[3.13] gh-117378: Only run the new multiprocessing SysPath test when appropriate (GH-126635)

Lib/test/_test_multiprocessing.py

@@ -258,6 +258,9 @@ def __call__(self, *args, **kwds):
 class BaseTestCase(object):
 
     ALLOWED_TYPES = ('processes', 'manager', 'threads')
+    # If not empty, limit which start method suites run this class.
+    START_METHODS: set[str] = set()
+    start_method = None  # set by install_tests_in_module_dict()
 
     def assertTimingAlmostEqual(self, a, b):
         if CHECK_TIMINGS:
@@ -6349,7 +6352,9 @@ def test_atexit(self):
 class _TestSpawnedSysPath(BaseTestCase):
     """Test that sys.path is setup in forkserver and spawn processes."""
 
-    ALLOWED_TYPES = ('processes',)
+    ALLOWED_TYPES = {'processes'}
+    # Not applicable to fork which inherits everything from the process as is.
+    START_METHODS = {"forkserver", "spawn"}
 
     def setUp(self):
         self._orig_sys_path = list(sys.path)
@@ -6361,11 +6366,8 @@ def setUp(self):
         sys.path[:] = [p for p in sys.path if p]  # remove any existing ""s
         sys.path.insert(0, self._temp_dir)
         sys.path.insert(0, "")  # Replaced with an abspath in child.
-        try:
-            self._ctx_forkserver = multiprocessing.get_context("forkserver")
-        except ValueError:
-            self._ctx_forkserver = None
-        self._ctx_spawn = multiprocessing.get_context("spawn")
+        self.assertIn(self.start_method, self.START_METHODS)
+        self._ctx = multiprocessing.get_context(self.start_method)
 
     def tearDown(self):
         sys.path[:] = self._orig_sys_path
@@ -6376,15 +6378,15 @@ def enq_imported_module_names(queue):
         queue.put(tuple(sys.modules))
 
     def test_forkserver_preload_imports_sys_path(self):
-        ctx = self._ctx_forkserver
-        if not ctx:
-            self.skipTest("requires forkserver start method.")
+        if self._ctx.get_start_method() != "forkserver":
+            self.skipTest("forkserver specific test.")
         self.assertNotIn(self._mod_name, sys.modules)
         multiprocessing.forkserver._forkserver._stop()  # Must be fresh.
-        ctx.set_forkserver_preload(
+        self._ctx.set_forkserver_preload(
             ["test.test_multiprocessing_forkserver", self._mod_name])
-        q = ctx.Queue()
-        proc = ctx.Process(target=self.enq_imported_module_names, args=(q,))
+        q = self._ctx.Queue()
+        proc = self._ctx.Process(
+                target=self.enq_imported_module_names, args=(q,))
         proc.start()
         proc.join()
         child_imported_modules = q.get()
@@ -6402,23 +6404,19 @@ def enq_sys_path_and_import(queue, mod_name):
             queue.put(None)
 
     def test_child_sys_path(self):
-        for ctx in (self._ctx_spawn, self._ctx_forkserver):
-            if not ctx:
-                continue
-            with self.subTest(f"{ctx.get_start_method()} start method"):
-                q = ctx.Queue()
-                proc = ctx.Process(target=self.enq_sys_path_and_import,
-                                   args=(q, self._mod_name))
-                proc.start()
-                proc.join()
-                child_sys_path = q.get()
-                import_error = q.get()
-                q.close()
-                self.assertNotIn("", child_sys_path)  # replaced by an abspath
-                self.assertIn(self._temp_dir, child_sys_path)  # our addition
-                # ignore the first element, it is the absolute "" replacement
-                self.assertEqual(child_sys_path[1:], sys.path[1:])
-                self.assertIsNone(import_error, msg=f"child could not import {self._mod_name}")
+        q = self._ctx.Queue()
+        proc = self._ctx.Process(
+                target=self.enq_sys_path_and_import, args=(q, self._mod_name))
+        proc.start()
+        proc.join()
+        child_sys_path = q.get()
+        import_error = q.get()
+        q.close()
+        self.assertNotIn("", child_sys_path)  # replaced by an abspath
+        self.assertIn(self._temp_dir, child_sys_path)  # our addition
+        # ignore the first element, it is the absolute "" replacement
+        self.assertEqual(child_sys_path[1:], sys.path[1:])
+        self.assertIsNone(import_error, msg=f"child could not import {self._mod_name}")
 
 
 class MiscTestCase(unittest.TestCase):
@@ -6615,6 +6613,8 @@ def install_tests_in_module_dict(remote_globs, start_method,
             if base is BaseTestCase:
                 continue
             assert set(base.ALLOWED_TYPES) <= ALL_TYPES, base.ALLOWED_TYPES
+            if base.START_METHODS and start_method not in base.START_METHODS:
+                continue  # class not intended for this start method.
             for type_ in base.ALLOWED_TYPES:
                 if only_type and type_ != only_type:
                     continue
@@ -6628,6 +6628,7 @@ class Temp(base, Mixin, unittest.TestCase):
                     Temp = hashlib_helper.requires_hashdigest('sha256')(Temp)
                 Temp.__name__ = Temp.__qualname__ = newname
                 Temp.__module__ = __module__
+                Temp.start_method = start_method
                 remote_globs[newname] = Temp
         elif issubclass(base, unittest.TestCase):
             if only_type: