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fix: add thread safety to preload_windows_gl_dlls #421

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fix: add thread safety to preload_windows_gl_dlls #421

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41 changes: 21 additions & 20 deletions envpool/python/glfw_context.py
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Original file line number Diff line number Diff line change
Expand Up @@ -67,26 +67,27 @@ def preload_windows_gl_dlls(
if resolved_dll_dir is None:
return
resolved_str = str(resolved_dll_dir)
if prepend_path:
path_entries = os.environ.get("PATH", "").split(os.pathsep)
if resolved_str not in path_entries:
filtered_entries = [entry for entry in path_entries if entry]
os.environ["PATH"] = os.pathsep.join([
resolved_str,
*filtered_entries,
])
if resolved_str not in _REGISTERED_DLL_DIRS:
_WINDOWS_DLL_HANDLES.append(os.add_dll_directory(resolved_str))
_REGISTERED_DLL_DIRS.add(resolved_str)
win_dll = getattr(ctypes, "WinDLL", None)
if win_dll is None:
return
for dll_name in ("libglapi.dll", "libgallium_wgl.dll", "opengl32.dll"):
dll_path = resolved_dll_dir / dll_name
dll_path_str = str(dll_path)
if dll_path.is_file() and dll_path_str not in _PRELOADED_DLL_PATHS:
_WINDOWS_DLL_HANDLES.append(win_dll(str(dll_path)))
_PRELOADED_DLL_PATHS.add(dll_path_str)
with _CONTEXT_LOCK:

@chatgpt-codex-connector chatgpt-codex-connector Bot May 28, 2026

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P1 Badge Avoid reacquiring the non-reentrant context lock

On Windows when ENVPOOL_DLL_DIR is set to a valid DLL directory, try_ensure_mujoco_glfw_context() already holds _CONTEXT_LOCK while constructing _GlfwContext, and _GlfwContext.__init__() calls preload_windows_gl_dlls(). This new with _CONTEXT_LOCK: therefore tries to acquire the same threading.Lock a second time in the same thread, causing the first GLFW context initialization to hang instead of rendering or falling back. Use an RLock or move the preload outside the outer locked region.

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if prepend_path:
path_entries = os.environ.get("PATH", "").split(os.pathsep)
if resolved_str not in path_entries:
filtered_entries = [entry for entry in path_entries if entry]
os.environ["PATH"] = os.pathsep.join([
resolved_str,
*filtered_entries,
])
if resolved_str not in _REGISTERED_DLL_DIRS:
_WINDOWS_DLL_HANDLES.append(os.add_dll_directory(resolved_str))
_REGISTERED_DLL_DIRS.add(resolved_str)
win_dll = getattr(ctypes, "WinDLL", None)
if win_dll is None:
return
for dll_name in ("libglapi.dll", "libgallium_wgl.dll", "opengl32.dll"):
dll_path = resolved_dll_dir / dll_name
dll_path_str = str(dll_path)
if dll_path.is_file() and dll_path_str not in _PRELOADED_DLL_PATHS:
_WINDOWS_DLL_HANDLES.append(win_dll(str(dll_path)))
_PRELOADED_DLL_PATHS.add(dll_path_str)


def _glfw_error_details(glfw: object) -> str:
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