentio#1306)

dialer.go

@@ -195,7 +195,7 @@ func (d *Dialer) LookupPartition(ctx context.Context, network string, address st
 			}
 		}
 
-		errch <- UnknownTopicOrPartition
+		errch <- unknownTopicOrPartition(topic, &partition)
 	}()
 
 	var prt Partition

error.go

@@ -655,6 +655,23 @@ func (e MessageTooLargeError) Error() string {
 	return MessageSizeTooLarge.Error()
 }
 
+type UnknownTopicOrPartitionError struct {
+	Topic     string
+	Partition *int
+}
+
+func unknownTopicOrPartition(topic string, partition *int) UnknownTopicOrPartitionError {
+	return UnknownTopicOrPartitionError{Topic: topic, Partition: partition}
+}
+
+func (e UnknownTopicOrPartitionError) Error() string {
+	return UnknownTopicOrPartition.Error()
+}
+
+func (e UnknownTopicOrPartitionError) Is(err error) bool {
+	return errors.Is(err, UnknownTopicOrPartition)
+}
+
 func makeError(code int16, message string) error {
 	if code == 0 {
 		return nil

writer.go

@@ -765,7 +765,7 @@ func (w *Writer) partitions(ctx context.Context, topic string) (int, error) {
 			return len(t.Partitions), nil
 		}
 	}
-	return 0, UnknownTopicOrPartition
+	return 0, unknownTopicOrPartition(topic, nil)
 }
 
 func (w *Writer) client(timeout time.Duration) *Client {