fix: 4.0.0 regression that broke speed limits for utp peers (#5086)

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 {};
         });