Fix fence completion race in virtio-gpu worker

When fences complete out of order (e.g., an immediate-retire for fence
N+1 arrives before the timeline signal for fence N), the unconditional
insert() would overwrite the higher fence_id with the lower one. This
causes fence N+1 to appear incomplete forever, hanging the guest.

Use entry().or_insert() with a max check so only strictly higher
fence_ids update the completed_fences map.

Signed-off-by: Adam Ford <adam.ford@anodize.com>

Author: Adam Ford

src/devices/src/virtio/gpu/virtio_gpu.rs

@@ -203,10 +203,13 @@ impl VirtioGpu {
                     i += 1;
                 }
             }
-            // Update the last completed fence for this context
-            fence_state
-                .completed_fences
-                .insert(ring, completed_fence.fence_id);
+            // Update the last completed fence for this context.
+            // Use max() to avoid a race where an out-of-order completion
+            // (e.g., immediate-retire for fence N+1 followed by timeline
+            // signal for fence N) would overwrite a higher fence_id with
+            // a lower one, causing fence N+1 to be stuck forever.
+            let entry = fence_state.completed_fences.entry(ring).or_insert(0);
+            *entry = (*entry).max(completed_fence.fence_id);
         })
     }