webseed: Close unused part responses after error

Also don't bother to read their response bodies.

webseed/client.go

@@ -94,7 +94,7 @@ func (ws *Client) NewRequest(r RequestSpec) Request {
 		Result: make(chan RequestResult, 1),
 	}
 	go func() {
-		b, err := readRequestPartResponses(requestParts)
+		b, err := readRequestPartResponses(ctx, requestParts)
 		req.Result <- RequestResult{
 			Bytes: b,
 			Err:   err,
@@ -112,12 +112,15 @@ func (me ErrBadResponse) Error() string {
 	return me.Msg
 }
 
-func recvPartResult(buf io.Writer, part requestPart) error {
+func recvPartResult(ctx context.Context, buf io.Writer, part requestPart) error {
 	result := <-part.result
 	if result.err != nil {
 		return result.err
 	}
 	defer result.resp.Body.Close()
+	if ctx.Err() != nil {
+		return ctx.Err()
+	}
 	switch result.resp.StatusCode {
 	case http.StatusPartialContent:
 	case http.StatusOK:
@@ -140,13 +143,29 @@ func recvPartResult(buf io.Writer, part requestPart) error {
 	return nil
 }
 
-func readRequestPartResponses(parts []requestPart) ([]byte, error) {
+func readRequestPartResponses(ctx context.Context, parts []requestPart) ([]byte, error) {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
 	var buf bytes.Buffer
-	for _, part := range parts {
-		err := recvPartResult(&buf, part)
-		if err != nil {
-			return buf.Bytes(), fmt.Errorf("reading %q at %q: %w", part.req.URL, part.req.Header.Get("Range"), err)
+	firstErr := make(chan error, 1)
+	go func() {
+		for _, part := range parts {
+			err := recvPartResult(ctx, &buf, part)
+			if err != nil {
+				// Ensure no further unnecessary response reads occur.
+				cancel()
+				select {
+				case firstErr <- fmt.Errorf("reading %q at %q: %w", part.req.URL, part.req.Header.Get("Range"), err):
+				default:
+				}
+			}
 		}
-	}
-	return buf.Bytes(), nil
+		select {
+		case firstErr <- nil:
+		default:
+		}
+	}()
+	// This can't be merged into the return statement, because buf.Bytes is called first!
+	err := <-firstErr
+	return buf.Bytes(), err
 }