367 lines
11 KiB
ReStructuredText
367 lines
11 KiB
ReStructuredText
:Author: Arvid Norberg, arvid@rasterbar.com
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libtorrent plugins
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==================
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.. contents::
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libtorrent has a plugin interface for implementing extensions to the protocol.
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These can be general extensions for transferring metadata or peer exchange
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extensions, or it could be used to provide a way to customize the protocol
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to fit a particular (closed) network.
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In short, the plugin interface makes it possible to:
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* register extension messages (sent in the extension handshake), see
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extensions_.
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* add data and parse data from the extension handshake.
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* send extension messages and standard bittorrent messages.
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* override or block the handling of standard bittorrent messages.
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* save and restore state via the session state
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* see all alerts that are posted
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.. _extensions: extension_protocol.html
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a word of caution
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-----------------
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Writing your own plugin is a very easy way to introduce serious bugs such as
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dead locks and race conditions. Since a plugin has access to internal
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structures it is also quite easy to sabotage libtorrent's operation.
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All the callbacks in this interface are called with the main libtorrent thread
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mutex locked. And they are always called from the libtorrent network thread. In
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case portions of your plugin are called from other threads, typically the main
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thread, you cannot use any of the member functions on the internal structures
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in libtorrent, since those require the mutex to be locked. Futhermore, you would
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also need to have a mutex on your own shared data within the plugin, to make
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sure it is not accessed at the same time from the libtorrent thread (through a
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callback). See `boost thread's mutex`_. If you need to send out a message from
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another thread, it is advised to use an internal queue, and do the actual
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sending in ``tick()``.
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Since the plugin interface gives you easy access to internal structures, it
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is not supported as a stable API. Plugins should be considered spcific to a
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specific version of libtorrent. Although, in practice the internals mostly
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don't change that dramatically.
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.. _`boost thread's mutex`: http://www.boost.org/doc/html/mutex.html
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plugin interface
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================
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The plugin interface consists of three base classes that the plugin may
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implement. These are called ``plugin``, ``torrent_plugin`` and ``peer_plugin``.
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They are found in the ``<libtorrent/extensions.hpp>`` header.
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These plugins are instantiated for each session, torrent and possibly each peer,
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respectively.
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For plugins that only need per torrent state, it is enough to only implement
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``torrent_plugin`` and pass a constructor function or function object to
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``session::add_extension()`` or ``torrent_handle::add_extension()`` (if the
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torrent has already been started and you want to hook in the extension at
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run-time).
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The signature of the function is::
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boost::shared_ptr<torrent_plugin> (*)(torrent*, void*);
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The first argument is the internal torrent object, the second argument
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is the userdata passed to ``session::add_torrent()`` or
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``torrent_handle::add_extension()``.
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The function should return a ``boost::shared_ptr<torrent_plugin>`` which
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may or may not be 0. If it is a null pointer, the extension is simply ignored
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for this torrent. If it is a valid pointer (to a class inheriting
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``torrent_plugin``), it will be associated with this torrent and callbacks
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will be made on torrent events.
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For more elaborate plugins which require session wide state, you would
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implement ``plugin``, construct an object (in a ``boost::shared_ptr``) and pass
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it in to ``session::add_extension()``.
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plugin
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======
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::
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struct plugin
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{
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virtual ~plugin();
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virtual boost::shared_ptr<torrent_plugin> new_torrent(torrent* t, void* user);
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virtual void added(boost::weak_ptr<aux::session_impl> s);
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virtual void on_alert(alert const* a);
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virtual void on_tick();
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virtual void save_state(entry& ent) const;
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virtual void load_state(lazy_entry const& ent);
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};
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torrent_plugin
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==============
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The synopsis for ``torrent_plugin`` follows::
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struct torrent_plugin
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{
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virtual ~torrent_plugin();
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virtual boost::shared_ptr<peer_plugin> new_connection(peer_connection*);
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virtual void on_piece_pass(int index);
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virtual void on_piece_failed(int index);
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virtual void tick();
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virtual bool on_pause();
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virtual bool on_resume();
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virtual void on_files_checked();
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virtual void on_state(int s);
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enum flags_t {
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first_time = 1,
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filtered = 2
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};
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virtual void on_add_peer(tcp::endpoint const& ip
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, int src, int flags);
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};
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This is the base class for a torrent_plugin. Your derived class is (if added
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as an extension) instantiated for each torrent in the session. The callback
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hook functions are defined as follows.
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new_connection()
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----------------
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::
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boost::shared_ptr<peer_plugin> new_connection(peer_connection*);
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This function is called each time a new peer is connected to the torrent. You
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may choose to ignore this by just returning a default constructed
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``shared_ptr`` (in which case you don't need to override this member
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function).
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If you need an extension to the peer connection (which most plugins do) you
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are supposed to return an instance of your ``peer_plugin`` class. Which in
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turn will have its hook functions called on event specific to that peer.
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The ``peer_connection`` will be valid as long as the ``shared_ptr`` is being
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held by the torrent object. So, it is generally a good idea to not keep a
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``shared_ptr`` to your own peer_plugin. If you want to keep references to it,
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use ``weak_ptr``.
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If this function throws an exception, the connection will be closed.
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on_piece_pass() on_piece_fail()
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-------------------------------
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::
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void on_piece_pass(int index);
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void on_piece_failed(int index);
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These hooks are called when a piece passes the hash check or fails the hash
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check, respectively. The ``index`` is the piece index that was downloaded.
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It is possible to access the list of peers that participated in sending the
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piece through the ``torrent`` and the ``piece_picker``.
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tick()
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------
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::
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void tick();
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This hook is called approximately once per second. It is a way of making it
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easy for plugins to do timed events, for sending messages or whatever.
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on_pause() on_resume()
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----------------------
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::
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bool on_pause();
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bool on_resume();
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These hooks are called when the torrent is paused and unpaused respectively.
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The return value indicates if the event was handled. A return value of
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``true`` indicates that it was handled, and no other plugin after this one
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will have this hook function called, and the standard handler will also not be
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invoked. So, returning true effectively overrides the standard behavior of
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pause or unpause.
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Note that if you call ``pause()`` or ``resume()`` on the torrent from your
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handler it will recurse back into your handler, so in order to invoke the
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standard handler, you have to keep your own state on whether you want standard
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behavior or overridden behavior.
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on_files_checked()
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------------------
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::
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void on_files_checked();
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This function is called when the initial files of the torrent have been
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checked. If there are no files to check, this function is called immediately.
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i.e. This function is always called when the torrent is in a state where it
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can start downloading.
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on_add_peer()
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-------------
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::
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enum flags_t {
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first_time = 1,
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filtered = 2
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};
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virtual void on_add_peer(tcp::endpoint const& ip
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, int src, int flags);
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This function is called whenever we hear about a peer from any peer source,
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such as the tracker, PEX, DHT or Local peer discovery.
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``src`` is a bitmask of ``peer_info::peer_source_flags``::
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enum peer_source_flags
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{
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tracker = 0x1,
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dht = 0x2,
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pex = 0x4,
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lsd = 0x8,
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resume_data = 0x10,
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incoming = 0x20
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};
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``flags`` is a bitmask of::
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enum flags_t {
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first_time = 1,
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filtered = 2
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};
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If the ``filtered`` flag is set, it means the peer wasn't added to the
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peer list because of and IP filter, port filter, reserved ports filter.
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peer_plugin
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===========
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::
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struct peer_plugin
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{
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virtual ~peer_plugin();
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virtual void add_handshake(entry&);
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virtual bool on_handshake(char const* reserved_bits);
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virtual bool on_extension_handshake(lazy_entry const& h);
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virtual bool on_choke();
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virtual bool on_unchoke();
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virtual bool on_interested();
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virtual bool on_not_interested();
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virtual bool on_have(int index);
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virtual bool on_bitfield(bitfield const& bits);
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virtual bool on_have_all();
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virtual bool on_have_none();
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virtual bool on_allowed_fast(int index);
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virtual bool on_request(peer_request const& req);
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virtual bool on_piece(peer_request const& piece, disk_buffer_holder& buffer);
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virtual bool on_cancel(peer_request const& req);
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virtual bool on_reject(peer_request const& req);
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virtual bool on_suggest(int index);
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virtual bool on_extended(int length
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, int msg, buffer::const_interval body);
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virtual bool on_unknown_message(int length, int msg
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, buffer::const_interval body);
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virtual void on_piece_pass(int index);
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virtual void on_piece_failed(int index);
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virtual void tick();
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virtual bool write_request(peer_request const& r);
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};
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disk_buffer_holder
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==================
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::
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struct disk_buffer_holder
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{
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disk_buffer_holder(aux::session_impl& s, char* b);
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~disk_buffer_holder();
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char* release();
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char* buffer();
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};
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The disk buffer holder acts like a ``scoped_ptr`` that frees a disk buffer
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when it's destructed, unless it's released. ``release`` returns the disk
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buffer and transferres ownership and responsibility to free it to the caller.
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A disk buffer is freed by passing it to ``session_impl::free_disk_buffer()``.
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``buffer()`` returns the pointer without transferring responsibility. If
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this buffer has been released, ``buffer()`` will return 0.
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custom alerts
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=============
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Since plugins are running within internal libtorrent threads, one convenient
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way to communicate with the client is to post custom alerts.
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The expected interface of any alert, apart from deriving from the ``alert``
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base class, looks like this:
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.. parsed-literal::
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const static int alert_type = *<unique alert ID>*;
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virtual int type() const { return alert_type; }
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virtual std::string message() const;
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virtual std::auto_ptr<alert> clone() const
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{ return std::auto_ptr<alert>(new name(\*this)); }
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const static int static_category = *<bitmask of alert::category_t flags>*;
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virtual int category() const { return static_category; }
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virtual char const* what() const { return *<string literal of the name of this alert>*; }
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The ``alert_type`` is used for the type-checking in ``alert_cast``. It must not collide with
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any other alert. The built-in alerts in libtorrent will not use alert type IDs greater than
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``user_alert_id``. When defining your own alert, make sure it's greater than this constant.
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``type()`` is the run-time equivalence of the ``alert_type``.
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The ``message()`` virtual function is expected to construct a useful string representation
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of the alert and the event or data it represents. Something convenient to put in a log file
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for instance.
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``clone()`` is used internally to copy alerts. The suggested implementation of simply
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allocating a new instance as a copy of ``*this`` is all that's expected.
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The static category is required for checking wether or not the category for a specific alert
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is enabled or not, without instantiating the alert. The ``category`` virtual function is
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the run-time equivalence.
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The ``what()`` virtual function may simply be a string literal of the class name of
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your alert.
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For more information, see the `alert section`_.
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.. _`alert section`: reference-Alerts.html
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