(opt-in) use curl for tracker communication

CMakeLists.txt

@@ -147,6 +147,7 @@ set(libtorrent_aux_include_files
 	bandwidth_socket.hpp
 	bencoder.hpp
 	bind_to_device.hpp
+	bitmask.hpp
 	bloom_filter.hpp
 	bt_peer_connection.hpp
 	buffer.hpp
@@ -156,6 +157,13 @@ set(libtorrent_aux_include_files
 	copy_ptr.hpp
 	cpuid.hpp
 	crc32c.hpp
+	curl.hpp
+	curl_basic_request.hpp
+	curl_boost_socket.hpp
+	curl_pool.hpp
+	curl_request.hpp
+	curl_tracker_manager.hpp
+	curl_tracker_request.hpp
 	deadline_timer.hpp
 	debug.hpp
 	debug_disk_thread.hpp
@@ -192,7 +200,9 @@ set(libtorrent_aux_include_files
 	http_parser.hpp
 	http_stream.hpp
 	http_tracker_connection.hpp
+	http_tracker_request.hpp
 	instantiate_connection.hpp
+	intrusive_list.hpp
 	invariant_check.hpp
 	io.hpp
 	io_bytes.hpp
@@ -203,6 +213,7 @@ set(libtorrent_aux_include_files
 	link.hpp
 	listen_socket_handle.hpp
 	lsd.hpp
+	memory.hpp
 	merkle.hpp
 	merkle_tree.hpp
 	mmap_storage.hpp
@@ -325,6 +336,12 @@ set(sources
 	cpuid.cpp
 	crc32c.cpp
 	create_torrent.cpp
+	curl_basic_request.cpp
+	curl_boost_socket.cpp
+	curl_pool.cpp
+	curl_request.cpp
+	curl_tracker_manager.cpp
+	curl_tracker_request.cpp
 	directory.cpp
 	disabled_disk_io.cpp
 	disk_buffer_holder.cpp
@@ -352,6 +369,7 @@ set(sources
 	http_connection.cpp
 	http_parser.cpp
 	http_tracker_connection.cpp
+	http_tracker_request.cpp
 	i2p_stream.cpp
 	identify_client.cpp
 	instantiate_connection.cpp
@@ -738,6 +756,8 @@ target_optional_compile_definitions(torrent-rasterbar PUBLIC FEATURE NAME i2p DE
 	DESCRIPTION "build with I2P support" DISABLED TORRENT_USE_I2P=0)
 target_optional_compile_definitions(torrent-rasterbar PUBLIC FEATURE NAME webtorrent DEFAULT OFF
 	DESCRIPTION "build with WebTorrent support" DISABLED TORRENT_USE_RTC=0)
+target_optional_compile_definitions(torrent-rasterbar PUBLIC FEATURE NAME curl DEFAULT OFF
+	DESCRIPTION "Enable support for connection reuse and advanced HTTP features towards trackers" ENABLED TORRENT_USE_CURL=1)
 target_optional_compile_definitions(torrent-rasterbar PUBLIC FEATURE NAME logging DEFAULT ON
 	DESCRIPTION "build with logging" DISABLED TORRENT_DISABLE_LOGGING)
 target_optional_compile_definitions(torrent-rasterbar PUBLIC FEATURE NAME mutable-torrents DEFAULT ON
@@ -858,6 +878,21 @@ if (webtorrent)
 	endif()
 endif()
 
+if (curl)
+	find_public_dependency(CURL REQUIRED)
+	set_package_properties(CURL
+		PROPERTIES
+		URL "https://curl.se/"
+		DESCRIPTION "Command line tool and library for transferring data with URLs"
+		TYPE RECOMMENDED
+		PURPOSE "Enable support for connection reuse and advanced HTTP features towards tracker"
+	)
+	target_link_libraries(torrent-rasterbar PUBLIC CURL::libcurl)
+
+	target_optional_compile_definitions(torrent-rasterbar PUBLIC FEATURE NAME curl_debug DEFAULT OFF
+		DESCRIPTION "Enable curl logging connection details to stderr" ENABLED TORRENT_DEBUG_LIBCURL=1)
+endif()
+
 # Boost
 find_public_dependency(Boost REQUIRED)
 target_link_libraries(torrent-rasterbar PUBLIC Boost::headers)

ChangeLog

@@ -1,5 +1,6 @@
 2.1.0 not released
 
+	* (opt-in) use curl for tracker communication
 	* make the save path for part files configurable
 	* retry failed SAM connection (for i2p)
 	* deprecated remap_files(), and prevent it from breaking v2 torrents

Jamfile

@@ -109,6 +109,11 @@ rule linking ( properties * )
 		result += <library>wolfssl ;
 	}
 
+	if <curl>on in $(properties)
+	{
+		result += <library>curl ;
+	}
+
 	if <target-os>windows in $(properties)
 		|| <target-os>cygwin in $(properties)
 	{
@@ -608,6 +613,26 @@ rule wolfssl-include-path ( properties * )
 	return $(result) ;
 }
 
+rule curl-lib-path ( properties * )
+{
+	local CURL_LIB = [ feature.get-values <curl-lib> : $(properties) ] ;
+	if $(CURL_LIB) != ""
+	{
+		return <search>$(CURL_LIB) ;
+	}
+	return ;
+}
+
+rule curl-include-path ( properties * )
+{
+	local CURL_INCLUDE = [ feature.get-values <curl-include> : $(properties) ] ;
+	if $(CURL_INCLUDE) != ""
+	{
+		return <include>$(CURL_INCLUDE) ;
+	}
+	return ;
+}
+
 path-constant blacklist-file : tools/sanitizer-blacklist.txt ;
 
 feature openssl-lib : : free path ;
@@ -642,6 +667,17 @@ feature webtorrent : off on : composite propagated ;
 feature.compose <webtorrent>on : <define>TORRENT_USE_RTC=1 ;
 feature.compose <webtorrent>off : <define>TORRENT_USE_RTC=0 ;
 
+feature curl : off on : composite propagated ;
+feature.compose <curl>on : <define>TORRENT_USE_CURL=1 ;
+feature.compose <curl>off : <define>TORRENT_USE_CURL=0 ;
+
+feature curl-lib : : free path ;
+feature curl-include : : free path ;
+
+feature curl_debug : off on : composite propagated ;
+feature.compose <curl_debug>on : <define>TORRENT_DEBUG_LIBCURL=1 ;
+feature.compose <curl_debug>off : <define>TORRENT_DEBUG_LIBCURL=0 ;
+
 feature asserts : off on production system : composite propagated ;
 feature.compose <asserts>on : <define>TORRENT_USE_ASSERTS=1 ;
 feature.compose <asserts>production : <define>TORRENT_USE_ASSERTS=1 <define>TORRENT_PRODUCTION_ASSERTS=1 ;
@@ -816,6 +852,9 @@ lib gnutls : : <name>gnutls <conditional>@gnutls-lib-path : :
 lib wolfssl : : <name>wolfssl <conditional>@wolfssl-lib-path : :
 	<conditional>@wolfssl-include-path ;
 
+lib curl : : <name>curl <conditional>@curl-lib-path : :
+	<conditional>@curl-include-path ;
+
 lib dbghelp : : <name>dbghelp ;
 
 # required for networking on beos
@@ -853,6 +892,12 @@ SOURCES =
 	cpuid
 	crc32c
 	create_torrent
+	curl_basic_request
+	curl_boost_socket
+	curl_pool
+	curl_request
+	curl_tracker_manager
+	curl_tracker_request
 	directory
 	disk_buffer_holder
 	disk_buffer_pool
@@ -928,6 +973,7 @@ SOURCES =
 	time
 	tracker_manager
 	http_tracker_connection
+	http_tracker_request
 	udp_tracker_connection
 	timestamp_history
 	udp_socket

Makefile

@@ -319,6 +319,12 @@ SOURCES = \
   cpuid.cpp                       \
   crc32c.cpp                      \
   create_torrent.cpp              \
+  curl_basic_request.cpp          \
+  curl_boost_socket.cpp           \
+  curl_pool.cpp                   \
+  curl_request.cpp                \
+  curl_tracker_manager.cpp        \
+  curl_tracker_request.cpp        \
   directory.cpp                   \
   disabled_disk_io.cpp            \
   disk_buffer_holder.cpp          \
@@ -346,6 +352,7 @@ SOURCES = \
   http_connection.cpp             \
   http_parser.cpp                 \
   http_tracker_connection.cpp     \
+  http_tracker_request.cpp        \
   i2p_stream.cpp                  \
   identify_client.cpp             \
   instantiate_connection.cpp      \
@@ -541,6 +548,7 @@ HEADERS = \
   aux_/bandwidth_socket.hpp         \
   aux_/bencoder.hpp                 \
   aux_/bind_to_device.hpp           \
+  aux_/bitmask.hpp                  \
   aux_/buffer.hpp                   \
   aux_/byteswap.hpp                 \
   aux_/bloom_filter.hpp             \
@@ -551,6 +559,13 @@ HEADERS = \
   aux_/copy_ptr.hpp                 \
   aux_/cpuid.hpp                    \
   aux_/crc32c.hpp                   \
+  aux_/curl.hpp                     \
+  aux_/curl_basic_request.hpp       \
+  aux_/curl_boost_socket.hpp        \
+  aux_/curl_pool.hpp                \
+  aux_/curl_request.hpp             \
+  aux_/curl_tracker_manager.hpp     \
+  aux_/curl_tracker_request.hpp     \
   aux_/deadline_timer.hpp           \
   aux_/debug.hpp                    \
   aux_/debug_disk_thread.hpp        \
@@ -589,7 +604,9 @@ HEADERS = \
   aux_/http_parser.hpp              \
   aux_/http_stream.hpp              \
   aux_/http_tracker_connection.hpp  \
+  aux_/http_tracker_request.hpp     \
   aux_/instantiate_connection.hpp   \
+  aux_/intrusive_list.hpp           \
   aux_/invariant_check.hpp          \
   aux_/io.hpp                       \
   aux_/io_bytes.hpp                 \
@@ -600,6 +617,7 @@ HEADERS = \
   aux_/link.hpp                     \
   aux_/listen_socket_handle.hpp     \
   aux_/lsd.hpp                      \
+  aux_/memory.hpp                   \
   aux_/merkle.hpp                   \
   aux_/merkle_tree.hpp              \
   aux_/mmap.hpp                     \

docs/building.rst

@@ -275,6 +275,10 @@ To customize the library path and include path for wolfSSL, set the features
 To disable linking against any SSL library, set the ``crypto`` build feature to
 ``built-in``. This will use an embedded version if SHA-1.
 
+When building with curl enabled, customize the library and include paths by
+setting the features ``curl-lib`` and ``curl-include``, respectively. To enable
+logging of curl's connection details to stderr, set ``curl_debug=on``.
+
 Build features
 ~~~~~~~~~~~~~~
 
@@ -460,6 +464,15 @@ Build features
 |                          | * ``off`` - disable mmap storage, and fall back to |
 |                          |   single-threaded, portable file operations.       |
 +--------------------------+----------------------------------------------------+
+| ``curl``                 | * ``off`` - default. HTTP connections to trackers  |
+|                          |   don't support connection- and SSL session reuse. |
+|                          | * ``on`` - libcurl is used for HTTP connections    |
+|                          |   to trackers, enabling connection- and SSL        |
+|                          |   session reuse. This includes support for the     |
+|                          |   default libcurl HTTP features.                   |
+|                          |   Note that libcurl needs to be build against      |
+|                          |   c-aris for async DNS support.                    |
++--------------------------+----------------------------------------------------+
 
 The ``variant`` feature is *implicit*, which means you don't need to specify
 the name of the feature, just the value.
@@ -599,6 +612,10 @@ Other build options are:
 
 Options are set on the ``cmake`` command line with the ``-D`` option or later on using ``ccmake`` or ``cmake-gui`` applications. ``cmake`` run outputs a summary of all available options and their current values.
 
+To link with a custom build of libcurl (or any other library), first build and install libcurl with
+custom dependencies to a custom prefix path. Then use the option ``CMAKE_PREFIX_PATH=/path/to/custom/dependencies/prefix``
+in libtorrent's cmake configuration.
+
 Step 2: Building libtorrent
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -748,6 +765,10 @@ own code that compiles and links with libtorrent.
 |                                        | peers are connected over authenticated SSL      |
 |                                        | streams.                                        |
 +----------------------------------------+-------------------------------------------------+
+| ``TORRENT_USE_CURL=1``                 | Libcurl is used for HTTP connections to         |
+|                                        | trackers. This will enable connection- and SSL  |
+|                                        | session reuse.                                  |
++----------------------------------------+-------------------------------------------------+
 
 .. _`BEP 38`: https://www.bittorrent.org/beps/bep_0038.html
 

include/libtorrent/aux_/bitmask.hpp

@@ -0,0 +1,43 @@
+/*
+
+Copyright (c) 2026, Arvid Norberg
+Copyright (c) 2026, The Baron Vladimir Harkonnen
+All rights reserved.
+
+You may use, distribute and modify this code under the terms of the BSD license,
+see LICENSE file.
+*/
+
+#ifndef TORRENT_BITMASK_HPP_INCLUDED
+#define TORRENT_BITMASK_HPP_INCLUDED
+
+#include <type_traits>
+
+template<typename E>
+class bitmask {
+	using U = std::underlying_type_t<E>;
+	U bits;
+public:
+	static_assert(std::is_enum_v<E>);
+	using option = E;
+
+	constexpr bitmask(E e = static_cast<E>(0)) noexcept : bits(static_cast<U>(e)) {}
+	constexpr explicit bitmask(U b) noexcept : bits(b) {}
+	[[nodiscard]] constexpr U raw() const noexcept { return bits; }
+
+	constexpr bool operator==(bitmask o) const noexcept { return bits == o.bits; }
+	constexpr bitmask operator|(bitmask o) const noexcept { return bitmask(bits | o.bits); }
+	constexpr bitmask operator&(bitmask o) const noexcept { return bitmask(bits & o.bits); }
+	constexpr bitmask operator^(bitmask o) const noexcept { return bitmask(bits ^ o.bits); }
+	constexpr bitmask operator~() const noexcept { return bitmask(~bits); }
+	constexpr bitmask& operator|=(bitmask o) noexcept { bits |= o.bits; return *this; }
+	constexpr bitmask& operator&=(bitmask o) noexcept { bits &= o.bits; return *this; }
+	constexpr bitmask& operator^=(bitmask o) noexcept { bits ^= o.bits; return *this; }
+
+	constexpr explicit operator bool() const noexcept { return bits != 0; }
+	[[nodiscard]] constexpr bool test(E e)       const noexcept { return (bits & static_cast<U>(e)) != 0; }
+	[[nodiscard]] constexpr bool test(bitmask o) const noexcept { return (bits & o.bits) != 0; }
+	constexpr void unset(E e) noexcept { bits &= ~static_cast<U>(e); }
+};
+
+#endif //TORRENT_BITMASK_HPP_INCLUDED

include/libtorrent/aux_/curl.hpp

@@ -0,0 +1,58 @@
+/*
+
+Copyright (c) 2026, Arvid Norberg
+Copyright (c) 2026, The Baron Vladimir Harkonnen
+All rights reserved.
+
+You may use, distribute and modify this code under the terms of the BSD license,
+see LICENSE file.
+*/
+
+#ifndef TORRENT_CURL_HPP_INCLUDED
+#define TORRENT_CURL_HPP_INCLUDED
+#include "libtorrent/config.hpp"
+
+#if TORRENT_USE_CURL
+#include <stdexcept>
+#include <string>
+#include <curl/curl.h>
+
+namespace libtorrent::aux {
+
+enum class curl_poll_t : int {
+	none   = CURL_POLL_NONE,
+	in     = CURL_POLL_IN,
+	out    = CURL_POLL_OUT,
+	remove = CURL_POLL_REMOVE,
+};
+static_assert(CURL_POLL_INOUT == (CURL_POLL_IN | CURL_POLL_OUT));
+
+enum class curl_cselect_t : int {
+	none = 0, // curl allows the value to be 0, but does not define a type for it
+	in   = CURL_CSELECT_IN,
+	out  = CURL_CSELECT_OUT,
+	err  = CURL_CSELECT_ERR,
+};
+
+class curl_easy_error: public std::runtime_error
+{
+	CURLcode code_;
+
+public:
+	curl_easy_error( CURLcode const ec, std::string const & prefix ):
+		std::runtime_error( prefix + ": " + curl_easy_strerror(ec) ), code_( ec ) {}
+
+	[[nodiscard]] CURLcode code() const noexcept
+	{
+		return code_;
+	}
+};
+
+inline bool curl_version_lower_than(unsigned int version) {
+	const auto ver = curl_version_info(CURLVERSION_NOW);
+	return !ver || ver->version_num < version;
+}
+}
+
+#endif //TORRENT_USE_CURL
+#endif //TORRENT_CURL_HPP_INCLUDED