fix(neshub): a picture switch waits for the new keyframe even behind a stalled writer (#425)

Follow-up to #423 from its re-review. The dgram writer now raises
awaiting_keyframe when it reaches the Resubscribe marker, in queue
order, and restart_picture sets the flag before queuing the marker.
Before this, an old keyframe still queued ahead of a primary switch
could clear the flag, letting the new picture's first delta out as the
first frame an old client saw after the switch. Needs a writer stalled
for at least 1 s to trigger.

Tested: cargo test -p neshub on nestripc-1, 78 passed.

apps/neshub/src/dgram.rs

@@ -318,6 +318,13 @@ pub async fn run_datagram_writer(
             MediaItem::Resubscribe => {
                 seq = 0;
                 resync = ResyncGate::default();
+                // Set here, in queue order, not only by whoever queued this: a
+                // keyframe from the old picture still queued ahead of it would
+                // otherwise clear the flag, and the new picture's first delta
+                // would go out as the first frame after the switch.
+                if let Some(ref awaiting) = awaiting_keyframe {
+                    awaiting.store(true, Ordering::Relaxed);
+                }
                 continue;
             }
             // No keyframe stream and no resync gate: every PyroWave frame

apps/neshub/src/session.rs

@@ -513,10 +513,10 @@ impl ClientSession {
     /// zero, hold deltas until a keyframe, and forget what it reported about
     /// the old one. The caller asks for the keyframe.
     pub fn restart_picture(&self) {
+        self.awaiting_keyframe.store(true, Ordering::Relaxed);
         if self.pending_video.is_none() {
             let _ = self.send_video.send(MediaItem::Resubscribe);
         }
-        self.awaiting_keyframe.store(true, Ordering::Relaxed);
         let _ = self.take_report();
     }