From 70ce33cab3caf8a53688c7b31b646393de567b70 Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Mon, 15 Jan 2024 12:43:55 +1100
Subject: [PATCH] Expect cancel acknowledgement from Transmission v4 clients

---
 peer.go     |  1 +
 peerconn.go | 20 +++++++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/peer.go b/peer.go
index 7d524b7f..c4b4fb9e 100644
--- a/peer.go
+++ b/peer.go
@@ -465,6 +465,7 @@ func (me *Peer) cancel(r RequestIndex) {
 		panic("request not existing should have been guarded")
 	}
 	if me._cancel(r) {
+		// Record that we expect to get a cancel ack.
 		if !me.requestState.Cancelled.CheckedAdd(r) {
 			panic("request already cancelled")
 		}
diff --git a/peerconn.go b/peerconn.go
index e2d944ff..7a1639f2 100644
--- a/peerconn.go
+++ b/peerconn.go
@@ -297,9 +297,23 @@ func (me *PeerConn) _request(r Request) bool {
 
 func (me *PeerConn) _cancel(r RequestIndex) bool {
 	me.write(makeCancelMessage(me.t.requestIndexToRequest(r)))
-	// Transmission does not send rejects for received cancels. See
-	// https://github.com/transmission/transmission/pull/2275.
-	return me.fastEnabled() && !me.remoteIsTransmission()
+	return me.remoteRejectsCancels()
+}
+
+// Whether we should expect a reject message after sending a cancel.
+func (me *PeerConn) remoteRejectsCancels() bool {
+	if !me.fastEnabled() {
+		return false
+	}
+	if me.remoteIsTransmission() {
+		// Transmission did not send rejects for received cancels. See
+		// https://github.com/transmission/transmission/pull/2275. Fixed in 4.0.0-beta.1 onward in
+		// https://github.com/transmission/transmission/commit/76719bf34c255da4fca991c2ad3fa4b65d2154b1.
+		// Peer ID prefix scheme described
+		// https://github.com/transmission/transmission/blob/7ec7607bbcf0fa99bd4b157b9b0f0c411d59f45d/CMakeLists.txt#L128-L149.
+		return me.PeerID[3] >= '4'
+	}
+	return true
 }
 
 func (cn *PeerConn) fillWriteBuffer() {
-- 
2.48.1