fix: use multiprocessing whenever fork context is available

Author: bhearsum

src/taskgraph/generator.py

@@ -7,7 +7,6 @@
 import logging
 import multiprocessing
 import os
-import platform
 from concurrent.futures import (
     FIRST_COMPLETED,
     ProcessPoolExecutor,
@@ -441,7 +440,9 @@ def _run(self):
         # redone in the new processes. Ideally this would be fixed, or we
         # would take another approach to parallel kind generation. In the
         # meantime, it's not supported outside of Linux.
-        if platform.system() != "Linux" or os.environ.get("TASKGRAPH_SERIAL"):
+        if "fork" not in multiprocessing.get_all_start_methods() or os.environ.get(
+            "TASKGRAPH_SERIAL"
+        ):
             all_tasks = self._load_tasks_serial(kinds, kind_graph, parameters)
         else:
             all_tasks = self._load_tasks_parallel(kinds, kind_graph, parameters)

test/test_generator.py

@@ -3,6 +3,7 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 
+import multiprocessing
 from concurrent.futures import ProcessPoolExecutor
 
 import pytest
@@ -12,6 +13,11 @@
 from taskgraph.generator import Kind, load_tasks_for_kind, load_tasks_for_kinds
 from taskgraph.loader.default import loader as default_loader
 
+forkonly = pytest.mark.skipif(
+    "fork" not in multiprocessing.get_all_start_methods(),
+    reason="requires 'fork' multiprocessing support",
+)
+
 
 class FakePPE(ProcessPoolExecutor):
     loaded_kinds = []
@@ -21,6 +27,7 @@ def submit(self, kind_load_tasks, *args):
         return super().submit(kind_load_tasks, *args)
 
 
+@forkonly
 def test_kind_ordering(mocker, maketgg):
     "When task kinds depend on each other, they are loaded in postorder"
     mocked_ppe = mocker.patch.object(generator, "ProcessPoolExecutor", new=FakePPE)