fix: audio decode against FFmpeg 6.x (channel count for layout stamping)

Frames decoded from PCM WAV carry an unspecified channel layout (RIFF has
no channel mask), so the layout is stamped with the default one for the
frame's channel count before the resampler is built. The count was read
back from the unspecified layout itself, which works on FFmpeg >= 7.0
(AVChannelLayout keeps nb_channels) but not on 6.x, where ffmpeg-next maps
ChannelLayout onto the legacy AVFrame.channel_layout bitmask: "unspecified"
is 0 there and reports 0 channels, the default layout for 0 channels is
empty again, and swr_init rejects it with EINVAL. Opus/Vorbis were
unaffected because their decoders report a native layout.

Take the channel count from the frame (falling back to the decoder), which
is populated on every FFmpeg version.

Verified on Ubuntu 24.04 with FFmpeg 6.1 system libs (the GitHub CI image)
and on FFmpeg 8.0.

Author: shaba

src/audio/mod.rs

@@ -90,9 +90,24 @@ fn decode_via_ffmpeg(data: &[u8]) -> Result<Vec<f32>, AudioError> {
         |decoder: &mut ff::decoder::Audio, pcm: &mut Vec<f32>| -> Result<(), AudioError> {
             let mut frame = ff::frame::Audio::empty();
             while decoder.receive_frame(&mut frame).is_ok() {
+                // swresample needs a concrete channel layout, and PCM in RIFF/WAV
+                // carries no channel mask, so libavcodec hands out frames with an
+                // unspecified layout that has to be stamped with the default one
+                // for the channel count. That count must come from the frame (or
+                // the decoder), never from the unspecified layout itself: on
+                // FFmpeg < 7.0 ffmpeg-next maps ChannelLayout onto the legacy
+                // AVFrame.channel_layout bitmask, where "unspecified" is 0 and so
+                // reports 0 channels; the default layout for 0 is empty again and
+                // swr_init then fails with EINVAL. FFmpeg >= 7.0 keeps the channel
+                // count in the layout even when unspecified, which is why this only
+                // broke against 6.x system libs.
                 if frame.channel_layout().is_empty() {
-                    let n = frame.channel_layout().channels();
-                    frame.set_channel_layout(ff::channel_layout::ChannelLayout::default(n));
+                    let n = match frame.channels() {
+                        0 => decoder.channels(),
+                        n => n,
+                    };
+                    let layout = ff::channel_layout::ChannelLayout::default(i32::from(n));
+                    frame.set_channel_layout(layout);
                 }
                 let resampler = match resampler.as_mut() {
                     Some(r) => r,