From 65d6e83b1a4dccdf3cbf42113f63f89a50003730 Mon Sep 17 00:00:00 2001
From: arvidn <arvid@libtorrent.org>
Date: Tue, 3 Mar 2020 13:54:45 +0100
Subject: [PATCH] add support for TCP_NOTSENT_LOWAT

---
 ChangeLog                                     |  1 +
 docs/hunspell/libtorrent.dic                  |  2 ++
 include/libtorrent/aux_/set_socket_buffer.hpp |  8 ++++++++
 include/libtorrent/settings_pack.hpp          |  5 +++++
 include/libtorrent/socket.hpp                 | 16 ++++++++++++++++
 src/session.cpp                               |  3 +--
 src/settings_pack.cpp                         |  1 +
 7 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index faf65d8e4..f51aeab74 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,4 @@
+	* support TCP_NOTSENT_LOWAT on Linux
 	* fix correct interface binding of local service discovery multicast
 	* fix issue with knowing which interfaces to announce to trackers and DHT
 	* undeprecate settings_pack::dht_upload_rate_limit
diff --git a/docs/hunspell/libtorrent.dic b/docs/hunspell/libtorrent.dic
index 06403eb9d..f4c316f02 100644
--- a/docs/hunspell/libtorrent.dic
+++ b/docs/hunspell/libtorrent.dic
@@ -557,3 +557,5 @@ fe80
 vcpkg
 leecher
 6881l
+NOTSENT
+LOWAT
diff --git a/include/libtorrent/aux_/set_socket_buffer.hpp b/include/libtorrent/aux_/set_socket_buffer.hpp
index e5cc71a07..06ac03514 100644
--- a/include/libtorrent/aux_/set_socket_buffer.hpp
+++ b/include/libtorrent/aux_/set_socket_buffer.hpp
@@ -42,6 +42,14 @@ namespace aux {
 	template <class Socket>
 	void set_socket_buffer_size(Socket& s, session_settings const& sett, error_code& ec)
 	{
+#ifdef TCP_NOTSENT_LOWAT
+		int const not_sent_low_watermark = sett.get_int(settings_pack::send_not_sent_low_watermark);
+		if (not_sent_low_watermark)
+		{
+			error_code ignore;
+			s.set_option(tcp_notsent_lowat(not_sent_low_watermark), ignore);
+		}
+#endif
 		int const snd_size = sett.get_int(settings_pack::send_socket_buffer_size);
 		if (snd_size)
 		{
diff --git a/include/libtorrent/settings_pack.hpp b/include/libtorrent/settings_pack.hpp
index e36d87cba..81fb5f04e 100644
--- a/include/libtorrent/settings_pack.hpp
+++ b/include/libtorrent/settings_pack.hpp
@@ -1729,6 +1729,11 @@ namespace aux {
 			// as zero.
 			resolver_cache_timeout,
 
+			// specify the not-sent low watermark for socket send buffers. This
+			// corresponds to the, Linux-specific, ``TCP_NOTSENT_LOWAT`` TCP socket
+			// option.
+			send_not_sent_low_watermark,
+
 			max_int_setting_internal
 		};
 
diff --git a/include/libtorrent/socket.hpp b/include/libtorrent/socket.hpp
index ffe17b23d..8254a61ab 100644
--- a/include/libtorrent/socket.hpp
+++ b/include/libtorrent/socket.hpp
@@ -246,6 +246,22 @@ namespace libtorrent {
 		int m_value;
 	};
 #endif // TORRENT_USE_NETLINK
+
+#ifdef TCP_NOTSENT_LOWAT
+	struct tcp_notsent_lowat
+	{
+		explicit tcp_notsent_lowat(int val) : m_value(val) {}
+		template<class Protocol>
+		int level(Protocol const&) const { return IPPROTO_TCP; }
+		template<class Protocol>
+		int name(Protocol const&) const { return TCP_NOTSENT_LOWAT; }
+		template<class Protocol>
+		int const* data(Protocol const&) const { return &m_value; }
+		template<class Protocol>
+		std::size_t size(Protocol const&) const { return sizeof(m_value); }
+		int m_value;
+	};
+#endif
 }
 
 #endif // TORRENT_SOCKET_HPP_INCLUDED
diff --git a/src/session.cpp b/src/session.cpp
index e3fb1bc11..eb08d5ce0 100644
--- a/src/session.cpp
+++ b/src/session.cpp
@@ -236,8 +236,7 @@ namespace {
 
 		set.set_int(settings_pack::max_rejects, 10);
 
-		set.set_int(settings_pack::recv_socket_buffer_size, 1024 * 1024);
-		set.set_int(settings_pack::send_socket_buffer_size, 1024 * 1024);
+		set.set_int(settings_pack::send_not_sent_low_watermark, 524288);
 
 		// don't let connections linger for too long
 		set.set_int(settings_pack::request_timeout, 10);
diff --git a/src/settings_pack.cpp b/src/settings_pack.cpp
index ee1f5932d..6dfcec422 100644
--- a/src/settings_pack.cpp
+++ b/src/settings_pack.cpp
@@ -348,6 +348,7 @@ constexpr int CLOSE_FILE_INTERVAL = 0;
 		SET(utp_cwnd_reduce_timer, 100, nullptr),
 		SET(max_web_seed_connections, 3, nullptr),
 		SET(resolver_cache_timeout, 1200, &session_impl::update_resolver_cache_timeout),
+		SET(send_not_sent_low_watermark, 16384, nullptr),
 	}});
 
 #undef SET