From: Matt Joiner <anacrolix@gmail.com>
Date: Tue, 5 Aug 2025 12:42:59 +0000 (+1000)
Subject: Propagate better webseed cancellation errors and cancel requests on peer context
X-Git-Tag: v1.59.0~2^2~54
X-Git-Url: http://www.git.stargrave.org/?a=commitdiff_plain;h=02bf07949520a77012a3dffc0621c3760322dcdc;p=btrtrc.git

Propagate better webseed cancellation errors and cancel requests on peer context
---

diff --git a/errors.go b/errors.go
index 10cbafc7..06430139 100644
--- a/errors.go
+++ b/errors.go
@@ -1 +1,8 @@
 package torrent
+
+// I don't trust errors.New with allocations, and I know I can use unique.Handle if I get desperate.
+type stringError string
+
+func (e stringError) Error() string {
+	return string(e)
+}
diff --git a/webseed-peer.go b/webseed-peer.go
index 230df90a..5dcef078 100644
--- a/webseed-peer.go
+++ b/webseed-peer.go
@@ -134,7 +134,7 @@ func (ws *webseedPeer) intoSpec(begin, end RequestIndex) webseed.RequestSpec {
 }
 
 func (ws *webseedPeer) spawnRequest(begin, end RequestIndex, logger *slog.Logger) {
-	extWsReq := ws.client.StartNewRequest(ws.intoSpec(begin, end), logger)
+	extWsReq := ws.client.StartNewRequest(ws.peer.closedCtx, ws.intoSpec(begin, end), logger)
 	wsReq := webseedRequest{
 		logger:  logger,
 		request: extWsReq,
@@ -263,7 +263,9 @@ func (ws *webseedPeer) maxChunkDiscard() RequestIndex {
 	return RequestIndex(int(intCeilDiv(webseed.MaxDiscardBytes, ws.peer.t.chunkSize)))
 }
 
-func (ws *webseedPeer) keepReading(wr *webseedRequest) bool {
+func (ws *webseedPeer) wantedChunksInDiscardWindow(wr *webseedRequest) bool {
+	// Shouldn't call this if request is at the end already.
+	panicif.GreaterThanOrEqual(wr.next, wr.end)
 	for ri := wr.next; ri < wr.end && ri <= wr.next+ws.maxChunkDiscard(); ri++ {
 		if ws.wantChunk(ri) {
 			return true
@@ -306,8 +308,10 @@ func (ws *webseedPeer) readChunks(wr *webseedRequest) (err error) {
 		err = ws.peer.receiveChunk(&msg)
 		stop := err != nil || wr.next >= wr.end
 		if !stop {
-			if !ws.keepReading(wr) {
-				wr.Cancel("finished or discarded")
+			if !ws.wantedChunksInDiscardWindow(wr) {
+				// This cancels the stream, but we don't stop su--reading to make the most of the
+				// buffered body.
+				wr.Cancel("no wanted chunks in discard window")
 			}
 		}
 		ws.peer.locker().Unlock()
diff --git a/webseed-request.go b/webseed-request.go
index c0db9dbe..bf267fa3 100644
--- a/webseed-request.go
+++ b/webseed-request.go
@@ -27,11 +27,11 @@ func (me *webseedRequest) Close() {
 }
 
 // Record that it was exceptionally cancelled.
-func (me *webseedRequest) Cancel(reason string) {
-	me.request.Cancel()
+func (me *webseedRequest) Cancel(cause string) {
+	me.request.Cancel(stringError(cause))
 	if !me.cancelled.Swap(true) {
 		if webseed.PrintDebug {
-			me.logger.Debug("cancelled", "reason", reason)
+			me.logger.Debug("cancelled", "cause", cause)
 		}
 	}
 }
diff --git a/webseed/client.go b/webseed/client.go
index 2076e2c5..c15b146e 100644
--- a/webseed/client.go
+++ b/webseed/client.go
@@ -46,14 +46,14 @@ type requestPart struct {
 }
 
 type Request struct {
-	cancel func()
+	cancel context.CancelCauseFunc
 	Body   io.Reader
 	// Closed with error to unstick copy routine when context isn't checked.
 	bodyPipe *io.PipeReader
 }
 
-func (r Request) Cancel() {
-	r.cancel()
+func (r Request) Cancel(cause error) {
+	r.cancel(cause)
 }
 
 func (r Request) Close() {
@@ -106,8 +106,8 @@ func (ws *Client) UrlForFileIndex(fileIndex int) string {
 	return urlForFileIndex(ws.Url, fileIndex, ws.info, ws.PathEscaper)
 }
 
-func (ws *Client) StartNewRequest(r RequestSpec, debugLogger *slog.Logger) Request {
-	ctx, cancel := context.WithCancel(context.TODO())
+func (ws *Client) StartNewRequest(ctx context.Context, r RequestSpec, debugLogger *slog.Logger) Request {
+	ctx, cancel := context.WithCancelCause(ctx)
 	var requestParts []requestPart
 	for i, e := range ws.fileIndex.LocateIter(r) {
 		req, err := newRequest(