update caching for free-threaded python compatibility

Author: ndgrigorian

dpctl/_sycl_device_factory.pyx

@@ -439,12 +439,27 @@ cdef class _DefaultDeviceCache:
         return _copy
 
 
+# no default, as would share a single mutable instance across threads and
+# concurrent access to the cache would not be thread-safe. Using ContextVar
+# without a default ensures each context gets its own instance.
 _global_default_device_cache = ContextVar(
     "global_default_device_cache",
-    default=_DefaultDeviceCache()
 )
 
 
+cdef _DefaultDeviceCache _get_default_device_cache():
+    """
+    Factory function to get or create a default device cache for the current
+    context
+    """
+    try:
+        return _global_default_device_cache.get()
+    except LookupError:
+        cache = _DefaultDeviceCache()
+        _global_default_device_cache.set(cache)
+        return cache
+
+
 cpdef SyclDevice _cached_default_device():
     """Returns a cached device selected by default selector.
 
@@ -453,7 +468,7 @@ cpdef SyclDevice _cached_default_device():
             A cached default-selected SYCL device.
 
     """
-    cdef _DefaultDeviceCache _cache = _global_default_device_cache.get()
+    cdef _DefaultDeviceCache _cache = _get_default_device_cache()
     d_, changed_ = _cache.get_or_create()
     if changed_:
         _global_default_device_cache.set(_cache)

dpctl/_sycl_queue_manager.pyx

@@ -75,12 +75,27 @@ cdef class _DeviceDefaultQueueCache:
         return _copy
 
 
+# no default, as would share a single mutable instance across threads and
+# concurrent access to the cache would not be thread-safe. Using ContextVar
+# without a default ensures each context gets its own instance.
 _global_device_queue_cache = ContextVar(
     "global_device_queue_cache",
-    default=_DeviceDefaultQueueCache()
 )
 
 
+cdef _DeviceDefaultQueueCache _get_device_queue_cache():
+    """
+    Factory function to get or create a default device queue cache for the
+    current context
+    """
+    try:
+        return _global_device_queue_cache.get()
+    except LookupError:
+        cache = _DeviceDefaultQueueCache()
+        _global_device_queue_cache.set(cache)
+        return cache
+
+
 cpdef object get_device_cached_queue(object key):
     """Returns a cached queue associated with given device.
 
@@ -97,7 +112,7 @@ cpdef object get_device_cached_queue(object key):
         TypeError: If the input key is not one of the accepted types.
 
     """
-    _cache = _global_device_queue_cache.get()
+    _cache = _get_device_queue_cache()
     q_, changed_ = _cache.get_or_create(key)
     if changed_:
         _global_device_queue_cache.set(_cache)