Fix OSX compatibility

esmvalcore/_task.py

@@ -785,6 +785,15 @@ def __repr__(self):
         return string
 
 
+def available_cpu_count() -> int:
+    """Return the number of available CPU cores."""
+    if hasattr(os, "sched_getaffinity"):
+        return len(os.sched_getaffinity(0))
+    if count := os.cpu_count():
+        return count
+    return 1
+
+
 class TaskSet(set):
     """Container for tasks."""
 
@@ -867,7 +876,7 @@ def _get_dask_config(self, max_parallel_tasks: int) -> dict:
             # No need to do anything when we are not running PreprocessingTasks.
             return {}
 
-        n_available_cpu_cores = len(os.sched_getaffinity(0))
+        n_available_cpu_cores = available_cpu_count()
         n_threaded_dask_schedulers = min(n_preproc_tasks, max_parallel_tasks)
         n_workers = max(
             1, round(n_available_cpu_cores / n_threaded_dask_schedulers)
@@ -890,7 +899,7 @@ def _run_parallel(self, scheduler_address, max_parallel_tasks):
         n_running = 0
 
         if max_parallel_tasks is None:
-            max_parallel_tasks = len(os.sched_getaffinity(0))
+            max_parallel_tasks = available_cpu_count()
         max_parallel_tasks = min(max_parallel_tasks, n_tasks)
         logger.info(
             "Running %s tasks using %s processes", n_tasks, max_parallel_tasks

tests/unit/task/test_available_cpu_count.py

@@ -0,0 +1,25 @@
+import pytest
+
+from esmvalcore import _task
+
+
+def test_available_cpu_count_linux(mocker):
+    mocker.patch.object(_task.os, "sched_getaffinity", return_value={0, 1})
+    result = _task.available_cpu_count()
+    assert result == 2
+    _task.os.sched_getaffinity.assert_called_once_with(0)
+
+
+@pytest.mark.parametrize(
+    "cpu_count,expected",
+    [
+        (None, 1),
+        (2, 2),
+    ],
+)
+def test_available_cpu_count_osx(monkeypatch, mocker, cpu_count, expected):
+    monkeypatch.delattr(_task.os, "sched_getaffinity")
+    mocker.patch.object(_task.os, "cpu_count", return_value=cpu_count)
+    result = _task.available_cpu_count()
+    assert result == expected
+    _task.os.cpu_count.assert_called_once_with()