xmms2 0.9.2 is out

February 11, 2023

Tl;DR: xmms2-0.9.2 is out and you can get it at https://github.com/xmms2/xmms2-devel/releases/tag/0.9.2!

xmms2 is still a music player daemon with various plugins to support stream decoding and transformation. See previous announcement on how to get started with xmms2.

Highlights

The guest star of this release is libvisual plugin!

xmms2 libvisual demo

Apart from that FLAC play back hangup fix went into the release. The rest of changes are cosmetic.

libvisual rabbit hole

In xmms2-0.9.1 libvisual plug-in did not work for me. I did not pay attention to that until Sebastian sent the PR #15 that fixes render stutter when player is on pause. I was not able to verify the fix and accepted the PR as is. The change looked reasonable. A while after Sebastian helped me to debug problems on my system and rule out the video subsystem.

Quiz question: why do you think libvisual did not work for me? Was it a misconfiguration on my side? A bug in xmms2 or somewhere else in its dependencies?

libvisual is a library to visualize your music! If you remember the times of Media Player in Windows 98 it’s that kind of visualization.

There are various plugins that render different videos. An example may look this way: https://www.youtube.com/watch?v=cAM-lhiSYcY.

xmms2 provides xmms2-libvisual client application that you can run and see the visualization. In theory you can just run it and get the result:

$ xmms2-libvisual
Controls: Arrow keys switch between plugins, TAB toggles fullscreen, ESC quits.
          Each plugin can has its own mouse/key bindings, too.
Note: you can give your favourite libvisual plugin as command line argument.
<hung>

But in my case nothing happened: the client app just hung.

By running strace I noticed that xmms2-libvisual used UDP protocol to connect to server. That was unusual. I expected xmms2 to use shared memory when ran on a local machine.

This was an xmmsclient bug: as some point build system stopped enabling semtimedop() presence and always fell back to UDP. Restoring semtimedop() was easy: we needed to set HAVE_SEMTIMEDOP define in visualization client:

--- a/src/clients/lib/xmmsclient/wscript
+++ b/src/clients/lib/xmmsclient/wscript
@@ -37,7 +37,10 @@ def build(bld):
         uselib = 'socket time',
         use = 'xmmsipc xmmssocket xmmsutils xmmstypes xmmsvisualization',
         vnum = '6.0.0',
-        defines = 'XMMSC_LOG_DOMAIN="xmmsclient"'
+        defines = [
+            'XMMSC_LOG_DOMAIN="xmmsclient"',
+            "HAVE_SEMTIMEDOP=%d" % int(bld.env.have_semtimedop),
+        ]
         )

After the fix the error changed:

$ xmms2-libvisual
Controls: Arrow keys switch between plugins, TAB toggles fullscreen, ESC quits.
          Each plugin can has its own mouse/key bindings, too.
Note: you can give your favourite libvisual plugin as command line argument.
Available plugins:
Error: Actor plugin not found!

This happens because libvisual itself does not render images without plugins. And it does not provide any default plugins. I needed to install libvisual-plugins to get something to render!

Due to some specifics of nixpkgs there is no easy way to share plugin load path by both libvisual and libvisual-plugins. To work it around I added a --libvisual-plugins flag to be able to load it from arbitrary location.

After that I got empty window. Sebastian helped me with that by configuring VIS output:

$ xmms2 server config effect.order.0 = visualization

And I got the video back:

xmms2 libvisual demo

Yay!

Later I returned to UDP protocol. It ought to work! I added a bit of logging to see if client gets anything from the server:

--- a/src/clients/lib/xmmsclient/visualization/udp.c
+++ b/src/clients/lib/xmmsclient/visualization/udp.c
@@ -21,7 +21,8 @@ udp_timediff (int32_t id, int socket) {
 	for (i = 0; i < 10; ++i) {
 		send (socket, packet, packet_d.size, 0);
 	}
-	printf ("Syncing ");
+	printf ("Syncing UDP time ");
+	fflush (stdout);
 	do {
 		if ((recv (socket, packet, packet_d.size, 0) == packet_d.size) && (*packet_d.__unaligned_type == 'T')) {
 			struct timeval rtv;
@@ -45,6 +46,7 @@ udp_timediff (int32_t id, int socket) {
 			       net2ts (packet_d.serverstamp), net2ts (packet_d.clientstamp), tv2ts (&time));
 			 end of debug */
 			putchar('.');
+			fflush (stdout);
 		}
 	} while (diffc < 10);
 	free (packet);

fflush () confirmed that server did not answer to client’s requests:

$ xmms2-libvisual
...
Syncing UDP time <cursror here>

This hangup happens due to an interesting case of handling dual-stack IPv4+IPv6 setup this machine has:

As a result IPv6 client was failing to connect IPv4 server on the same machine.

The fix is straightforward: server should bind not to the first getaddrinfo() address, but all the getaddrinfo() addresses. The only caveat is that binding on IPv6+IPv4 fails if IPv4 was already bound. Setting IPV6_V6ONLY allowed disabling dual-stack default on bound socket.

The change is a bit long as it added support for multiple server sockets in the event loop. Its gist is around:

--- a/src/xmms/visualization/udp.c
+++ b/src/xmms/visualization/udp.c
@@ -122,41 +119,76 @@ init_udp (xmms_visualization_t *vis, int32_t id, xmms_error_t *err)
 		hints.ai_flags = AI_PASSIVE;
 		hints.ai_protocol = 0;

 		if ((s = getaddrinfo (NULL, G_STRINGIFY (XMMS_DEFAULT_UDP_PORT), &hints, &result)) != 0)
 		// ...

+		for (rp = result; rp != NULL; rp = rp->ai_next) {
+			int sock;
+			xmms_vis_server_t *s = &servers[opened_servers];
+
+			sock = socket (rp->ai_family, rp->ai_socktype, rp->ai_protocol);
+			if (!xmms_socket_valid (sock)) {
 				continue;
 			}
-			if (bind (vis->socket, rp->ai_addr, rp->ai_addrlen) != -1) {
-				break;
-			} else {
-				close (vis->socket);
+			if (bind (sock, rp->ai_addr, rp->ai_addrlen) == -1) {
+				/* In case we already bound v4 socket
+				 * and v6 are attempting to set up
+				 * dual-stack v4+v6. Try again v6-only
+				 * mode. */
+				if (rp->ai_family == AF_INET6 && errno == EADDRINUSE) {
+					int v6only = 1;
+					if (setsockopt (sock, IPPROTO_IPV6, IPV6_V6ONLY, &v6only, sizeof (v6only)) == -1) {
+						close (socket);
+						continue;
+					}
+					if (bind (sock, rp->ai_addr, rp->ai_addrlen) == -1) {
+						close (socket);
+						continue;
+					}

After the change I got working visualization over UDP as well.

libvisual UDP protocol

While debugging stuck visualization stream I got the following picture of how visual UDP clients are expected to interact with xmms2 server on the wire level:

  1. First xmms2-libvisual connects over TCP (or UNIX socket) to xmms2 server using standard XMMS_PATH environment variable. Let’s call it “control channel” socket.
  2. Then over “control channel” client sends a visualization.register() RPC and gets the numeric ID back. At this point client does not know if it’s an SHM or UDP transport.
  3. Then client tries to open an SHM object with this numeric ID by sending visualization.init_shm(id) RPC. If it fails client falls back to try UDP instead.
  4. The client sends visualization.init_udp(id) RPC.
  5. As a response to client RPC server opens UDP socket on a fixed 9667 port and waits for incoming messages.

Now client and server are ready to negotiate VIS-specific UDP channel. There are a few types of messages that pass around:

Timestamps are 8 bytes long and consist of a par of 2 values: 4 bytes for seconds and 4 bytes for nanoseconds. Each data sample is a 16-bit value. Usually of PCM format.

UDP side of the protocol looks like that:

It’s not a very robust or secure protocol. But it’s so simple. It allows for multiple parallel visualization clients to co-exist.

Parting words

xmms2 is back with libvisual support! VIS protocol is not very complicated.

To answer the quiz question above the problem was in all the listed cases:

After fixing all the above I hope more people will be able to play with libvisual.

Have fun!