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

The first version had it running two forkserver and one spawn tests underneath each of the _fork, _forkserver, and _spawn test suites that build off the generic one.

This adds to the existing complexity of the multiprocessing test suite by offering BaseTestCase classes another attribute to control which suites they are invoked under. Practicality vs purity here. :/

Net result: we don't over-run the new test and their internal logic is simplified.

Lib/test/_test_multiprocessing.py

@@ -258,6 +258,9 @@ class TimingWrapper(object):
 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:
@@ -6403,7 +6406,9 @@ class _TestAtExit(BaseTestCase):
 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)
@@ -6415,11 +6420,8 @@ class _TestSpawnedSysPath(BaseTestCase):
         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
@@ -6430,15 +6432,15 @@ class _TestSpawnedSysPath(BaseTestCase):
         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()
@@ -6456,23 +6458,19 @@ class _TestSpawnedSysPath(BaseTestCase):
             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):
@@ -6669,6 +6667,8 @@ 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
@@ -6682,6 +6682,7 @@ def install_tests_in_module_dict(remote_globs, start_method,
                     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: