[3.15] gh-159538: Fix deep chain leak in test_deeply_dotted_name (GH-159576) (#159609)

gh-159538: Fix deep chain leak in test_deeply_dotted_name (GH-159576)

test_deeply_dotted_name() builds a 10,000-deep chain of lazy import
placeholders in a thread with a 256 KiB C stack.  Freeing the chain
there defers most deallocations to the trashcan, which only drains them
once the stack margin recovers.  With a stack that small the margin
never does, so the deferred objects were leaked when the thread exited,
which -R reports as a reference leak on platforms with larger frames
such as PPC64LE.

Clear the namespace in a cleanup, which runs on the main thread.
(cherry picked from commit c84d4b5)

Co-authored-by: Brittany Reynoso <breynoso@meta.com>

Author: miss-islington

Lib/test/test_lazy_import/__init__.py

@@ -325,6 +325,9 @@ def test_deeply_dotted_name(self):
         m.m = m
         ns = {"__builtins__": {"__lazy_import__": __lazy_import__,
                                "__import__": lambda *args: m}}
+        # Free the chain on the main thread: a thread with a small C stack
+        # postpones deep deallocations and never gets back to them.
+        self.addCleanup(ns.clear)
         exec("lazy import " + ".".join(["m"] * 10_000) + " as x", ns)
         with contextlib.suppress(RecursionError):
             ns["x"].resolve()