[core] Queue::drop is acquiring the snatch guard as well (#8691)

Author: uael

wgpu-core/src/device/resource.rs

@@ -854,7 +854,20 @@ impl Device {
                 user_closures.mappings,
                 user_closures.blas_compact_ready,
                 queue_empty,
-            ) = queue_result
+            ) = queue_result;
+            // DEADLOCK PREVENTION: We must drop `snatch_guard` before `queue` goes out of scope.
+            //
+            // `Queue::drop` acquires the snatch guard. If we still hold it when `queue` is dropped
+            // at the end of this block, we would deadlock. This can happen in the following scenario:
+            //
+            // - Thread A calls `Device::maintain` while Thread B holds the last strong ref to the queue.
+            // - Thread A calls `self.get_queue()`, obtaining a new strong ref, and enters this branch.
+            // - Thread B drops its strong ref, making Thread A's ref the last one.
+            // - When `queue` goes out of scope here, `Queue::drop` runs and tries to acquire the
+            //   snatch guard — but Thread A (this thread) still holds it, causing a deadlock.
+            drop(snatch_guard);
+        } else {
+            drop(snatch_guard);
         };
 
         // Based on the queue empty status, and the current finished submission index, determine the result of the poll.
@@ -909,7 +922,6 @@ impl Device {
 
         // Don't hold the locks while calling release_gpu_resources.
         drop(fence);
-        drop(snatch_guard);
 
         if should_release_gpu_resource {
             self.release_gpu_resources();