fix: threadpool configuration

src/zarr/core/sync.py

@@ -54,9 +54,7 @@ def _get_executor() -> ThreadPoolExecutor:
     global _executor
     if not _executor:
         max_workers = config.get("threading.max_workers", None)
-        print(max_workers)
-        # if max_workers is not None and max_workers > 0:
-        #     raise ValueError(max_workers)
+        logger.debug("Creating Zarr ThreadPoolExecutor with max_workers=%s", max_workers)
         _executor = ThreadPoolExecutor(max_workers=max_workers, thread_name_prefix="zarr_pool")
         _get_loop().set_default_executor(_executor)
     return _executor
@@ -118,6 +116,9 @@ def sync(
         # NB: if the loop is not running *yet*, it is OK to submit work
         # and we will wait for it
         loop = _get_loop()
+    if _executor is None and config.get("threading.max_workers", None) is not None:
+        # trigger executor creation and attach to loop
+        _ = _get_executor()
     if not isinstance(loop, asyncio.AbstractEventLoop):
         raise TypeError(f"loop cannot be of type {type(loop)}")
     if loop.is_closed():
@@ -153,6 +154,7 @@ def _get_loop() -> asyncio.AbstractEventLoop:
             # repeat the check just in case the loop got filled between the
             # previous two calls from another thread
             if loop[0] is None:
+                logger.debug("Creating Zarr event loop")
                 new_loop = asyncio.new_event_loop()
                 loop[0] = new_loop
                 iothread[0] = threading.Thread(target=new_loop.run_forever, name="zarr_io")

tests/test_sync.py

@@ -12,6 +12,7 @@
     _get_lock,
     _get_loop,
     cleanup_resources,
+    loop,
     sync,
 )
 from zarr.storage import MemoryStore
@@ -148,11 +149,20 @@ def test_open_positional_args_deprecate():
 
 
 @pytest.mark.parametrize("workers", [None, 1, 2])
-def test_get_executor(clean_state, workers) -> None:
+def test_threadpool_executor(clean_state, workers: int | None) -> None:
     with zarr.config.set({"threading.max_workers": workers}):
-        e = _get_executor()
-        if workers is not None and workers != 0:
-            assert e._max_workers == workers
+        _ = zarr.zeros(shape=(1,))  # trigger executor creation
+        assert loop != [None]  # confirm loop was created
+        if workers is None:
+            # confirm no executor was created if no workers were specified
+            # (this is the default behavior)
+            assert loop[0]._default_executor is None
+        else:
+            # confirm executor was created and attached to loop as the default executor
+            # note: python doesn't have a direct way to get the default executor so we
+            # use the private attribute
+            assert _get_executor() is loop[0]._default_executor
+            assert _get_executor()._max_workers == workers
 
 
 def test_cleanup_resources_idempotent() -> None: