From 7e9f955b4023887439eba681f14bbb81b40c1357 Mon Sep 17 00:00:00 2001
From: Juan Pizarro <jpizarrom@gmail.com>
Date: Thu, 17 Jul 2025 17:01:48 +0200
Subject: [PATCH] fix(hil-serl): drain queue on get_last_item_from_queue
 (#1524)

* fix(hil-serl): drain queue on get_last_item_from_queue

* parametrize queue tests

* revert changes for Darwin

* revert parametrize queue tests

* add test_get_last_item_multiple_items_with_torch_queue

* update test_get_last_item_multiple_items_with_torch_queue

* update test_get_last_item_multiple_items_with_torch_queue
---
 src/lerobot/utils/queue.py | 21 +++++++++++++++++----
 tests/utils/test_queue.py  | 16 ++++++++++++++++
 2 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/src/lerobot/utils/queue.py b/src/lerobot/utils/queue.py
index ceb30e2b..864d798a 100644
--- a/src/lerobot/utils/queue.py
+++ b/src/lerobot/utils/queue.py
@@ -14,6 +14,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import platform
+from contextlib import suppress
 from queue import Empty
 from typing import Any
 
@@ -30,10 +32,21 @@ def get_last_item_from_queue(queue: Queue, block=True, timeout: float = 0.1) ->
         item = None
 
     # Drain queue and keep only the most recent parameters
-    try:
-        while True:
+    if platform.system() == "Darwin":
+        # On Mac, avoid using `qsize` due to unreliable implementation.
+        # There is a comment on `qsize` code in the Python source:
+        # Raises NotImplementedError on Mac OSX because of broken sem_getvalue()
+        try:
+            while True:
+                item = queue.get_nowait()
+        except Empty:
+            pass
+
+        return item
+
+    # Details about using qsize in https://github.com/huggingface/lerobot/issues/1523
+    while queue.qsize() > 0:
+        with suppress(Empty):
             item = queue.get_nowait()
-    except Empty:
-        pass
 
     return item
diff --git a/tests/utils/test_queue.py b/tests/utils/test_queue.py
index 0a0d2177..6e42acdb 100644
--- a/tests/utils/test_queue.py
+++ b/tests/utils/test_queue.py
@@ -18,6 +18,8 @@ import threading
 import time
 from queue import Queue
 
+from torch.multiprocessing import Queue as TorchMPQueue
+
 from lerobot.utils.queue import get_last_item_from_queue
 
 
@@ -46,6 +48,20 @@ def test_get_last_item_multiple_items():
     assert queue.empty()
 
 
+def test_get_last_item_multiple_items_with_torch_queue():
+    """Test getting the last item when queue has multiple items."""
+    queue = TorchMPQueue()
+    items = ["first", "second", "third", "fourth", "last"]
+
+    for item in items:
+        queue.put(item)
+
+    result = get_last_item_from_queue(queue)
+
+    assert result == "last"
+    assert queue.empty()
+
+
 def test_get_last_item_different_types():
     """Test with different data types in the queue."""
     queue = Queue()