doc: Update MultiProcPlugin.__init__ docstring to explain app_config

Author: effigies

niworkflows/engine/plugin.py

@@ -401,6 +401,16 @@ def __init__(self, pool=None, plugin_args=None):
         """
         Initialize the plugin.
 
+        If passed a nipreps-style configuration object in `plugin_args["app_config"]`,
+        the following fields must be present:
+
+        app_config.environment.total_memory : :obj:`float`
+            Memory available to the workflow in gigabytes.
+        app_config._process_initializer : :obj:`callable`
+            A function that accepts a file path and returns None, to be run in each worker.
+        app_config.file_path : :obj:`str`
+            The path to a file that will be passed to the initializer.
+
         Arguments
         ---------
         pool : :obj:`~concurrent.futures.Executor`

niworkflows/engine/tests/test_plugin.py

@@ -75,7 +75,7 @@ def test_plugin_args_noconfig(workflow, caplog):
     workflow.run(plugin=MultiProcPlugin(plugin_args={"n_procs": 2, "memory_gb": 0.1}))
 
 
-def touch_file(file_path):
+def touch_file(file_path: str) -> None:
     """Module-level functions play more nicely with multiprocessing."""
     with open(file_path, "w") as f:
         f.write("flag")
@@ -89,7 +89,7 @@ def test_plugin_app_config(tmp_path, workflow, caplog):
     app_config = SimpleNamespace(
         environment=SimpleNamespace(total_memory=1),
         _process_initializer=touch_file,
-        file_path=init_flag,
+        file_path=str(init_flag),
     )
     caplog.set_level(logging.INFO, logger="nipype.workflow")
     workflow.run(