Gstreamer decodebin not linking to the audioconvert - rust

Question

I'm trying to play a sound file with gstreamer in Rust (using the gstreamer crate).
Here's my code:
gst::init().unwrap();
let source = gst::ElementFactory::make("filesrc", Some("source")).expect("Could not create source");
source.set_property_from_str("location", "/home/yuutsuna/Music/m3.mp3");
let decodebin = gst::ElementFactory::make("decodebin", Some("decodebin")).expect("Could not create decodebin element");
let audioconvert = gst::ElementFactory::make("audioconvert", Some("audioconvert")).expect("Could not create audioconvert element");
let sink = gst::ElementFactory::make("pulsesink", None).expect("SINK");
sink.set_property_from_str("device", "alsa_output.pci-0000_02_02.0.analog-stereo");
let pipeline = gst::Pipeline::new(Some("music-pipeline"));
pipeline.add_many(&[&source, &decodebin, &audioconvert, &sink]).unwrap();
gst::Element::link(&source, &decodebin).expect("could not link source and decodebin");
gst::Element::link(&decodebin, &audioconvert).expect("Could not link decodebin and audioconvert");
gst::Element::link(&audioconvert, &sink).expect("Could not link audioconvert and sink");
I get the error Could not link decodebin and audioconvert
However the following command which do the same is working:
gst-launch-1.0 filesrc location="/home/yuutsuna/Music/m3.mp3" ! decodebin ! audioconvert ! pulsesink device=alsa_output.pci-0000_02_02.0.analog-stereo
After looking into the logs and the documentation I found out gstreamer is not able to link the pads between the decodebin and the audioconvert
0:00:00.098766751 13567 0x7fd40c004600 INFO GST_ELEMENT_PADS gstutils.c:1816:gst_element_link_pads_full: trying to link element decodebin:(any) to element audioconvert:(any)
0:00:00.098808081 13567 0x7fd40c004600 INFO GST_PADS gstpad.c:4357:gst_pad_peer_query:<audioconvert:src> pad has no peer
0:00:00.098874314 13567 0x7fd40c004600 INFO structure gststructure.c:2917:gst_structure_get_valist: Expected field 'channel-mask' in structure: audio/x-raw, rate=(int)[ 1, 2147483647 ], channels=(int)[ 1, 2147483647 ];
0:00:00.099085743 13567 0x7fd40c004600 INFO GST_ELEMENT_PADS gstelement.c:1013:gst_element_get_static_pad: no such pad 'src_%u' in element "decodebin"
0:00:00.099126988 13567 0x7fd40c004600 INFO GST_ELEMENT_PADS gstutils.c:1270:gst_element_get_compatible_pad:<decodebin> Could not find a compatible pad to link to audioconvert:sink
The decodebin source pad is a dynamic pad so it's not available right at the creation of the element. I confirmed it by running the request_pad method which returned None. So I tried to delete the line where I linked the elements then added an action on the event pad_added like shown in the documentation in order to directly link the pads. However my lambda is never called. Here's the line I used:
decodebin.connect("pad_added", true, |value| { info!("new pad {:?}", value); None });
I'm not sure if I'm listening to the event correctly. The only examples I found were in C. Either that or there's another issue...
EDIT: After a good sleep I just found an example where instead it uses the connect_pad_added method. I'll test this later but I think it's the solution.

Answer 1

Ok so I replaced this line:
gst::Element::link(&decodebin, &audioconvert).expect("Could not link decodebin and audioconvert");
into:
let audioconvert_weak = audioconvert.downgrade();
decodebin.connect_pad_added(move |_, src_pad| {
println!("new pad {:?}", src_pad);
let sink_pad = match audioconvert_weak.upgrade() {
None => return,
Some(s) => s.static_pad("sink").expect("cannot get sink pad from sink")
};
src_pad.link(&sink_pad).expect("Cannot link the decodebin source pad to the audioconvert sink pad");
});
Instead of linking the 2 elements I set a closure which is called when a pad is added for the decodebin element. The code in the closure will then print a message, get the sink pad from my audioconvert element (that I passed using a weak reference) and finally link the pads.