Desktop: ask GStreamer for pipewiresink, or Linux plays nothing on Bluetooth

docs/DESKTOP.md

@@ -61,6 +61,27 @@ sudo apt install gstreamer1.0-plugins-good gstreamer1.0-libav          # Debian
 
 Check with `gst-inspect-1.0 autoaudiosink`. If that says "No such element", the app will be silent.
 
+**And `pulsesink` is silent on Bluetooth.** Having the plugins is not the end of it. WebKit asks
+`autoaudiosink` to choose the output element; autoaudiosink prefers `pulsesink`; and pulsesink
+talking to PipeWire's PulseAudio compatibility layer plays nothing at all on a Bluetooth sink. The
+failure is invisible from inside the page — the stream opens, unmuted, at full volume, the
+AudioContext says `running`, `currentTime` advances, and the samples go nowhere. Measured on the
+same page with the same packages, same Bluetooth device:
+
+| sink element | peak |
+|---|---|
+| `autoaudiosink` → `pulsesink` | 0.0000 |
+| `pipewiresink` ranked MAX | 0.3349 |
+
+So `src-tauri/src/lib.rs` sets `GST_PLUGIN_FEATURE_RANK=pipewiresink:MAX` before the webview
+starts, which makes the sink follow the desktop's default output. Set that variable yourself to
+override it. Browsers never use GStreamer, which is why chopdeck.com plays on a machine where the
+desktop app is mute — a difference worth remembering before blaming the machine.
+
+One more, not a bug: WebKit reports the AudioContext state as `interrupted` (not a standard state)
+whenever the window does not have focus, and freezes `currentTime`. An unfocused window is silent
+by design; it resumes on focus.
+
 The builds are not code-signed yet. macOS will refuse to open the app until you right-click it and
 choose Open once; Windows SmartScreen asks for confirmation. Signing (an Apple Developer ID and a
 Windows certificate) is the step to take before listing on itch.io or Steam, and it is only secrets

src-tauri/src/lib.rs

@@ -1,8 +1,46 @@
 // Chop Deck desktop. Everything the machine does lives in the web build under ../dist; this shell only
 // adds what a browser tab cannot: a native save dialog for SAVE mode, opening links in the system
-// browser, and a second window for the owner's manual.
+// browser, a second window for the owner's manual, and on Linux one nudge to GStreamer so the machine
+// is audible.
+
+/// WebKitGTK has no audio stack of its own: Web Audio leaves through GStreamer, and WebKit asks
+/// `autoaudiosink` to choose the output element. autoaudiosink prefers `pulsesink`, and pulsesink
+/// talking to PipeWire's PulseAudio compatibility layer is **silent on Bluetooth outputs** -- the
+/// stream opens, unmuted, at full volume, the AudioContext reports "running", and not one sample
+/// arrives. Measured on one machine, same page, same packages, same Bluetooth sink:
+///
+/// | sink element              | peak   |
+/// |---------------------------|--------|
+/// | autoaudiosink -> pulsesink| 0.0000 |
+/// | pipewiresink ranked MAX   | 0.3349 |
+///
+/// Ranking the native `pipewiresink` top fixes it and costs nothing on wired outputs, where
+/// pulsesink happened to work anyway. Browsers are unaffected because they never use GStreamer,
+/// which is why chopdeck.com plays on a machine where this app did not.
+///
+/// An existing `GST_PLUGIN_FEATURE_RANK` is respected: we append rather than replace, and do
+/// nothing at all if the user has already said something about pipewiresink. This must run before
+/// the webview starts, because GStreamer reads the variable when it builds its registry.
+#[cfg(target_os = "linux")]
+fn prefer_pipewire_audio_sink() {
+    const KEY: &str = "GST_PLUGIN_FEATURE_RANK";
+    let existing = std::env::var(KEY).unwrap_or_default();
+    if existing.contains("pipewiresink") {
+        return;
+    }
+    let next = if existing.is_empty() {
+        "pipewiresink:MAX".to_owned()
+    } else {
+        format!("{existing},pipewiresink:MAX")
+    };
+    std::env::set_var(KEY, next);
+}
+
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
+    #[cfg(target_os = "linux")]
+    prefer_pipewire_audio_sink();
+
     tauri::Builder::default()
         .plugin(tauri_plugin_dialog::init())
         .plugin(tauri_plugin_fs::init())