Do webseed request parts sequentially

This means we can treat the number of bytes in the result with enough accuracy to decide if we should count it as a wasted chunk. Also I'm not sure why it was a good idea to do parts of a request in parallel anyway, it could just lead to spikes in outstanding requests to the webseed, rather than sticking to the predictable maxRequests limit.

webseed-peer.go

@@ -125,10 +125,14 @@ func (ws *webseedPeer) onClose() {
 
 func (ws *webseedPeer) requestResultHandler(r Request, webseedRequest webseed.Request) {
 	result := <-webseedRequest.Result
+	close(webseedRequest.Result) // one-shot
 	// We do this here rather than inside receiveChunk, since we want to count errors too. I'm not
 	// sure if we can divine which errors indicate cancellation on our end without hitting the
 	// network though.
-	ws.peer.doChunkReadStats(int64(len(result.Bytes)))
+	if len(result.Bytes) != 0 || result.Err == nil {
+		// Increment ChunksRead and friends
+		ws.peer.doChunkReadStats(int64(len(result.Bytes)))
+	}
 	ws.peer.readBytes(int64(len(result.Bytes)))
 	ws.peer.t.cl.lock()
 	defer ws.peer.t.cl.unlock()
@@ -138,6 +142,7 @@ func (ws *webseedPeer) requestResultHandler(r Request, webseedRequest webseed.Re
 	if result.Err != nil {
 		if !errors.Is(result.Err, context.Canceled) && !ws.peer.closed.IsSet() {
 			ws.peer.logger.Printf("Request %v rejected: %v", r, result.Err)
+			// // Here lies my attempt to extract something concrete from Go's error system. RIP.
 			// cfg := spew.NewDefaultConfig()
 			// cfg.DisableMethods = true
 			// cfg.Dump(result.Err)

webseed/client.go

@@ -26,6 +26,7 @@ type requestPart struct {
 	req    *http.Request
 	e      segments.Extent
 	result chan requestPartResult
+	start  func()
 }
 
 type Request struct {
@@ -79,13 +80,15 @@ func (ws *Client) NewRequest(r RequestSpec) Request {
 			result: make(chan requestPartResult, 1),
 			e:      e,
 		}
-		go func() {
-			resp, err := ws.HttpClient.Do(req)
-			part.result <- requestPartResult{
-				resp: resp,
-				err:  err,
-			}
-		}()
+		part.start = func() {
+			go func() {
+				resp, err := ws.HttpClient.Do(req)
+				part.result <- requestPartResult{
+					resp: resp,
+					err:  err,
+				}
+			}()
+		}
 		requestParts = append(requestParts, part)
 		return true
 	}) {
@@ -116,6 +119,8 @@ func (me ErrBadResponse) Error() string {
 
 func recvPartResult(ctx context.Context, buf io.Writer, part requestPart) error {
 	result := <-part.result
+	// Make sure there's no further results coming, it should be a one-shot channel.
+	close(part.result)
 	if result.err != nil {
 		return result.err
 	}
@@ -165,29 +170,15 @@ func recvPartResult(ctx context.Context, buf io.Writer, part requestPart) error
 	}
 }
 
-func readRequestPartResponses(ctx context.Context, parts []requestPart) ([]byte, error) {
-	ctx, cancel := context.WithCancel(ctx)
-	defer cancel()
+func readRequestPartResponses(ctx context.Context, parts []requestPart) (_ []byte, err error) {
 	var buf bytes.Buffer
-	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:
-				}
-			}
+	for _, part := range parts {
+		part.start()
+		err = recvPartResult(ctx, &buf, part)
+		if err != nil {
+			err = fmt.Errorf("reading %q at %q: %w", part.req.URL, part.req.Header.Get("Range"), err)
+			break
 		}
-		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
 }