fix gpu shutdown panic

src/main.rs

···
         handle.join().unwrap();
     }
     if let Some(handle) = gpu_handle {
-        handle.join().unwrap();
+        if let Err(e) = handle.join() {
+            println!("GPU thread exited with error: {:?}", e);
+        }
     }
 
     println!("Finished heating for {:?}", start_time.elapsed());
···
             });
             compute_pass.set_pipeline(&compute_pipeline);
             compute_pass.set_bind_group(0, &bind_group, &[]);
-            // Dispatch enough work to keep GPU busy
-            compute_pass.dispatch_workgroups(256, 256, 1);
+            // Dispatch enough work to keep GPU busy. Use a smaller grid to
+            // avoid very long-running single submissions which can trigger
+            // backend timeouts during shutdown.
+            compute_pass.dispatch_workgroups(128, 128, 1);
         }
 
+        // Gracefully handle errors on shutdown
+        // Submit work
         queue.submit(Some(encoder.finish()));
-        // Queue submission handles synchronization
-
         iteration += 1;
-        //if iteration % 100 == 0 {
-        //    println!("GPU iterations: {}", iteration);
-        //}
+    }
+
+    // Wait for all submitted work to complete before returning to avoid
+    // backend timeouts during shutdown. `on_submitted_work_done` takes a
+    // callback, so use a channel and wait with a timeout to avoid hanging
+    // indefinitely.
+    let (tx, rx) = std::sync::mpsc::sync_channel(1);
+    queue.on_submitted_work_done(move || {
+        // best-effort send; ignore error if receiver was dropped
+        let _ = tx.send(());
+    });
+    match rx.recv_timeout(Duration::from_secs(30)) {
+        Ok(()) => (),
+        Err(std::sync::mpsc::RecvTimeoutError::Timeout) => {
+            eprintln!("Warning: timed out waiting for GPU work to complete during shutdown");
+        }
+        Err(e) => {
+            eprintln!("Warning: error while waiting for GPU shutdown: {:?}", e);
+        }
     }
 
     println!("GPU heater stopped after {} iterations", iteration);