Poll appsink instead of relying on the new-sample signal

The last round of testing ruled out the pipeline construction itself:
even with the sink bin rebuilt from explicit, individually-checked
link() calls (no thrown errors, playbin reaches PLAYING), the
'new-sample' signal still never fired inside gnome-shell — while an
equivalent pipeline worked fine standalone via gst-launch-1.0.

The remaining, gnome-shell-specific difference: appsink emits
'new-sample' from GStreamer's own streaming thread, not the main
thread. GJS's JS engine isn't safe to call into from an arbitrary
background thread, and a cross-thread signal emission can be silently
dropped rather than invoked or crashed on — which looks exactly like
"the signal never fires" from here, even though the pipeline is
actually running.

Replaces the signal entirely with polling: a ~30fps GLib.timeout_add()
on the main thread calls appsink.try_pull_sample(0), which is
explicitly documented as safe to call from any thread since we are now
the ones calling into GStreamer rather than the reverse. Also updates
the no-frames watchdog to report the poll count instead of a signal
fire count.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019RDqbdjsiisSU7CbQke4g8

lib/liveWallpaper.js

@@ -50,7 +50,8 @@ export class LiveWallpaperManager {
         this._upowerSignalId = 0;
 
         this._frameCount = 0;
-        this._newSampleCallCount = 0;
+        this._pollCount = 0;
+        this._pollTimeoutId = 0;
         this._noFrameWatchdogId = 0;
     }
 
@@ -154,7 +155,11 @@ export class LiveWallpaperManager {
         }
 
         capsfilter.set_property('caps', Gst.Caps.from_string('video/x-raw,format=RGBA'));
-        appsink.set_property('emit-signals', true);
+        // emit-signals is deliberately left off: appsink's 'new-sample' fires
+        // from GStreamer's streaming thread, not gnome-shell's main thread,
+        // and a cross-thread call into the JS engine can be silently dropped
+        // rather than invoked. We poll with try_pull_sample() from a main
+        // thread GLib timer instead, which is safe to call from any thread.
         appsink.set_property('max-buffers', 2);
         appsink.set_property('drop', true);
         appsink.set_property('sync', true);
@@ -199,7 +204,6 @@ export class LiveWallpaperManager {
             this._layoutActor();
 
             const sinkBin = this._buildSinkBin();
-            this._appsink.connect('new-sample', sink => this._onNewSample(sink));
 
             this._playbin = Gst.ElementFactory.make('playbin', 'benthicbloom-live-wallpaper');
             this._playbin.set_property('video-sink', sinkBin);
@@ -214,16 +218,20 @@ export class LiveWallpaperManager {
             this._logger.debug(`playbin.set_state(PLAYING) returned ${stateChangeResult}`);
 
             this._frameCount = 0;
-            this._newSampleCallCount = 0;
+            this._pollCount = 0;
+            // ~30fps polling of the appsink from the main thread. See the
+            // comment on appsink's properties above for why this replaces
+            // a 'new-sample' signal handler.
+            this._pollTimeoutId = GLib.timeout_add(GLib.PRIORITY_DEFAULT, 33, () => this._pollForSample());
+
             this._noFrameWatchdogId = GLib.timeout_add_seconds(GLib.PRIORITY_DEFAULT, 4, () => {
                 this._noFrameWatchdogId = 0;
                 if (this._active && this._frameCount === 0) {
                     this._logger.warn(
                         `Live wallpaper: no frames received 4s after starting playback ` +
-                        `(new-sample signal fired ${this._newSampleCallCount} times). ` +
+                        `(polled appsink ${this._pollCount} times). If that count is 0, the poll timer ` +
-                        'If that count is 0, the signal itself never fired (check for a stalled ' +
+                        'itself never ran; if it is nonzero, try_pull_sample() keeps returning nothing ' +
-                        'pipeline above); if it is nonzero, pull_sample()/buffer processing is failing ' +
+                        '— check for a "Failed to poll live wallpaper frame" error above.');
-                        '— check for a "Failed to process live wallpaper frame" error above.');
                 }
                 return GLib.SOURCE_REMOVE;
             });
@@ -238,18 +246,19 @@ export class LiveWallpaperManager {
         }
     }
 
-    _onNewSample(sink) {
+    _pollForSample() {
+        if (!this._appsink)
+            return GLib.SOURCE_REMOVE;
+
         const Gst = this._Gst;
-        this._newSampleCallCount = (this._newSampleCallCount ?? 0) + 1;
+        this._pollCount++;
-        if (this._newSampleCallCount === 1)
+        if (this._pollCount === 1)
-            this._logger.debug('Live wallpaper: new-sample signal fired for the first time');
+            this._logger.debug('Live wallpaper: appsink polling started');
 
         try {
-            const sample = sink.pull_sample();
+            const sample = this._appsink.try_pull_sample(0);
-            if (!sample) {
+            if (!sample)
-                this._logger.debug('Live wallpaper: pull_sample() returned no sample');
+                return GLib.SOURCE_CONTINUE;
-                return Gst.FlowReturn.OK;
-            }
 
             const buffer = sample.get_buffer();
             const structure = sample.get_caps().get_structure(0);
@@ -259,7 +268,7 @@ export class LiveWallpaperManager {
             const [ok, mapInfo] = buffer.map(Gst.MapFlags.READ);
             if (!ok) {
                 this._logger.debug('Live wallpaper: buffer.map() failed');
-                return Gst.FlowReturn.OK;
+                return GLib.SOURCE_CONTINUE;
             }
 
             try {
@@ -273,10 +282,10 @@ export class LiveWallpaperManager {
                 buffer.unmap(mapInfo);
             }
         } catch (e) {
-            this._logger.error(e, 'Failed to process live wallpaper frame (new-sample handler)');
+            this._logger.error(e, 'Failed to poll live wallpaper frame');
         }
 
-        return Gst.FlowReturn.OK;
+        return GLib.SOURCE_CONTINUE;
     }
 
     _updateFrame(data, width, height) {
@@ -431,6 +440,11 @@ export class LiveWallpaperManager {
             this._noFrameWatchdogId = 0;
         }
 
+        if (this._pollTimeoutId) {
+            GLib.source_remove(this._pollTimeoutId);
+            this._pollTimeoutId = 0;
+        }
+
         if (this._playbin) {
             const Gst = this._Gst;
             const bus = this._playbin.get_bus();