From 6132e74a36792acc564a27a5267ef327b039f508 Mon Sep 17 00:00:00 2001
From: Charles Kerr <charles@charleskerr.com>
Date: Mon, 27 Feb 2023 14:32:40 -0600
Subject: [PATCH] fix: 4.0.0 regression that broke speed limits for utp peers

Fixes #4701.
---
 libtransmission/peer-io.cc | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/libtransmission/peer-io.cc b/libtransmission/peer-io.cc
index 9e631de80e4..219f8af1beb 100644
--- a/libtransmission/peer-io.cc
+++ b/libtransmission/peer-io.cc
@@ -733,7 +733,16 @@ void tr_peerIo::utp_init([[maybe_unused]] struct_utp_context* ctx)
         {
             if (auto const* const io = static_cast<tr_peerIo*>(utp_get_userdata(args->socket)); io != nullptr)
             {
-                return std::size(io->inbuf_);
+                // We use this callback to enforce speed limits by telling
+                // libutp to read no more than `target_dl_bytes` bytes.
+                auto const target_dl_bytes = io->bandwidth_.clamp(TR_DOWN, RcvBuf);
+
+                // libutp's private function get_rcv_window() allows libutp
+                // to read up to (UTP_RCVBUF - READ_BUFFER_SIZE) bytes and
+                // UTP_RCVBUF is set to `RcvBuf` by tr_peerIo::utp_init().
+                // So to limit dl to `target_dl_bytes`, we need to return
+                // N where (`target_dl_bytes` == RcvBuf - N).
+                return RcvBuf - target_dl_bytes;
             }
             return {};
         });