restore find_plugin API (#2026)

include/libtorrent/extensions.hpp

@@ -370,6 +370,10 @@ namespace libtorrent {
 		// hidden
 		virtual ~peer_plugin() {}
 
+		// This function is expected to return the name of
+		// the plugin.
+		virtual string_view type() const { return {}; }
+
 		// can add entries to the extension handshake
 		// this is not called for web seeds
 		virtual void add_handshake(entry&) {}

include/libtorrent/peer_connection.hpp

@@ -303,6 +303,7 @@ namespace aux {
 
 #ifndef TORRENT_DISABLE_EXTENSIONS
 		void add_extension(std::shared_ptr<peer_plugin>);
+		peer_plugin const* find_plugin(string_view type);
 #endif
 
 		// this function is called once the torrent associated

include/libtorrent/peer_connection_handle.hpp

@@ -57,6 +57,7 @@ struct TORRENT_EXPORT peer_connection_handle
 	connection_type type() const;
 
 	void add_extension(std::shared_ptr<peer_plugin>);
+	peer_plugin const* find_plugin(string_view type);
 
 	bool is_seed() const;
 

src/peer_connection.cpp

@@ -518,6 +518,14 @@ namespace libtorrent {
 		TORRENT_ASSERT(is_single_thread());
 		m_extensions.push_back(ext);
 	}
+
+	peer_plugin const* peer_connection::find_plugin(string_view type)
+	{
+		TORRENT_ASSERT(is_single_thread());
+		auto p = std::find_if(m_extensions.begin(), m_extensions.end()
+			, [&](std::shared_ptr<peer_plugin> const& e) { return e->type() == type; });
+		return p != m_extensions.end() ? p->get() : nullptr;
+	}
 #endif
 
 	void peer_connection::send_allowed_set()

src/peer_connection_handle.cpp

@@ -57,6 +57,18 @@ void peer_connection_handle::add_extension(std::shared_ptr<peer_plugin> ext)
 #endif
 }
 
+peer_plugin const* peer_connection_handle::find_plugin(string_view type)
+{
+#ifndef TORRENT_DISABLE_EXTENSIONS
+	std::shared_ptr<peer_connection> pc = native_handle();
+	TORRENT_ASSERT(pc);
+	return pc->find_plugin(type);
+#else
+	TORRENT_UNUSED(type);
+	return nullptr;
+#endif
+}
+
 bool peer_connection_handle::is_seed() const
 {
 	std::shared_ptr<peer_connection> pc = native_handle();