bpo-30028: make test.support.temp_cwd() fork-safe (GH-1066)

Make test.support.temp_cwd() fork-safe. The context manager test.support.temp_cwd() no longer removes the temporary directory when executing in a process other than the parent it entered from.
If a forked child exits the context manager it won't do the cleanup.
(cherry picked from commit 33dddac00b)

Co-authored-by: Anselm Kruis <a.kruis@science-computing.de>

Lib/test/test_support.py

@@ -9,9 +9,11 @@ import stat
 import subprocess
 import sys
 import tempfile
+import textwrap
 import time
 import unittest
 from test import support
+from test.support import script_helper
 
 TESTFN = support.TESTFN
 
@@ -165,6 +167,33 @@ class TestSupport(unittest.TestCase):
                                         f'temporary directory {path!r}: '),
                         warn)
 
+    @unittest.skipUnless(hasattr(os, "fork"), "test requires os.fork")
+    def test_temp_dir__forked_child(self):
+        """Test that a forked child process does not remove the directory."""
+        # See bpo-30028 for details.
+        # Run the test as an external script, because it uses fork.
+        script_helper.assert_python_ok("-c", textwrap.dedent("""
+            import os
+            from test import support
+            with support.temp_cwd() as temp_path:
+                pid = os.fork()
+                if pid != 0:
+                    # parent process (child has pid == 0)
+
+                    # wait for the child to terminate
+                    (pid, status) = os.waitpid(pid, 0)
+                    if status != 0:
+                        raise AssertionError(f"Child process failed with exit "
+                                             f"status indication 0x{status:x}.")
+
+                    # Make sure that temp_path is still present. When the child
+                    # process leaves the 'temp_cwd'-context, the __exit__()-
+                    # method of the context must not remove the temporary
+                    # directory.
+                    if not os.path.isdir(temp_path):
+                        raise AssertionError("Child removed temp_path.")
+        """))
+
     # Tests for change_cwd()
 
     def test_change_cwd(self):