src: lib: stream: sink: image_sink: Fix thumbnail cold-start timeout

Request an upstream ForceKeyUnit event before waiting for a snapshot
frame. The identity filter drops all non-keyframes (DELTA_UNIT), so
without this the appsink must wait for the encoder's next natural
keyframe which can take 8+ seconds with default key-int-max settings.

Also increase the timeout from 2s to 15s as a safety net for slow
pipelines.

Author: joaoantoniocardoso

src/lib/stream/sink/image_sink.rs

@@ -435,12 +435,21 @@ impl ImageSink {
                 .remove_probe(blocker);
         }
 
-        // Trigger the snapshot
+        // Request an immediate keyframe so the identity filter (which drops
+        // non-keyframes via drop-buffer-flags = DELTA_UNIT) lets a frame
+        // through without waiting for the encoder's natural key-int-max.
+        if let Some(queue_src_pad) = self.queue.static_pad("src") {
+            let event = gst_video::UpstreamForceKeyUnitEvent::builder()
+                .all_headers(true)
+                .build();
+            queue_src_pad.send_event(event);
+        }
+
         let mut receiver = self.flat_samples_sender.subscribe();
 
-        // Wait for the snapshot to be taken, with a timeout
-        // Here we'd have the raw snapshot, we just need to convert it to the final format/size
-        tokio::time::timeout(tokio::time::Duration::from_secs(2), receiver.recv())
+        // Wait for the snapshot with enough headroom for the keyframe to
+        // arrive even on slow pipelines or high key-int-max settings.
+        tokio::time::timeout(tokio::time::Duration::from_secs(15), receiver.recv())
             .await??
             .map_err(|e| anyhow!(e.to_string()))
     }