Reclaim a connection on reconnect

Don't let the client wait too long when it reconnects because of a
broken connection.

src/core/uplink.c

@@ -562,6 +562,11 @@ static void handle_buffer(struct uplink *uplink) {
 					uplink->auth_status = AUTHENTICATED; // We are authenticated if the server writes to us
 			} else {
 				if (command == 'C' && uplink->auth_status == NOT_STARTED) {
+					// The server is sending a challenge.
+					// We send a „sesssion ID“ ‒ our PID. This way, the server will know if it's the same process reconnecting and drop the old connection sooner.
+					ulog(LLOG_DEBUG, "Sending session ID\n");
+					uint32_t sid = htonl(getpid());
+					uplink_send_message(uplink, 'S', &sid, sizeof sid);
 					ulog(LLOG_DEBUG, "Sending login info\n");
 					// Prepare data
 #define HALF_SIZE 16

src/core/uplink.txt

@@ -124,3 +124,9 @@ which means the login failed.
 
 Any of the above commands from above are forbidden before the
 authentication step is complete.
+
+During the authentication phase, the client may send an `S` message,
+followed by a 4-byte integer. The integer is a session ID which serves
+to distinguish  a reconnect of the same client from the situation when
+two instances on the same client machine fight over the connection.
+Currently, the PID of the client is used.

src/master/client.py

@@ -66,6 +66,7 @@ class ClientConn(twisted.protocols.basic.Int32StringReceiver):
 		self.__plugin_versions = {}
 		self.MAX_LENGTH = 1024 * 1024 * 1024 # A gigabyte should be enough
 		self.last_pong = time.time()
+		self.session_id = None
 
 	def has_plugin(self, plugin_name):
 		return plugin_name in self.__available_plugins
@@ -187,6 +188,12 @@ class ClientConn(twisted.protocols.basic.Int32StringReceiver):
 				else:
 					login_failure('Asked for session before loging in')
 					return
+			elif msg == 'S':
+				if len(params) != 4:
+					logger.warn("Wrong session ID length on client %s: %s", self.cid(), len(params))
+					return
+				(self.session_id,) = struct.unpack("!I", params)
+				logger.debug("Client %s uses session ID %s", self.cid(), self.session_id)
 			return
 		elif msg == 'P': # Ping. Answer pong.
 			self.sendString('p' + params)

src/master/plugin.py

@@ -171,6 +171,8 @@ class Plugins:
 			if self.__clients[client.cid()].last_pong + 900 < time.time():
 				# The client seems connected, but it didn't pong for really long time, kill it
 				logger.warn('Stray connection from %s, dropping old connection', client.cid())
+			if client.session_id is not None and self.__clients[client.cid()].session_id == client.session_id:
+				logger.warn('Taking over previous connection from session %s on client %s', client.session_id, client.cid())
 			else:
 				logger.warn("%s already connected, dropping connection", client.cid())
 				return False