bpo-30028: make test.support.temp_cwd() fork-safe

Improve the context manager test.support.temp_cwd() to not remove the temporary
directory, if a forked child terminates.

Lib/test/support/__init__.py

@@ -960,10 +960,12 @@ def temp_dir(path=None, quiet=False):
             warnings.warn(f'tests may fail, unable to create '
                           f'temporary directory {path!r}: {exc}',
                           RuntimeWarning, stacklevel=3)
+    if dir_created:
+        pid = os.getpid()
     try:
         yield path
     finally:
-        if dir_created:
+        if dir_created and pid == os.getpid():
             rmtree(path)
 
 @contextlib.contextmanager

Lib/test/test_regrtest.py

@@ -821,5 +821,20 @@ def test_crashed(self):
                                   randomize=True)
 
 
+class TempCwdTestCase(unittest.TestCase):
+    @unittest.skipUnless(hasattr(os, "fork"), "test requires os.fork")
+    def test_forked_child(self):
+        import sys
+        with support.temp_cwd() as cwd:
+            pid = os.fork()
+            if pid != 0:
+                # parent
+                os.waitpid(pid, 0)
+                self.assertTrue(os.path.isdir(cwd), "directory was removed "+cwd)
+        if pid == 0:
+            # terminate the child in order to not confuse the test runner
+            os._exit(0)
+
+
 if __name__ == '__main__':
     unittest.main()